Class 8 Science Chapter 4 Electricity: Magnetic and Heating Effects (New Course)

Key Concept: Magnetic Effect of Electric Current

b) The compass needle deflects from its original direction.
[Solution Description]
When an electric current flows through a conductor, it generates a magnetic field around it. The compass needle is a small magnet that aligns itself along the direction of the magnetic field. Thus, when the current flows, the compass needle deflects due to the magnetic field created by the current-carrying wire. When the circuit is opened and no current flows, the magnetic field disappears, and the compass needle returns to its original position aligned with Earth’s magnetic field.

Key Concept: Magnetic Effect of Electric Current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that an electric current flowing through a wire deflects a nearby compass needle, which is true as observed in Activity 4.1 where the compass needle deflected when the circuit was closed. The reason given is that a current-carrying conductor produces a magnetic field around it, which is also true and correctly explains why the compass needle gets deflected. The magnetic field generated by the current interacts with the compass needle’s magnetic field, causing deflection. Thus, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Electromagnet Strength, Factors Affecting Magnetic Field

d) Increasing the diameter of the wire while keeping the number of turns constant
[Solution Description]
The strength of the magnetic field of a solenoid depends on: (1) number of turns per unit length, (2) current passing through it, and (3) core material. Increasing the number of turns or current strengthens the field, while using a soft iron core enhances it due to high permeability. However, increasing the diameter of the wire without changing other parameters does not affect the field strength because it’s the number of turns and current that matter.

Key Concept: Magnetic Field Direction, Right-Hand Rule

b) West
[Solution Description]
First, recall that the magnetic field lines around a current-carrying straight wire form concentric circles. The direction can be determined using the right-hand rule: if the thumb points in the direction of the current, the fingers curl in the direction of the magnetic field. For a vertical wire with current flowing upwards, the magnetic field to the north of the wire would point west (since the compass needle deflects opposite to the field direction). Therefore, the correct answer is west.

Key Concept: Magnetic Field Calculation, Deflection of Compass Needle

b) $10^{-5}$ T
[Solution Description]
To find the magnetic field strength ($B$), we use the formula:
$B = \frac{\mu_0 I}{2 \pi r}$
Given:
Current ($I$) = 5 A
Distance ($r$) = 10 cm = 0.1 m
$\mu_0 = 4\pi \times 10^{-7}$ Tm/A
Substituting the values:
$B = \frac{(4\pi \times 10^{-7}) \times 5}{2 \pi \times 0.1}$
Simplify the equation:
$B = \frac{20\pi \times 10^{-7}}{0.2\pi}$
$B = 100 \times 10^{-7} = 10^{-5} \text{ T}$
The magnetic field strength is $10^{-5}$ T.

Key Concept: Magnetic Effect of Electric Current, Hans Christian Oersted’s Experiment

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion states that the deflection of a compass needle when current flows through a wire indicates the production of a magnetic field by the current-carrying wire. This is correct because Oersted’s experiment demonstrated that an electric current creates a magnetic field around the conductor, causing the compass needle to deflect. The reason claims that a magnetic field only exists in the presence of a permanent magnet, which is false. Magnetic fields can be produced by both permanent magnets and current-carrying conductors. Therefore, the assertion is true, but the reason is false.

Key Concept: Disappearance of Magnetic Field

b) When the current stops flowing.
[Solution Description]
The magnetic field around a current-carrying wire exists only as long as the current flows through the wire. When the current stops, the magnetic field disappears.

Key Concept: Magnetic Effect of Electric Current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When an electric current flows through a wire, it produces a magnetic field around it, as discovered by Hans Christian Oersted. This magnetic field interacts with the compass needle (which is a tiny magnet), causing it to deflect from its original position. When the current is switched off, the magnetic field disappears, and the compass needle returns to its original direction. Thus, both Assertion (A) and Reason (R) are true, and Reason correctly explains Assertion.

Key Concept: Magnetic effect of electric current

c) The compass needle deflects when current flows and returns to its original position when current stops
[Solution Description]
The compass needle deflects due to the magnetic field generated by the current-carrying wire, as discovered by Hans Christian Oersted. When the current flows, the magnetic field is produced, causing deflection. Once the current stops, the magnetic field disappears, and the needle returns to its original position.

Key Concept: Magnetic field around a current-carrying wire

c) Due to the magnetic field created by the electric current in the wire
[Solution Description]
The current-carrying wire produces a magnetic field around it, which exerts a force on the magnetic compass needle, causing it to deflect. This is because the compass needle aligns itself along the direction of the resultant magnetic field, which includes both Earth’s magnetic field and the wire’s magnetic field.

