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: Magnetic Effect of Current, Oersted's Experiment

c) The needle will deflect in the opposite direction with the same magnitude
[Solution Description]
When the current direction is reversed, the direction of the magnetic field around the wire also reverses (right-hand rule). The compass needle, which aligns with the net magnetic field (Earth's field + wire's field), will now deflect in the opposite direction but with the same magnitude, assuming all other conditions remain unchanged.

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

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: 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 a compass needle deflects when an electric current flows through a nearby wire. This is true because, as discovered by Hans Christian Oersted, an electric current generates a magnetic field around the wire, which interacts with the magnetic compass needle, causing it to deflect. The reason states that an electric current produces a magnetic field around the conductor, which affects the compass needle. This is also true and correctly explains why the compass needle deflects. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

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: 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: 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: 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, Strength of electromagnet

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that increasing the number of turns in the coil of an electromagnet (while keeping current constant) increases its magnetic field strength. This is true because more turns mean more loops for the current to create a magnetic field, thus enhancing the overall magnetic effect. The reason states that the magnetic field strength is directly proportional to both the number of turns and the current. While this is also true, the reason does not correctly explain the assertion since the assertion specifically mentions keeping the current constant, whereas the reason introduces current as a variable. Therefore, the reason is not the correct explanation for the assertion under the given condition.

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 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: Direction of deflection

b) The needle deflects in the opposite direction.
[Solution Description]
The direction of the magnetic field around a current-carrying wire depends on the direction of the current. Reversing the current reverses the polarity of the magnetic field, which causes the compass needle to deflect in the opposite direction compared to its initial deflection.

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 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 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: 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: 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, 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: 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: Polarity of Electromagnet

c) By observing which compass pole is attracted to the coil's end.
[Solution Description]
The polarity of an electromagnet can be determined by placing a magnetic compass near one end of the coil. If the north pole of the compass needle is attracted to that end, it indicates the end is a south pole (since unlike poles attract). Similarly, the other end will have the opposite polarity (north).

Key Concept: Electromagnets, Magnetic Field Strength

c) Increase the number of turns and use more cells
[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 the number of turns or the current will strengthen the electromagnet. Using more cells increases the current. Therefore, both increasing the number of turns and using more cells will enhance the magnetic field.

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: 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: Strength of Electromagnet

b) Color of the wire used in the coil
[Solution Description]The strength of an electromagnet depends on the amount of electric current passing through the coil and the number of turns in the coil. However, the color of the wire has no effect on the magnetic field generated. Increasing current or turns increases the magnetic strength.

Key Concept: Lifting Electromagnets - Principle

c) The electromagnet attracts iron or steel objects
[Solution Description]
When an electric current flows through the coil of a lifting electromagnet, it creates a magnetic field around it. This magnetic field attracts ferromagnetic materials like iron or steel. The strength of the magnet depends on the current. When the current is turned off, the magnetic field disappears, and the objects are released.

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: 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: 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: 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: 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: Heating effect, Material resistance

d) Nichrome wire; higher resistance to current flow
[Solution Description]
The heating effect of current depends on the resistance of the material. Nichrome has higher resistivity compared to copper. For the same length and thickness, resistance $R$ is given by $R = \rho \frac{L}{A}$, where $\rho$ is resistivity, $L$ is length, and $A$ is cross-sectional area. Since nichrome has higher $\rho$, it will resist current flow more, converting more electrical energy into heat energy. Thus, the nichrome wire heats up more.

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

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

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 of conductors and its effect on heating

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 more than a copper wire of the same dimensions when the same current flows through them. This is true because nichrome has a higher resistivity than copper, leading to greater resistance for wires of the same size and length. The reason given is also correct as it explains why nichrome heats up more—due to its higher resistance. Therefore, the reason correctly explains the assertion.

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: Resistance and 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 a thicker copper wire produces less heat than a thinner nichrome wire for the same current and length. This is true because the heating effect depends on the resistance of the wire ($H = I^2 R t$). Copper has lower resistivity than nichrome, and a thicker wire has lower resistance due to $R = \rho \frac{L}{A}$. The reason correctly explains the assertion since resistance decreases with increasing cross-sectional area ($A$) for the same material and length.

Key Concept: Heating effect applications, Safety

b) To prevent overheating and potential fires[Solution Description]Underrated components can overheat due to excessive current, leading to melting/fire. Properly rated components ensure safe operation by handling expected current without overheating, preventing energy loss and hazards.

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: Current, Heat energy relationship

b) It becomes one-fourth
[Solution Description]
The heat produced ($H$) is given by $H = I^2 R t$. Reducing the current to half ($I' = \frac{I}{2}$) affects the heat produced as $H' = (\frac{I}{2})^2 R t = \frac{1}{4} I^2 R t = \frac{H}{4}$. Thus, the heat reduces to one-fourth of the original value when current is halved.

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: Factors affecting heating in wires

c) Color of the wire[Solution Description]The heat generated in a wire depends on the material (resistivity), length, thickness (cross-sectional area), and current magnitude. The color of the wire does not affect the heating effect.

