Key Concept: Applications of Electromagnetism

c) Electromagnet
[Solution Description]
Devices such as electromagnets, electric motors, and loudspeakers use the principle of the magnetic effect of electric current. These devices rely on the interaction between the magnetic field produced by the current and permanent magnets or other magnetic materials to function.

Key Concept: Electromagnet

c) It disappears.
[Solution Description]
An electromagnet works on the principle of the magnetic effect of electric current. When current flows through the wire wound around a magnetic material (like iron), it creates a magnetic field, making the material behave like a magnet. However, once the current is turned off, the magnetic field ceases to exist, and the material loses its magnetic property.

Key Concept: Magnetic Effect of Electric Current

b) The compass needle deflects from its original direction.
[Solution Description]
When electric current flows through a conductor, it produces a magnetic field around it. A compass needle is a tiny magnet that aligns itself with the magnetic field. Therefore, when current flows through the wire near the compass, the needle deflects due to the magnetic effect of the current. When the current stops, the magnetic field disappears, and the needle returns to its original position.

Key Concept: Electromagnetism

b) Magnetic effect of electric current
[Solution Description]
The deflection of the compass needle is caused by the magnetic effect of electric current. According to Oersted’s discovery, an electric current produces a magnetic field around the conductor. This magnetic field interacts with the compass needle, causing it to deflect. The effect ceases when the current stops flowing.

Key Concept: Applications of Electromagnets

b) Wristwatch.
[Solution Description]
Electromagnets are used in electric bells, motors, fans, loudspeakers, and lifting heavy iron objects like in scrap yard cranes. A wristwatch typically uses permanent magnets or quartz mechanisms, not electromagnets.

Key Concept: Electromagnets, Magnetic Effect of Electric Current

c) 4.8 T
[Solution Description]
The magnetic field ($B$) produced by an electromagnet depends on the current ($I$) and the number of turns per unit length ($n$). The relationship is given by:
$B = \mu n I$
Initially, $B_1 = 0.8$ T for $I_1 = 4$ A.
When the current is doubled ($I_2 = 8$ A) and the number of turns is tripled ($n_2 = 3n_1$), the new magnetic field ($B_2$) is:
$B_2 = \mu (3n_1)(8)$
Since $\mu$ and $n_1$ are constants, we can write:
$\frac{B_2}{B_1} = \frac{24n_1}{4n_1} = 6$
So, $B_2 = 6 \times 0.8 = 4.8$ T.

Key Concept: Oersted's Experiment

d) Hans Christian Oersted
[Solution Description]
Hans Christian Oersted discovered in 1820 that an electric current produces a magnetic field. He observed that a compass needle deflected when an electric circuit was closed or opened, proving the connection between electricity and magnetism.

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: 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, Electromagnetism

c) The needle deflects when the circuit is 'ON' and returns to original position when 'OFF'.
[Solution Description]
When the circuit is switched 'ON', current flows through the wire, creating a magnetic field around it. This magnetic field interacts with the compass needle (a tiny magnet), causing it to deflect. When the circuit is switched 'OFF', 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: Discovery of the magnetic effect of electric current

d) Hans Christian Oersted
[Solution Description]
Hans Christian Oersted discovered that an electric current produces a magnetic field around it when he observed the deflection of a compass needle near a current-carrying wire in 1820.

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: 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 when an electric current flows through a conductor, the compass needle deflects. This is true because a current-carrying conductor produces a magnetic field (as observed in Activity 4.1). The reason explains that this deflection happens due to the magnetic field created by the current-carrying conductor, which aligns with Oersted’s discovery. Hence, both the assertion and reason are correct, and the reason correctly explains the assertion.

Key Concept: Magnetic field strength, Distance dependence

b) Becomes half
[Solution Description]
The magnetic field strength ($B$) around a straight current-carrying conductor is given by $B = \frac{\mu_0 I}{2 \pi r}$, where $r$ is the distance from the wire. If the distance $r$ is doubled, the magnetic field strength becomes $B' = \frac{\mu_0 I}{2 \pi (2r)} = \frac{B}{2}$. Thus, the magnetic field strength reduces to half its original value.