Key Concept: Magnetic Effect of Current

b) The compass needle deflects from its original direction
[Solution Description]
When electric current flows through a wire, it produces a magnetic field around the wire. This magnetic field interacts with the compass needle (which is a tiny magnet itself), causing deflection. Once the current stops, the magnetic field disappears, and the needle returns to its original position.

Key Concept: Oersted’s discovery, Magnetic effect of electric current

c) Electric current produces a magnetic field around the conductor.
[Solution Description]
Oersted discovered that an electric current produces a magnetic field around the conductor carrying it. This was observed when a compass needle deflected near a current-carrying wire. His discovery established the link between electricity and magnetism, leading to further studies in electromagnetism.

Key Concept: Magnetic field direction

a) Towards north
[Solution Description]
Using the right-hand thumb rule: Point your right thumb in the direction of current (upwards). The curling fingers show the direction of magnetic field lines. For a point east of the wire, the field lines would be directed north.
$B$ direction is tangential to circular field lines around wire: east $\rightarrow$ north orientation.

Key Concept: Deflection observation

b) Needle deflection doubles
[Solution Description]
Magnetic field strength is directly proportional to current ($B \propto I$). Doubling the current doubles the magnetic field strength. Since compass deflection depends on field strength, the deflection angle increases (but not exactly double as it’s a vector addition with Earth’s magnetic field).

Key Concept: Field strength calculation

b) $1 \times 10^{-5}$ T
[Solution Description]
Using the formula:
$B = \frac{\mu_0 I}{2 \pi r}$
Convert cm to m: $4cm = 0.04m$
Substitute values:
$B = \frac{(4\pi \times 10^{-7}) \times 2}{2 \pi \times 0.04}$
Simplify:
$B = \frac{8\pi \times 10^{-7}}{0.08\pi} = 1 \times 10^{-5}$ T

Key Concept: Magnetic effect of current, Compass needle deflection

b) Towards west
[Solution Description]
The magnetic field due to the current-carrying wire can be determined using the right-hand thumb rule. If the current flows from north to south, the magnetic field lines around the wire are clockwise when viewed from the south. Since the compass needle was initially pointing towards geographic north, the north pole of the compass needle will be deflected towards the west.

Key Concept: Electromagnet Polarity

b) The end is a South pole
[Solution Description]
The compass needle’s North pole is attracted to the opposite pole of the electromagnet (South pole). Therefore, if the North pole of the compass points towards the electromagnet, that end must be the South pole.

Key Concept: Electromagnets, Magnetic Field Strength

d) The magnetic field strength will quadruple
[Solution Description]
The strength of an electromagnet depends on the current flowing through the coil and the number of turns. Doubling the number of turns increases the field strength proportionally. Connecting two cells in series doubles the voltage, which also increases the current (assuming negligible resistance). Thus, the combined effect is a fourfold increase in magnetic field strength.

Key Concept: Electromagnets, Core Material

d) The wooden-core electromagnet will be much weaker
[Solution Description]
Iron is a ferromagnetic material that greatly enhances the magnetic field when it forms the core of an electromagnet. Wood is non-magnetic and does not contribute to the magnetic field. Thus, the wooden-core electromagnet will have a significantly weaker magnetic field than the iron-core one for the same current and number of turns.

Key Concept: Magnetic poles of electromagnets

c) By observing which compass pole is attracted to the electromagnet’s end
[Solution Description]
To determine the polarity of an electromagnet using a compass, place the compass near one end of the electromagnet while the current is flowing. If the north pole of the compass needle is attracted to that end, then that end is the south pole of the electromagnet (since unlike poles attract). Similarly, the other end will be the north pole.

Key Concept: Electromagnets and Magnetic Poles

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When electric current flows through the coil of an electromagnet, it generates a magnetic field, making it behave like a magnet. This is why the electromagnet can attract iron clips. However, when the current is switched off, the magnetic field ceases to exist, and the electromagnet no longer exhibits magnetic properties. Therefore, both Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Strength of Electromagnet

d) Reducing the number of turns in the coil
[Solution Description]
The strength of an electromagnet depends on the current flowing through the coil and the number of turns in the coil. Increasing either of these factors increases the strength. However, reducing the number of turns in the coil decreases the magnetic field strength, as fewer turns result in a weaker magnetic effect.

Key Concept: Electromagnet Basics

c) The coil acts as a magnet only while current flows.
[Solution Description]
When an electric current flows through the coil wound around an iron nail, the coil behaves as a magnet and attracts nearby magnetic materials like iron paper clips. The iron core enhances the magnetic effect. When the current is stopped, the magnetic effect disappears.

Key Concept: Factors Affecting Electromagnet Strength

c) Increasing the number of turns in the coil.
[Solution Description]
The strength of an electromagnet depends on the amount of current flowing through the coil and the number of turns in the coil. Increasing either the current or the number of turns strengthens the magnetic field. Using an iron core also enhances the magnetic effect.