Key Concept: Resistance and Heating

c) Nichrome has higher resistance than copper
[Solution Description]
Nichrome has higher resistance compared to copper, which causes more electrical energy to be converted into heat energy when current flows through it.

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) 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: Safety Concerns Due to Heating Effect

d) Using wires rated for the specified current and avoiding circuit overloading
[Solution Description]
Overheating in household wiring can be prevented by using wires with appropriate current ratings, ensuring proper insulation, and avoiding overloading circuits. Using thicker wires reduces resistance and minimizes heat generation.

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: Dry cells, Rechargeable batteries

b) The chemical reactions in dry cells are irreversible
[Solution Description]
Dry cells use an irreversible chemical reaction where the electrolyte (a moist paste) reacts completely with the electrodes, depleting the reactants permanently. The zinc container corrodes, and the manganese dioxide gets reduced, leaving no way to reverse the reaction. In contrast, rechargeable batteries like lithium-ion cells use reversible reactions, allowing the original materials to be restored during recharging.

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: Chemical Reaction in Voltaic Cell

c) $\text{Zn (s) + Cu}^{2+}\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq) + Cu (s)}$
[Solution Description]
In a Voltaic cell with zinc (\text{Zn}) and copper (\text{Cu}) electrodes, the reaction is: $\text{Zn (s) + Cu}^{2+}\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq) + Cu (s)}$. Here, zinc oxidizes to $\text{Zn}^{2+}$ while $\text{Cu}^{2+}$ reduces to copper metal.

Key Concept: Voltaic cell components and function

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In a lemon cell, the copper and iron electrodes are inserted into the lemon, which contains acidic juice acting as the electrolyte. When these electrodes come into contact with the lemon juice, a chemical reaction occurs where zinc (from the iron nail) loses electrons and copper gains them. This electron flow generates an electric current, producing electricity. Thus, both Assertion (A) and Reason (R) are true, and Reason correctly explains why the lemon cell generates electricity.

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: 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: 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: Rechargeability of Dry Cells

b) Their chemical reactions are irreversible and exhaustible
[Solution Description]
Dry cells are single-use because the chemical reactions that produce electricity are irreversible. Once the reactants are exhausted, the cell cannot be recharged and must be disposed of. Rechargeable batteries use reversible reactions, allowing them to be reused multiple times.

Key Concept: Rechargeable Batteries

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that rechargeable batteries can be recharged and reused multiple times, which is true as mentioned in the syllabus. The reason provided is also true because reusing batteries reduces wastage and leads to cost savings over time. Additionally, the reason correctly explains why rechargeable batteries are beneficial, directly linking their reusability to economic and environmental savings.

Key Concept: Rechargeable battery economics

b) Rechargeable Li-ion batteries (\$40)
[Solution Description]
Calculate total energy needed: 2000mAh/day × 365 days × 5 years = 3,650,000mAh required.
Single-use batteries:
3,650,000mAh ÷ 1200mAh/battery = 3041.67 → 3042 batteries needed.
Total cost = 3042 × \$0.50 = \$1521.
Rechargeable batteries:
Each battery provides 2000mAh × 500 cycles = 1,000,000mAh over lifetime.
3,650,000mAh ÷ 1,000,000mAh/battery = 3.65 → 4 batteries needed.
Total cost = 4 × \$10 = \$40.
The rechargeable option is far cheaper.

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: 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: 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: 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: 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: Chemical reactions in voltaic cells, electrode materials

d) The chemicals involved in the reaction get used up
[Solution Description]
A voltaic cell stops working when the chemical reactants (electrodes or electrolyte) are depleted due to the redox reactions occurring at the electrodes. Over time, these reactions consume the active materials, reducing their ability to produce electrons or ions, leading to a depletion of electrical energy. This state is referred to as a "dead" cell.

Key Concept: Lithium-ion batteries, Solid-state batteries

c) They provide higher energy density and safer operation.
[Solution Description]
Solid-state batteries use solid electrolytes instead of liquid or paste-like electrolytes found in lithium-ion batteries. This design improves safety by reducing risks of leakage or combustion, allows faster charging rates, and increases energy density for longer-lasting performance.
Step 1: Lithium-ion batteries contain flammable liquid electrolytes, posing safety risks. Step 2: Solid-state batteries use non-flammable solid electrolytes, enhancing safety. Step 3: The solid structure enables faster ion movement, leading to quicker charging. Step 4: Solid-state designs also support higher energy densities, increasing battery life. Thus, solid-state batteries offer improved safety, charging speed, and longevity compared to lithium-ion batteries.

Key Concept: Chemical reactions, Dry cells

c) It loses electrons and slowly corrodes away.
[Solution Description]
In a dry cell, the zinc container undergoes oxidation (loses electrons) to produce zinc ions ($\text{Zn}^{2+}$). This reaction provides electrons that flow through the external circuit, generating electricity. Over time, the zinc container corrodes as it dissolves into the electrolyte.
Step 1: Zinc reacts with the moist paste electrolyte. Step 2: The oxidation half-reaction occurs at the anode (zinc): $\text{Zn} \rightarrow \text{Zn}^{2+} + 2\text{e}^-$. Step 3: Electrons flow out via the negative terminal, powering devices. Step 4: Zinc gradually degrades as it forms $\text{Zn}^{2+}$ ions. Therefore, the zinc container corrodes while supplying electrons for current.