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: 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 Effect of Current

c) The iron nail enhances the magnetic field strength
[Solution Description]
Inserting an iron nail increases the magnetic permeability of the core, making the magnetic field stronger. The iron core acts as a magnet itself, amplifying the overall magnetic effect of the coil, leading to greater deflection of the compass needle.

Key Concept: Electromagnets

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] An electromagnet works on the principle that a current-carrying coil produces a magnetic field. The assertion states that the electromagnet loses its magnetism when the current is switched off, which is true because the magnetic field disappears once the current stops flowing. The reason correctly explains the assertion by stating that the magnetic field is temporary and dependent on the current. Thus, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Assertion-Reason

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to Activity 4.2, when electric current flows through the coil, the iron nail becomes magnetized and attracts iron clips. When the current stops, the magnetic effect vanishes, and the clips fall down. This confirms that the Assertion is true. The Reason correctly explains why the electromagnet works—it behaves like a magnet only when current flows, which aligns with the observation. Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Lifting Electromagnets, Applications

c) To increase the magnetic field strength
[Solution Description]
Iron cores enhance the magnetic field produced by the current-carrying coil, making the electromagnet stronger. This allows it to lift heavier metal objects efficiently. Without the iron core, the magnetic field would be weaker, reducing its lifting capacity.

Key Concept: Electromagnets, Polarity

c) The compass needle's south pole points towards the nail
[Solution Description]
The polarity of an electromagnet reverses when the direction of the current is reversed. Initially, the north pole of the compass points towards the nail, meaning the nail's end was a south pole. Reversing the current will switch the poles; hence, the compass's south pole will now point towards the nail.

Key Concept: Electromagnets

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In an electromagnet, the magnetic effect is produced due to the flow of electric current through the coil. When the current is switched off, the magnetic field disappears because there is no longer any current flowing to generate it. Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Electromagnets, Magnetic Effect of Current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Increasing the number of turns in the coil or the current through it enhances the magnetic field strength of an electromagnet, as the magnetic field $B$ is given by $B = \mu_0 n I$, where $\mu_0$ is permeability, $n$ is number of turns per unit length, and $I$ is current. Thus, both Assertion and Reason are true, and 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: Electromagnet Polarity

d) By observing the deflection of a magnetic compass
[Solution Description]
To determine the polarity of an electromagnet, place a magnetic compass near one end of the energized coil. If the north pole of the compass needle is attracted to that end, the end is the south pole of the electromagnet (since unlike poles attract). Reversing the direction of current changes the polarity.

Key Concept: Lifting electromagnets, Magnetic effect of current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that reducing the number of turns in the coil decreases the lifting capacity of the electromagnet for a given current. This is true because the magnetic field strength ($B$) of an electromagnet depends on both the current ($I$) and the number of turns per unit length ($n$) in the coil, as given by $B = \mu_0 n I$, where $\mu_0$ is the permeability of free space. Thus, fewer turns result in a weaker magnetic field, reducing the lifting capacity. The reason correctly explains this relationship, stating that the magnetic field strength is directly proportional to the number of turns in the coil.

Key Concept: Electromagnetism

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, lifting electromagnets work on the principle that when electric current flows through a conductor (like a wire), it produces a magnetic field around it. When the current is turned ON, the electromagnet lifts the iron/steel objects; when the current is switched OFF, the magnetic field disappears, and the objects are released. Therefore, the Assertion correctly states that lifting electromagnets lose their magnetic effect when the current is switched OFF, and the Reason accurately explains this phenomenon as the disappearance of the magnetic field due to the stopping of the current.

Key Concept: Factors affecting heat production, Ohm's Law

d) Heat produced quadruples
[Solution Description]
According to Joule's law, $H = I^2 R t$. If current $I$ is doubled, the heat produced becomes $H' = (2I)^2 R t = 4I^2 R t = 4H$. Therefore, the heat produced increases by four times.

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 Electric Current

b) The wire becomes warm.
[Solution Description]
When an electric current passes through a conductor like a nichrome wire, it gets heated due to the resistance offered by the wire. This is known as the heating effect of electric current. Thus, the wire becomes warm.

Key Concept: Heating Effect of Electric Current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When electric current flows through a conductor like nichrome wire, it faces resistance. Due to its high resistance property, nichrome converts more electrical energy into heat energy compared to other conductors like copper. This explains why the wire heats up. Therefore, both Assertion and Reason are true, and Reason correctly explains Assertion.