Key Concept: Factors affecting electromagnet strength, Current effect

c) Student Y’s electromagnet will be stronger
[Solution Description]
The strength of an electromagnet increases with the number of turns in the coil because more turns create a stronger magnetic field for the same current. Since both students use the same cell (same current), Student Y’s coil with twice the number of turns will produce a stronger magnetic field than Student X’s, resulting in greater deflection of a compass needle or attracting more iron clips.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
An electromagnet’s strength depends on the magnetic field generated by the current in the coil. When an iron core is inserted, it becomes magnetized due to the current-induced magnetic field, thereby significantly increasing the overall magnetic strength. Thus, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Application

c) In factories and scrap yards for moving metal objects
[Solution Description]
Lifting electromagnets are widely used in factories and scrap yards for moving, lifting, and sorting heavy metal objects efficiently. Their ability to control magnetism via electric current makes them ideal for handling large quantities of metal without physical contact.

Key Concept: Strength Control

b) By increasing or decreasing the current
[Solution Description]
The lifting capacity of an electromagnet can be controlled by adjusting the electric current. Increasing the current strengthens the magnetic field, allowing heavier objects to be lifted. Conversely, decreasing the current weakens the magnetic field, reducing the lifting capacity. Turning the current OFF completely removes the magnetic field, releasing any attached objects.

Key Concept: Resistance and Heating

a) It is a good conductor of electricity and generates more heat.
[Solution Description]
Nichrome wire is preferred in heating devices because it offers high resistance to the flow of electric current. Due to this high resistance, more heat is generated when current passes through it compared to other conductors like copper of the same size and length. It is also a good conductor of electricity, but the primary reason is its high resistance leading to higher heat generation.

Key Concept: Concept: Heating Effect of Electric Current

d) Resistance in the wire converts electrical energy into heat energy.
[Solution Description]
When electric current flows through a conductor like nichrome wire, it faces resistance, which causes some electrical energy to be converted into heat. This heating effect occurs due to the material’s resistivity. Nichrome has high resistance compared to other conductors, leading to more heat generation when current passes through it. Thus, the wire gets warm.

Key Concept: Heat Generation with Multiple Cells

c) The wire heats up more.
[Solution Description]
Using a battery of 2 cells increases the voltage, which results in a higher current flowing through the wire. Since the heat generated depends on the magnitude of the current, the wire will heat up more with 2 cells compared to 1 cell for the same duration.

Key Concept: Heating effect of current, Resistance in conductors

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The heating effect of electric current is given by Joule’s law: $H = I^2 R t$, where $H$ is heat generated, $I$ is current, $R$ is resistance, and $t$ is time. The resistance of a wire depends on its material resistivity ($\rho$) as per $R = \frac{\rho l}{A}$, where $l$ is length and $A$ is cross-sectional area. Since nichrome has higher resistivity than copper, for the same dimensions, nichrome wire will have higher resistance. With the same current passing through both wires, the nichrome wire generates more heat due to its higher resistance. Thus, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Safety precautions, Wire selection

d) To prevent overheating; wire may melt or cause hazards
[Solution Description]
Thinner wires have higher resistance per unit length (since $R \propto 1/A$, where $A$ is cross-sectional area). For a given current, higher resistance causes more heat generation ($H = I^2 R t$). If the wire is too thin, excessive heat can melt insulation, damage plugs/sockets, or cause fires. Hence, using appropriately rated wires ensures safe heat dissipation.

Key Concept: Current magnitude, Heating duration

c) Temperature increases four times due to squared current effect
[Solution Description]
The heat generated ($H$) in a conductor is directly proportional to the square of the current ($I$) and the resistance ($R$) for a given time ($t$), according to Joule’s law: $H = I^2 R t$. When the voltage doubles, the current also doubles (assuming resistance remains constant as per Ohm’s law $V = IR$). Since $H \propto I^2$, doubling the current quadruples the heat produced, leading to a significantly higher temperature in the wire.

Key Concept: Household applications of heating effect

d) Electric fan
[Solution Description]
Household appliances like electric irons, water heaters, and hair dryers use the heating effect of electric current to function. However, an electric fan works on the principle of converting electrical energy into mechanical energy to rotate the blades, not primarily for heating.

Key Concept: Heating effect of electric current

c) The wire will get hotter.
[Solution Description]
Increasing the number of cells increases the voltage in the circuit, which in turn increases the current flowing through the wire according to Ohm’s Law ($V = IR$). The heat generated in the wire depends on the magnitude of the current ($H = I^2Rt$), so more current leads to more heating. Thus, the wire will get hotter.

Key Concept: Resistance and Heating Effect

c) Heat increases
[Solution Description]
The heating effect of electric current increases with the magnitude of the current, as per Joule’s law of heating. Heat generated ($H = I^2 R t$) is directly proportional to the square of the current (I). Therefore, increasing the current leads to more heat production.