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: Dry cell components

c) Zinc container
[Solution Description]
A dry cell consists of a zinc container which acts as the negative terminal and a carbon rod at the center that acts as the positive terminal. The electrolyte is a paste-like substance inside the zinc container.

Key Concept: Structure of Dry Cells, Working Principle

c) The gas bubbles increase internal resistance, reducing voltage output.
[Solution Description]
Hydrogen gas forms due to side reactions at the carbon rod (positive terminal). This gas buildup creates resistance, known as polarization, which reduces the efficiency and voltage output of the cell over time.

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: 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: 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: Function of Voltaic cell

d) To generate electric current through chemical reactions[Solution Description]A Voltaic cell generates electric current due to chemical reactions between the electrodes and the electrolyte.

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: Electrolyte in Voltaic cell

b) Weak acid or salt solution[Solution Description]The electrolyte in a Voltaic cell is usually a weak acid or salt solution, such as sulfuric acid or saltwater.

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 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: 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: Applications of Voltaic cells

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Voltaic cells are used in medical devices like pacemakers, which is true as they provide portable and long-term power. The reason explains that Voltaic cells offer a steady and reliable source of electricity, which is also true and directly supports the assertion since medical implants require consistent power. Hence, the reason correctly explains the assertion.

Key Concept: Redox reactions, Applications of voltaic cells

b) 3,839 C
[Solution Description]
First, calculate the moles of zinc oxidized:
$\text{Moles of Zn} = \frac{\text{Mass}}{\text{Molar Mass}} = \frac{1.30\,\text{g}}{65.38\,\text{g/mol}} \approx 0.0199\,\text{mol}$
The oxidation reaction is:
$\text{Zn (s)} \rightarrow \text{Zn}^{2+} + 2e^-$
From stoichiometry, 1 mole of Zn produces 2 moles of electrons. So, total charge $Q$ is:
$Q = n \times F \times \text{moles of electrons per mole} = 0.0199\,\text{mol} \times 96,485\,\text{C/mol} \times 2 \approx 3,839\,\text{C}$

Key Concept: Construction of dry cells

b) Serves as the negative terminal and undergoes oxidation
[Solution Description]
In a dry cell, the zinc container serves as the anode (negative terminal). It undergoes oxidation, releasing electrons into the external circuit. The chemical reaction at the anode is: $Zn \rightarrow Zn^{2+} + 2e^-$. The zinc container also physically holds the electrolyte paste and other components.

Key Concept: Characteristics of Dry Cells

c) They cannot be recharged
[Solution Description]
One of the limitations of dry cells, as per the syllabus, is that they cannot be recharged. Once the reactants are exhausted, the cell becomes 'dead' and must be disposed of.

Key Concept: Rechargeable batteries, Environmental impact

c) They reduce total waste through multiple recharge cycles
[Solution Description]
While both battery types contain toxic materials, lithium-ion batteries have two key environmental advantages: 1) They can be recharged hundreds of times, reducing the total number of batteries needing disposal per unit energy delivered. 2) Their longer lifespan means fewer resources are consumed in manufacturing replacements. However, proper recycling is still essential for both types as improper disposal of lithium-ion batteries can cause fires and environmental contamination from cobalt/lithium leaching.

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: 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: Lithium-Ion Batteries, Resource Constraints

c) They use rare and expensive materials like lithium and cobalt
[Solution Description]
Lithium-ion batteries rely on special metals like lithium and cobalt, which are mined and processed in limited parts of the world. This scarcity and the complex manufacturing process make them more expensive compared to dry cells, which use more commonly available materials like zinc and carbon.

Key Concept: Battery Usage

c) Flashlights[Solution Description]Dry cells are commonly used in small portable devices such as flashlights, toys, remote controls, and wall clocks due to their compact design and convenience for everyday use.

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: 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 Battery Lifespan

b) Chemical reactions cause permanent changes in electrode materials
[Solution Description]
Rechargeable batteries degrade over time due to chemical reactions that cause irreversible changes in the electrode materials and electrolyte. Each cycle causes minor structural damage, reducing capacity gradually.

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: 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: 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: Concept: Future Battery Technologies

c) They replace liquid electrolytes with solid materials
[Solution Description]
Solid-state batteries replace liquid or paste-like electrolytes with solid materials. This makes them safer, capable of faster charging, and longer-lasting compared to traditional lithium-ion batteries.

Key Concept: Concept: Lithium-ion Battery Characteristics

c) Lightweight and ability to recharge multiple times
[Solution Description]
Lithium-ion batteries are popular because they are lightweight, have a high energy density, and can be recharged many times before wearing out. They are commonly used in devices like phones and laptops due to these advantages.