Key Concept: Factors Affecting Heat Generation

c) Color of the wire
[Solution Description]
The heat generated in a wire depends on the material, thickness, length, duration of current flow, and magnitude of the current. The color of the wire does not affect the heating effect.

Key Concept: Practical Applications

c) Electric iron
[Solution Description]
Household appliances like electric irons utilize the heating effect of electric current by converting electrical energy into heat energy to perform tasks like ironing clothes.

Key Concept: Factors affecting heat generation

c) The heat produced decreases.
[Solution Description]
The resistance of a wire is directly proportional to its length ($R \propto L$). Doubling the length doubles the resistance ($R_{\text{new}} = 2R$). Since $H = I^2Rt$ and $I = V/R$, doubling the resistance reduces the current by half ($I_{\text{new}} = V/(2R) = I/2$). Substituting these values, the new heat produced becomes $H_{\text{new}} = (I/2)^2(2R)t = (1/2)I^2Rt$, which is half the original heat. Thus, the heat produced decreases.

Key Concept: Heating Effect of Electric Current

b) Because of the conversion of electrical energy into heat energy due to resistance
[Solution Description]
When electric current flows through a conductor, it encounters resistance, which converts some electrical energy into heat energy. This is known as the heating effect of electric current.

Key Concept: Heating effect of electric current

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
When electric current flows through a conductor like nichrome, the resistance of the material causes some electrical energy to be converted into heat energy. This is called the heating effect of electric current. Nichrome has higher resistance than copper, so more heat is generated in nichrome for the same current. Thus, both assertion and reason are true, and the reason correctly explains why the nichrome wire gets hot.

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: Factors Affecting Heat Generation

c) It doubles
[Solution Description]
The resistance of a wire is given by $R = \rho \frac{L}{A}$, where $\rho$ is resistivity, $L$ is length, and $A$ is cross-sectional area. Doubling the length doubles the resistance. From Joule's law, $H = I^2Rt$, if $I$ remains constant and $R$ doubles, the heat generated also doubles.

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: Heating effect of electric current

d) Nichrome wire has higher resistance than copper wire.
[Solution Description]
Nichrome wire is preferred over copper wire for heating devices because it has a higher resistance compared to copper wire of the same dimensions. The higher resistance causes more heat to be generated when current flows through it, making it suitable for heating applications.

Key Concept: Heating Effect of Current in Nichrome Wire

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that nichrome wire is preferred over copper wire for making heating elements, which is true because nichrome's high resistance makes it suitable for generating heat efficiently. The reason given is also correct since nichrome indeed has higher resistance compared to copper, resulting in greater heat production for the same current. Moreover, the reason correctly explains why nichrome is preferred as a heating element material. Therefore, both assertion and reason are true, and the reason provides the correct explanation for the assertion.

Key Concept: Practical application of heating effect

b) Electrical energy is converted into heat energy.
[Solution Description]
When an electric current passes through the heating element (usually made of nichrome), the resistance of the wire converts electrical energy into heat energy due to the heating effect of current. This heat is then used to warm the base of the iron.

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

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: Heating Effect of Electric Current

b) It has high resistivity and can withstand high temperatures without oxidizing
[Solution Description]
Nichrome wire is chosen as a heating element due to its high resistivity, which allows it to generate more heat when current passes through it. It also has a high melting point and does not oxidize easily at high temperatures, making it suitable for heating applications.

Key Concept: Heating effect of current, Resistance, Power dissipation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that nichrome wire is used in heating elements because it generates more heat than copper for the same current. This is true as nichrome has higher resistivity ($\rho_{\text{nichrome}} > \rho_{\text{copper}}$), causing higher power dissipation $$ P = I^2 R$$, where $I$ is current and $R$ is resistance. The Reason correctly explains the Assertion since higher resistivity directly leads to higher resistance $$ R = \rho \frac{L}{A}$$ for the same dimensions ($L$ = length, $A$ = cross-sectional area), resulting in more heat production. Thus, both 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: 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

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

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, Electrolyte, Rechargeable Batteries

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In a Voltaic cell, electricity is generated through a chemical reaction between the electrodes and the electrolyte. When the chemicals (reactants) are fully consumed, the reaction stops, and no more electrons are transferred, halting the current flow. Thus, Assertion (A) is true. The Reason (R) correctly explains that the current flow depends on this chemical reaction, making it the correct explanation for Assertion (A). Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Chemical reaction in Voltaic cell

c) The cell stops generating electricity
[Solution Description]
In a Voltaic cell, the electric current is generated due to the chemical reaction between the electrodes and the electrolyte. Over time, the chemicals (electrodes or electrolyte) get used up, leading to the termination of the electrochemical reaction.
As a result, the cell stops producing electricity and becomes a 'dead' cell. Thus, no further current flows through the circuit.