Key Concept: Magnetic + Heating effects

c) Both heating and magnetic effects
[Solution Description]
Current produces both magnetic field (deflecting compass) and heating effect (due to resistance). Both phenomena occur simultaneously when current flows.

Key Concept: Factors Affecting Heating

c) Color of the conductor
[Solution Description]
The heating effect depends on the current (I), resistance (R), and time (t) as per $H = I^2 R t$. The color of the conductor does not influence the heat produced

Key Concept: Application of Heating Effect

b) It has high resistivity and high melting point
[Solution Description]
Nichrome is preferred because it has high resistivity, which means it resists the flow of current more than conductors like copper, leading to more heat generation. It also has a high melting point and does not oxidize easily at high temperatures, making it ideal for heating applications.

Key Concept: Resistance, Material property comparison

c) Nichrome has higher resistivity and durability at high temperatures
[Solution Description]
Nichrome has higher resistivity ($\rho$) compared to copper. A high-resistivity material like nichrome generates more heat for the same current flow due to greater opposition ($R = \rho \frac{L}{A}$). Additionally, nichrome has a high melting point and does not oxidize easily at high temperatures, making it durable for heating applications.

Key Concept: Nichrome wire heating effect

c) Due to the conversion of electrical energy into heat energy
[Solution Description]
When electric current flows through a conductor like nichrome wire, it faces resistance. This resistance converts some of the electrical energy into heat energy due to the Joule heating effect. Therefore, the wire gets hot.

Key Concept: Heating Effect of Electric Current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that a nichrome wire heats up when an electric current is passed through it, which is true because of the heating effect of electric current. The reason given is that nichrome has high resistance, which causes electrical energy to convert into heat energy. This is also true and correctly explains why the nichrome wire heats up. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Heating effect of current, Nichrome wire properties

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to Joule’s law of heating, the heat produced ($H$) in a conductor is given by $H = I^2 R t$, where $I$ is current, $R$ is resistance, and $t$ is time. Since nichrome has higher resistivity compared to copper, its resistance ($R$) is higher for the same dimensions. Thus, for the same current ($I$) and duration ($t$), more heat is generated in nichrome due to its higher resistance. Therefore, both Assertion (A) and Reason (R) are true, and Reason correctly explains Assertion.

Key Concept: Heating Effect of Current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that an electric iron gets hot when current passes through its heating element. This is true because appliances like electric irons utilize the heating effect of electric current to generate heat. The Reason states that this heating effect occurs due to the resistance in the conductor, which converts electrical energy into heat energy. This is also true and correctly explains why the assertion holds. Hence, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Energy conversion, Efficiency, Safety

b) 0.3 kWh
[Solution Description]
Total energy consumed per hour = $1500 \text{ W} \times 3600 \text{ s} = 5,400,000 \text{ J}$.
Energy converted to heat = $80\% \times 5,400,000 = 4,320,000 \text{ J}$.
Energy lost = Total energy – Heat energy = $5,400,000 – 4,320,000 = 1,080,000 \text{ J}$.
Converting to kWh: $\frac{1,080,000}{3,600,000} = 0.3 \text{ kWh}$.

Key Concept: Heating Effect of Electric Current

b) Electric iron
[Solution Description]
Household appliances like electric irons, room heaters, and kettles work based on the heating effect of electric current, where electrical energy is converted to heat energy due to resistance in the conductor.

Key Concept: Resistance, Material properties, Industrial applications

b) Increases by 50%
[Solution Description]
Resistance ($R$) is given by $R = \rho \frac{L}{A}$, where $\rho$ is resistivity, $L$ is length, and $A$ is area.
Original resistance: $R_1 = \rho \frac{L}{A}$.
New length: $L_2 = L + 0.5L = 1.5L$.
New resistance: $R_2 = \rho \frac{1.5L}{A} = 1.5R_1$.
Thus, resistance increases by 50%.

Key Concept: Voltaic Cell and Dry Cell

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion is true as per section 4.3.1 and 4.3.2 – Voltaic cells indeed use a liquid electrolyte, whereas dry cells use a moist paste electrolyte. The Reason is also true, since the absence of liquid in dry cells makes them more portable and less prone to leakage or spillage compared to Voltaic cells. Moreover, the Reason correctly explains why dry cells are more portable, which is directly related to the nature of their electrolyte.

Key Concept: Basic Concept

c) They can be recharged and reused
[Solution Description]
Rechargeable batteries can be reused multiple times by recharging them, which reduces waste and saves money in the long run compared to single-use dry cells that must be disposed of after use.

Key Concept: Voltaic Cell Components

c) It facilitates the chemical reaction between the electrodes.
[Solution Description]
The electrolyte in a Voltaic cell facilitates the chemical reaction between the electrodes by allowing ions to move freely, thus enabling the generation of electric current.

Key Concept: Basic Concept

b) Electrolyte
[Solution Description]
The Voltaic cell uses a liquid solution called an electrolyte, which is usually a weak acid or salt solution. This electrolyte facilitates the chemical reaction that produces electricity.

Key Concept: Voltaic cell components

d) Two different metal plates with a conducting liquid
[Solution Description]
A Voltaic cell consists of two different metal electrodes and an electrolyte, which can be a weak acid or salt solution. The chemical reaction between these components generates electricity.
Copper and zinc are common electrode materials, while lemon juice or saltwater is used as the electrolyte.
Therefore, both electrodes and an electrolyte are necessary for a functional Voltaic cell.

Key Concept: Electrode Polarity, Chemical Reaction

b) Zinc electrode because it undergoes oxidation
[Solution Description]
Zinc acts as the negative terminal because it is more reactive than copper. In the electrochemical series, zinc has a higher tendency to lose electrons and undergo oxidation compared to copper. When zinc reacts with the acidic lemon juice (electrolyte), it releases electrons that flow through the external circuit to the copper electrode (positive terminal). This electron flow constitutes the electric current.

Key Concept: Electrochemical reactions, Electrode potential

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In a voltaic cell, the metal with the higher standard reduction potential tends to act as the positive electrode (cathode) because it has a greater tendency to gain electrons (undergo reduction). Copper has a standard reduction potential of $+0.34$ V, while zinc has $-0.76$ V. Since copper’s potential is higher, it attracts electrons more strongly and becomes the cathode (positive terminal). Thus, both Assertion and Reason are true, and Reason correctly explains Assertion.

Key Concept: Electrochemical Series, Cell Performance

c) Cell B due to greater reactivity difference
[Solution Description]
Cell B (magnesium/copper) will produce a higher voltage because magnesium is more reactive than aluminum in the electrochemical series. The greater the difference in reactivity between the two metals, the higher the potential difference (voltage) generated. Magnesium’s stronger tendency to lose electrons compared to aluminum results in a larger electron flow and thus a higher voltage output when paired with copper.

Key Concept: Rechargeable Batteries

c) Lithium-ion battery
[Solution Description]
Lithium-ion (Li-ion) batteries are the most common type of rechargeable batteries today. They are used in many devices such as smartphones, laptops, and electric vehicles. Unlike dry cells, they can be recharged multiple times.

Key Concept: Electrolyte in Dry Cell

b) Thick moist paste of ammonium chloride and zinc chloride
[Solution Description]
The electrolyte in a dry cell is not a liquid but a thick, moist paste. This is why it is called a ‘dry’ cell. The paste consists of ammonium chloride and zinc chloride, which facilitate the chemical reaction necessary for electricity generation.

Key Concept: Dry Cells – Structure and Function

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In a dry cell, the zinc container serves as the negative terminal because it undergoes oxidation (loses electrons) during the chemical reaction that generates electricity. The carbon rod acts as the positive terminal. The reason correctly explains why zinc is used as the negative terminal, as it readily loses electrons due to its position in the electrochemical series.

Key Concept: Dry Cells

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that the dry cell is called ‘dry’ because its electrolyte is a liquid. However, this is false as the dry cell is called ‘dry’ because its electrolyte is a thick moist paste, not a liquid. The Reason correctly explains why it is called a dry cell by stating that the electrolyte is a thick moist paste and not a liquid. Therefore, the Assertion is false, but the Reason is true.

Key Concept: Basic Concept

c) They can be recharged and reused multiple times.
[Solution Description]
Rechargeable batteries can be recharged and reused multiple times, reducing waste and saving money over time compared to single-use batteries.

Key Concept: Rechargeable Battery Chemistry

a) Graphite
[Solution Description]
Lithium-ion batteries use graphite (carbon) as the anode. During charging, lithium ions move from the cathode to the anode and embed themselves between the layers of graphite. This reversible process allows Li-ion batteries to be rechargeable.

Key Concept: Battery Applications

c) Safer operation due to non-flammable solid electrolytes[Solution Description]Solid-state batteries use solid electrolytes instead of liquid or paste-like electrolytes. This makes them safer by reducing leakage and combustion risks, enables faster charging, and improves battery lifespan compared to traditional Li-ion batteries.

Key Concept: Rechargeable Batteries

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The Assertion states that Lithium-ion batteries are the most commonly used rechargeable batteries today. This is true as mentioned in the syllabus under “A step further” section. The Reason states that Lithium-ion batteries use special metals like lithium and cobalt, which are mined in limited parts of the world. This is also true as per the syllabus. However, the Reason does not explain why Lithium-ion batteries are commonly used; it only describes a characteristic of these batteries. Therefore, both statements are true, but the Reason is not the correct explanation of the Assertion.