Key Concept: Function of Electrolyte

c) To allow ion movement for chemical reactions
[Solution Description]
The electrolyte in a Voltaic cell is a conducting liquid (such as a salt solution or weak acid) that allows ions to move between the electrodes, facilitating 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: Dry cells, Rechargeable batteries

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Assertion (A) is true because in a dry cell, the zinc container oxidizes (loses electrons) to form zinc ions, making it the negative terminal. Reason (R) is also true because the carbon rod gains electrons from the electrolyte, acting as the positive terminal. However, Reason (R) does not explain why the zinc container is the negative terminal—it merely describes the role of the carbon rod. Thus, both are true but Reason is not the correct explanation for Assertion.

Key Concept: Terminal Materials in Dry Cell

c) Zinc container
[Solution Description]
The negative terminal of a dry cell is made of zinc. The zinc container itself serves as the negative terminal, while the carbon rod with a metal cap acts as the positive terminal.

Key Concept: Energy conversion, Disposable vs rechargeable

b) Disposable: \$0.17/h; Rechargeable: \$0.005/h
[Solution Description]
Complex calculation requiring multiple steps:
Step 1: Disposable runtime = $\frac{3000mAh}{500mA} = 6$ hours (\$1 total)
Step 2: Rechargeable per-cycle runtime = $\frac{2000mAh}{500mA} = 4$ hours
Step 3: Total rechargeable lifespan = $4h \times 500 = 2000$ hours (\$10)
Step 4: Cost comparison:
- Disposable: $\frac{\$1}{6h} = \$0.167/h$
- Rechargeable: $\frac{\$10}{2000h} = \$0.005/h$
Must account for both capacity and reusability.

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: 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: Rechargeable batteries, Lithium-ion batteries

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion claims that lithium-ion batteries are common due to easily accessible materials, but this is false because lithium and cobalt are actually scarce and concentrated in few regions, as mentioned in the syllabus. The reason correctly states that these materials are mined in limited parts of the world, which is true according to the syllabus. Thus, the assertion is false, but the reason is true.

Key Concept: Environmental Impact

c) They contain harmful chemicals that pollute the environment
[Solution Description]
Rechargeable batteries contain hazardous materials like acids, lithium, lead, or cadmium, which can cause environmental pollution or fires if improperly disposed of. Many components are also valuable and recyclable, making proper disposal essential.

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: 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: Electrode potential, metal pairs in voltaic cells

d) Magnesium and Copper
[Solution Description]
The voltage produced by a voltaic cell depends on the difference in electrode potentials of the two metals. Among the given options, the pair with the largest potential difference will generate the highest voltage. Standard reduction potentials show that magnesium has a much lower (more negative) potential than copper, leading to a higher cell voltage compared to other pairs like zinc/copper or iron/copper.

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: Voltaic Cell Experiment

b) To act as an electrolyte
[Solution Description]
Lemon juice acts as an electrolyte in the lemon battery experiment, facilitating the flow of electric current between the copper and iron electrodes.

Key Concept: Electrolyte in dry cells

d) Thick moist paste
[Solution Description]
Unlike Voltaic cells, dry cells are called 'dry' because the electrolyte is not in liquid form but is a thick moist paste, making it more portable and less prone to leakage.

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: Solid-state batteries

d) They are safer and charge faster
[Solution Description]
Solid-state batteries use solid electrolytes instead of liquid or paste-like electrolytes, making them safer (reducing fire risks), faster to charge, and longer-lasting compared to conventional lithium-ion batteries.

Key Concept: Dry cells and rechargeable batteries

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Dry cells are called “dry” because they do not contain a free-flowing liquid electrolyte like wet cells. Instead, the electrolyte is present in the form of a thick moist paste. This paste allows the chemical reaction needed to produce electric current, but it does not spill easily. Therefore, Assertion (A) is true, and Reason (R) correctly explains why dry cells are called dry.