Key Concept: Voltaic Cell and Electrolyte

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
A Voltaic cell requires an electrolyte, which is a conducting liquid, to facilitate the chemical reaction that generates electricity. The electrolyte allows the movement of ions between the two electrodes (positive and negative), completing the internal circuit. This ion movement is essential for the flow of electric current in the external circuit. Thus, both Assertion (A) and Reason (R) are true, and Reason correctly explains why the electrolyte is necessary in a Voltaic cell.

Key Concept: Voltaic Cell Components

b) Electrolyte
[Solution Description]
The liquid in a Voltaic cell that helps conduct electricity is called an electrolyte. It facilitates the chemical reaction between the metal plates (electrodes) to produce electricity.

Key Concept: Voltaic Cell Working Principle, Chemical Reactions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In a voltaic cell, the zinc electrode acts as the anode where oxidation occurs. The reaction is $\text{Zn (s)} \rightarrow \text{Zn}^{2+} (\text{aq}) + 2e^-$. As zinc atoms lose electrons and form $\text{Zn}^{2+}$ ions, they dissolve into the electrolyte, causing a decrease in the mass of the zinc electrode. The reason correctly explains why the assertion is true, as the dissolution of zinc due to oxidation directly causes the observed mass loss.

Key Concept: Purpose of Electrolyte

c) It enables ion movement to sustain the chemical reaction
[Solution Description]
The electrolyte facilitates the movement of ions between the electrodes, enabling the chemical reaction that produces electricity. Without it, the redox reactions cannot occur.

Key Concept: Dry cells and rechargeable batteries

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the zinc container acts as the negative terminal in a dry cell because zinc is more reactive than carbon. This is correct since zinc loses electrons more easily (higher reactivity) and serves as the anode (negative terminal). The reason explains the process by which zinc provides electrons through oxidation, which is the correct explanation for why zinc is used as the negative terminal. Thus, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Rechargeable batteries

c) Lithium-ion (Li-ion) battery
[Solution Description]
Lithium-ion (Li-ion) batteries are the most widely used rechargeable batteries today due to their high energy density, lightweight, and ability to be recharged multiple times. They are commonly found in smartphones, laptops, and other portable electronic devices.

Key Concept: Rechargeable batteries

c) Lithium-ion battery
[Solution Description]
The lithium-ion (Li-ion) battery is the most common type of rechargeable battery found in almost all modern devices due to its efficiency, rechargeability, and widespread availability.

Key Concept: Dry cells, Rechargeable batteries

c) Rechargeable batteries can be reused, reducing waste and cost.
[Solution Description]
Dry cells are single-use and cannot be recharged. Once their chemical reactions deplete the electrolyte, they must be disposed of. Rechargeable batteries can be recharged multiple times by reversing the chemical reaction through an external power source. This makes them cost-effective and environmentally friendly over time, as they reduce waste.
First, identify that dry cells have a limited lifespan due to irreversible chemical reactions. Second, understand that rechargeable batteries allow reversal of chemical reactions when charged. Third, conclude that using rechargeable batteries reduces waste and long-term costs since they can be reused. Hence, replacing a dry cell with a rechargeable battery is beneficial for sustainability and economy.

Key Concept: Structure of Dry Cell

c) Carbon rod with a metal cap
[Solution Description]
In a dry cell, the carbon rod covered with a metal cap serves as the positive terminal. The chemical reactions occur around this rod, reducing manganese dioxide and ammonium ions.

Key Concept: Features of Dry Cells

c) They are portable and do not leak due to the paste-like electrolyte.
[Solution Description]
Dry cells are portable, lightweight, and leak-proof due to their paste-like electrolyte. They do not require maintenance and are widely used in small electronic devices like flashlights and remote controls.

Key Concept: Applications, Rechargeable Batteries

c) Rechargeable batteries reduce long-term costs and environmental impact.
[Solution Description]
Rechargeable batteries can be reused multiple times by recharging, which reduces long-term costs and environmental waste. Dry cells are single-use and would require frequent replacement, making them impractical for high-energy applications like electric vehicles.

Key Concept: Dry cell structure and function

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the zinc container in a dry cell acts as the negative terminal due to electron donation during the chemical reaction, which is true. The reason explains that zinc’s higher reactivity compared to carbon causes it to lose electrons and act as the negative terminal, which is also correct and provides the scientific basis for the assertion. Therefore, both statements are true, and the reason correctly explains the assertion.