Key Concept: Working principle and structure of dry cells

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, which is true as per the given syllabus. The reason explains that this is because the zinc reacts with the electrolyte to release electrons, which is also correct. Moreover, the reason directly explains why the zinc container functions as the negative terminal. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Working Principle, Structure of Dry Cells

c) The potential difference drops to zero as the chemical reaction halts.
[Solution Description]
The zinc container acts as the negative terminal in a dry cell. When it corrodes completely, the chemical reaction stops because there is no more zinc to act as the anode. This means no more electrons are released, and the potential difference drops to zero.

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: Chemical Reaction in Dry Cell

c) Zinc loses electrons to form zinc ions.
[Solution Description]
At the zinc electrode, oxidation occurs where zinc loses electrons to form zinc ions. The reaction is: $Zn \rightarrow Zn^{2+} + 2e^-$. This confirms that zinc acts as the negative terminal by releasing electrons.

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, Electrolyte, Chemical Reactions

a) Electrons flow from zinc to copper; zinc oxidizes to $Zn^{2+}$; hydrogen gas forms at copper.
[Solution Description]
In a Voltaic cell with zinc and copper electrodes:
1. Zinc acts as the negative electrode (anode) because it is more reactive than copper. It loses electrons: $Zn \rightarrow Zn^{2+} + 2e^-$.
2. Copper acts as the positive electrode (cathode). The electrons released by zinc flow through the external circuit to the copper electrode, where they combine with hydrogen ions from the electrolyte: $2H^+ + 2e^- \rightarrow H_2(g)$.
3. The overall reaction involves zinc dissolving in the electrolyte and hydrogen gas forming at the copper electrode.

Key Concept: Electrodes in Voltaic cell

c) Electrodes[Solution Description]The two metal plates in a Voltaic cell are called electrodes. One acts as the positive electrode and the other as the negative electrode.

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: Voltaic Cells and their Applications

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that a Voltaic cell cannot be recharged after its chemicals are depleted, which is true because it is a primary cell. The Reason explains that the chemical reaction in a Voltaic cell is irreversible, which causes the reactants to deplete over time and makes it non-rechargeable. Here, the Reason correctly explains why the Assertion is true since irreversible reactions prevent recharging. Therefore, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Chemical Reaction in Voltaic Cells

b) Oxidation of zinc
[Solution Description]
In a voltaic cell, oxidation occurs at the anode. For example, zinc (Zn) loses electrons to form zinc ions ($\text{Zn}^{2+}$), as shown in the reaction:
$\text{Zn (s)} \rightarrow \text{Zn}^{2+} \text{(aq)} + 2e^-$

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: Rechargeable Batteries

c) Lithium-ion batteries
[Solution Description]
Rechargeable batteries, such as lithium-ion batteries, are a type of voltaic cell that can be recharged and reused multiple times, making them ideal for devices like mobile phones and laptops.

Key Concept: Construction of a Dry Cell

d) Zinc container
[Solution Description]
The negative terminal in a dry cell is the zinc container, which acts as the anode and loses electrons during the chemical reaction. This is clearly mentioned in the syllabus under "Construction of a Dry Cell."

Key Concept: Chemical reactions in dry cells

b) $2MnO_2 + 2NH_4^+ + 2e^- \rightarrow Mn_2O_3 + 2NH_3 + H_2O$
[Solution Description]
In a zinc-carbon dry cell, the cathode reaction occurs at the carbon rod (positive terminal), where manganese dioxide ($MnO_2$) reacts with ammonium ions ($NH_4^+$) and electrons to form manganese(III) oxide ($Mn_2O_3$), ammonia ($NH_3$), and water ($H_2O$). The correct reaction is: $2MnO_2 + 2NH_4^+ + 2e^- \rightarrow Mn_2O_3 + 2NH_3 + H_2O$

Key Concept: Construction 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 is indeed the negative terminal because zinc undergoes oxidation (loses electrons), which is characteristic of the anode in an electrochemical cell. The reason correctly explains why the zinc container acts as the negative terminal, as it describes the underlying chemical process.
Therefore, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

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: Battery Usage in Daily Life

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Dry cells are indeed widely used in flashlights due to their ability to provide portable electrical energy, and they are designed for single-use as the chemical reaction inside them ($\text{Zn} + 2\text{MnO}_2 + 2\text{NH}_4\text{Cl} \rightarrow \text{Zn(NH}_3\text{)}_2\text{Cl}_2 + \text{Mn}_2\text{O}_3 + \text{H}_2\text{O}$) is irreversible. This irreversibility prevents recharging, validating both the Assertion and Reason. Furthermore, the Reason correctly explains why dry cells are single-use, linking the assertion about their usage to the underlying chemical property.

Key Concept: Dry Cells, Rechargeable Batteries

c) Ability to be recharged and reused multiple times
[Solution Description]
The main advantage of using a rechargeable lithium-ion battery over a dry cell is that it can be recharged and reused multiple times, reducing waste and cost over time. Dry cells are single-use and must be disposed of after depletion, whereas lithium-ion batteries can be recharged hundreds of times before wearing out.

Key Concept: Battery Safety

c) To prevent environmental harm
[Solution Description]
Used batteries may contain harmful materials like acids and metals (lead, cadmium, nickel, or lithium). Proper disposal through e-waste recycling facilities prevents environmental harm and reduces pollution.

Key Concept: Rechargeable Batteries

b) They can be recharged and reused multiple times
[Solution Description]
Rechargeable batteries can be reused multiple times by recharging them with electricity, saving money and reducing waste. Unlike single-use dry cells, they do not need frequent replacement, making them ideal for long-term use in devices like mobile phones.

Key Concept: Lithium-ion Batteries

b) They have higher energy density than most alternatives
[Solution Description]
Lithium-ion batteries have higher energy density, meaning they can store more energy per unit weight than many other rechargeable batteries. This makes them ideal for portable devices.

Key Concept: Rechargeable batteries, Lithium-ion battery properties

d) 22 cycles
[Solution Description]
First, calculate the initial capacity reduction: 80% of 4000 mAh is 3200 mAh. The battery starts at 4000 mAh and loses 1% per full cycle. Each full cycle consumes 100% of the capacity (4000 mAh). To find the number of cycles until the capacity reaches 3200 mAh:
Let n be the number of cycles.
Capacity after $n$ cycles = $4000 \times (0.99)^n$.
Set this equal to 3200 mAh:
$4000 \times (0.99)^n = 3200$.
Divide both sides by 4000:
$(0.99)^n = 0.8$.
Take natural logarithm on both sides:
$n \ln(0.99) = \ln(0.8)$.
Solve for $n$:
$n = \frac{\ln(0.8)}{\ln(0.99)} \approx 22.4$.
Since cycles must be whole numbers, the battery can undergo **22** full charge cycles.

Key Concept: Rechargeable Batteries, Lithium-ion Batteries, Environmental Considerations

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] The Assertion states that solid-state batteries are safer and charge faster than traditional lithium-ion batteries due to the replacement of liquid electrolytes with solid materials. This is true as solid-state batteries indeed offer enhanced safety and faster charging. The Reason explains that solid-state batteries eliminate the risk of leakage and thermal runaway associated with liquid electrolytes, which is also correct. Moreover, the Reason directly supports the Assertion by providing a specific explanation for why solid-state batteries are safer.
Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

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: Solid-state batteries, Future battery technology

d) They replace liquid electrolytes with non-flammable solid materials
[Solution Description]
Solid-state batteries replace liquid/paste electrolytes with solid materials, eliminating flammable components that can leak or ignite. This structural change prevents thermal runaway reactions, a major hazard in traditional Li-ion batteries, while enabling higher energy densities and faster charging without compromising safety.

Key Concept: Rechargeable batteries and their lifespan

b) Their performance degrades over time
[Solution Description]
According to the syllabus, rechargeable batteries do not last forever. After being charged and used many times, they slowly wear out. Hence, the correct answer is that their performance degrades over time.

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.

Key Concept: Lithium-ion Batteries

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that lithium-ion batteries are widely used in modern electronic devices due to their high energy density, which is true as mentioned in the syllabus. The reason provided is that high energy density allows these batteries to store more energy per unit mass, which is also true and correctly explains why lithium-ion batteries are preferred in electronic devices.