Key Concept: Electrode Materials, Lemon Cell Experiment

c) Lemon juice acts as an electrolyte; LED glows due to electron flow from iron to copper.
[Solution Description]
In the lemon cell experiment:
1. The lemon juice acts as the electrolyte, enabling the flow of ions between the electrodes.
2. Copper (less reactive) acts as the cathode, and iron (more reactive) acts as the anode. Iron loses electrons: $Fe \rightarrow Fe^{2+} + 2e^-$.
3. The electrons flow through the external circuit (LED) to the copper electrode, generating current.
4. The LED glows only when its positive terminal is connected to the copper electrode and the negative terminal to the iron electrode, ensuring correct current direction.

Key Concept: Electrodes and Electrolyte in Voltaic Cells

d) Assertion is false, but Reason is true.
[Solution Description]
When constructing a lemon battery, the copper electrode acts as the positive terminal (cathode) and the iron/nail electrode acts as the negative terminal (anode) due to their differing electrochemical potentials. The LED will only glow when its positive terminal is connected to the cathode (copper) and negative terminal to the anode (iron). If connections are reversed, the LED won’t glow because it blocks reverse current. However, reversing connections doesn’t change the inherent polarity of the electrodes – copper remains the cathode and iron the anode. The direction of current is indeed determined by the electrodes’ electrochemical potentials, but the Assertion incorrectly states that swapping connections changes electrode polarity.

Key Concept: Voltaic Cell Electrodes

b) Acts as the negative terminal and releases electrons.
[Solution Description]
In a Voltaic cell, the zinc electrode undergoes oxidation, losing electrons to form zinc ions ($Zn^{2+}$). The reaction at the zinc electrode is:
$Zn \rightarrow Zn^{2+} + 2e^-$
Therefore, the zinc electrode acts as the negative terminal (anode), supplying electrons to the external circuit.

Key Concept: Electrodes and Electrolyte in Voltaic Cells

c) Assertion is true, but Reason is false.
[Solution Description]
In a Voltaic cell, the electric current indeed flows from the positive electrode to the negative electrode through the external circuit (Assertion is true). However, the Reason given is false because it is the negative electrode (e.g., zinc) that has a higher tendency to lose electrons, not the positive electrode (e.g., copper). The positive electrode attracts electrons due to its lower tendency to oxidize. Therefore, Assertion is true, but Reason is false.

Key Concept: Medical applications, Battery limitations

c) They provide higher energy density and longer lifespan.
[Solution Description]
Lithium batteries offer higher energy density (more energy per unit mass), longer lifespan, and stable voltage output, which is critical for medical devices. Traditional voltaic cells (e.g., zinc-carbon) deplete faster and may leak harmful chemicals, making them unsuitable for implants.

Key Concept: Educational Demonstrations

c) The conversion of chemical energy to electrical energy via redox reactions.
[Solution Description]
The lemon battery demonstrates how two dissimilar metals (such as zinc and copper) immersed in an electrolyte (lemon juice) produce a voltage due to redox reactions. It helps students understand the conversion of chemical energy to electrical energy in a simple setup.

Key Concept: Portable Power Sources

c) They provide continuous power through spontaneous chemical reactions.
[Solution Description]
Portable devices like flashlights require small, self-contained power sources that can convert chemical energy directly into electrical energy without external input. Voltaic cells fulfill this need by providing a steady current due to spontaneous redox reactions.

Key Concept: Emergency Power Backup

d) They operate independently using internal chemical reactions.
[Solution Description]
Emergency lighting systems must operate reliably during power outages. Voltaic cells do not require an external power source as they function independently using internal redox reactions, making them ideal for backup power in emergencies.

Key Concept: Dry cell construction, Electrochemical reactions

c) The cell would produce much lower voltage
[Solution Description]
The zinc container in a dry cell serves as both the negative terminal and participates in the oxidation reaction ($\text{Zn} \rightarrow \text{Zn}^{2+} + 2\text{e}^-$). If replaced with less reactive copper (which has higher reduction potential than zinc), the electrochemical potential difference between electrodes would decrease significantly. This would result in much lower voltage output because the redox reaction becomes less favorable. The carbon rod (positive electrode) requires a more reactive metal (like zinc) to maintain proper cell function.

Key Concept: Electrochemical cells, Multi-concept

c) Replacing the iron nail with a zinc-coated nail
[Solution Description]
Increasing electrode surface area (A), using stronger electrolyte (B), or connecting multiple cells in series (D) would all increase current. However, replacing the iron nail with a zinc-coated nail (C) would actually decrease the potential difference because zinc has similar reactivity to iron (standard reduction potentials: Zn²⁺/Zn = -0.76V, Fe²⁺/Fe = -0.44V), whereas the Cu²⁺/Cu = +0.34V pairing creates greater voltage difference with iron than with zinc.

Key Concept: Construction and working of dry cells

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The zinc container in a dry cell serves as the negative terminal because zinc oxidizes more readily (loses electrons easily). This oxidation process provides the electrons for the current flow, making it the negative terminal. The Reason correctly explains why zinc is chosen as the negative terminal since its reactivity leads to oxidation, which is essential for generating electricity in the cell.
Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Electrolyte in dry cells

d) To prevent leakage and improve portability
[Solution Description]
The electrolyte in a dry cell is a moist paste instead of a liquid to prevent leakage, make the cell portable, and ensure stability during movement or tilting. A liquid electrolyte could spill, causing damage or short circuits. The thick paste maintains conductivity while being safer and more convenient for everyday use.

Key Concept: Battery Basics

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that dry cells are commonly used in flashlights and toys due to their lightweight and portable nature, which is true. The reason explains that dry cells convert chemical energy into electrical energy via redox reactions, which is also correct and directly explains why they are suitable for portable devices like flashlights and toys. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Dry Cell Structure

d) It serves as the positive terminal
[Solution Description]
In a dry cell, the zinc container acts as the negative terminal while the carbon rod serves as the positive terminal. The carbon rod is surrounded by a moist paste-like electrolyte and helps in the flow of electrons to complete the circuit.

Key Concept: Rechargeable Batteries

b) Mobile phone
[Solution Description]
Rechargeable batteries are widely used in portable electronic devices such as laptops, mobile phones, cameras, inverters, and vehicles. These batteries can be reused multiple times by recharging.

Key Concept: Battery Chemistry, Battery Applications

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion is true because during the discharge of a dry cell, zinc oxidizes and forms $\text{Zn}^{2+}$ ions, which dissolve into the electrolyte, causing the zinc container to lose mass over time.
Reason is also true because the chemical reaction involves the oxidation of zinc ($\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^{-}$), which is consistent with the assertion.
Additionally, the reason correctly explains why the zinc container loses mass (oxidation leads to dissolution). Thus, both statements are true, and the reason provides the correct explanation.

Key Concept: Battery Recycling

a) It prevents hazardous materials from contaminating the environment
[Solution Description]
Rechargeable batteries contain hazardous materials like heavy metals that can contaminate soil and water if not disposed properly. Recycling recovers valuable materials while preventing pollution.

Key Concept: Rechargeable batteries, Environmental impact

c) Their materials can cause environmental harm if not recycled properly.
[Solution Description]
The syllabus highlights that lithium-ion batteries contain materials like lithium and cobalt, which are mined in limited parts of the world. These materials can cause environmental harm if not disposed of properly, as they may lead to fires or contaminate the environment. Recycling these batteries is crucial to prevent harm and reuse valuable materials.

Key Concept: Rechargeable Batteries

d) They are safer, charge faster, and last longer
[Solution Description]
Scientists are working on solid-state batteries because they replace liquid or paste-like electrolytes with solid materials, making them safer, faster to charge, and longer-lasting, as mentioned in “A step further” in the syllabus.

Key Concept: Rechargeable Batteries

c) Dispose of it at an e-waste recycling facility
[Solution Description]
Used rechargeable batteries should be disposed of at special e-waste recycling facilities to prevent environmental harm and allow material recycling, as mentioned in “Snapshots” and “A step further.”

Key Concept: Rechargeable batteries, Environmental impact

c) To prevent toxic metal pollution and conserve natural resources
[Solution Description]
Recycling lithium-ion batteries is crucial because they contain toxic heavy metals like cobalt, nickel, and lithium, which can leach into soil and water if disposed improperly. Moreover, recovering these valuable metals reduces the need for environmentally damaging mining operations while preventing hazardous waste accumulation.

Key Concept: Types of rechargeable batteries

c) Lithium-Ion (Li-ion)
[Solution Description]
The most common type of rechargeable battery used in devices like smartphones and laptops is the lithium-ion (Li-ion) battery. This is explicitly mentioned in the syllabus under “A step further” section where it states: “Today, the lithium-ion (Li-ion) battery is the most common type of rechargeable battery, found in almost all devices that use batteries.”

Key Concept: Lithium-ion batteries, recycling

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that lithium-ion batteries contain valuable materials like lithium and cobalt, which aligns with the syllabus mentioning these metals are mined and processed in limited parts of the world. Hence, recycling them for reuse is economically sound. The Reason highlights the environmental benefit of recycling these batteries by preventing leakage of harmful substances, which is also emphasized in the syllabus. Moreover, the syllabus explicitly mentions that improper disposal of batteries can lead to environmental hazards due to their toxic components. The Reason correctly explains why the Assertion is important, making it a valid explanation.

Key Concept: Concept: Recycling Rechargeable Batteries

b) Prevents pollution and allows valuable materials to be reused
[Solution Description]
Recycling rechargeable batteries prevents hazardous materials like lead, cadmium, nickel, or lithium from polluting the environment. It also allows valuable materials to be reused, reducing the need for mining new resources.