Chapter 4: Electricity: Magnetic and Heating Effects - Solutions

Question 1

Fill in the blanks:

(i) The solution used in a Voltaic cell is called electrolyte.

(ii) A current carrying coil behaves like a magnet.

Question 2

Choose the correct option:

• (i) Dry cells are less portable compared to Voltaic cells. (False)
• (ii) A coil becomes an electromagnet only when electric current flows through it. (True)
• (iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (False)

Magnetic Strength ∝ Number of Cells in series (Voltage)
Strength (2 cells) > Strength (1 cell)

Question 3

An electric current flows through a nichrome wire for a short time. (i) The wire becomes warm. (ii) A magnetic compass placed below the wire is deflected. Choose the correct option:

• (a) Only (i) is correct
• (b) Only (ii) is correct
• (c) Both (i) and (ii) are correct
• (d) Both (i) and (ii) are not correct

Answer: (c) Both (i) and (ii) are correct.

Question 4

Match the items in Column A with those in Column B.

Column A	Column B
(i) Voltaic cell	(d) chemical reactions
(ii) Electric iron	(c) Works on heating effect of electric current
(iii) Nichrome wire	(a) Best suited for electric heater
(iv) Electromagnet	(b) Works on magnetic effect of electric current

Question 5

Nichrome wire is commonly used in electrical heating devices because it:

• (i) is a good conductor of electricity.
• (ii) generates more heat for a given current.
• (iii) is cheaper than copper.
• (iv) is an insulator of electricity.

Answer: (ii) generates more heat for a given current.

Joule's Law of Heating:
H = I²Rt
Nichrome has high resistance (R), which allows it to generate more heat energy (H) for a given electric current (I) compared to regular conductors.

Question 6

Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Give reason(s) to support this statement considering societal impact.

Answer:

• Electric heaters and stoves do not produce smoke, soot, or ash, unlike firewood or charcoal. This leads to cleaner air and reduces indoor air pollution, which is a major cause of respiratory problems.
• Electric devices start heating instantly and are easy to operate with switches or thermostats.
• Burning wood or charcoal contributes to deforestation and carbon emissions. Electric heating, especially when powered by renewable energy, is more eco-friendly.
• Electric devices reduce the risk of accidental fires and burns. They also prevent harmful exposure to smoke, especially for women and children in rural households.

Question 7

[Figure 4.4a: A magnetic compass placed near an electromagnet coil ]

Look at the Fig. 4.4a. If the compass placed near the coil deflects: (i) Draw an arrow on the diagram to show the path of the electric current. (ii) Explain why the compass needle moves when current flows. (iii) Predict what would happen to the deflection if you reverse the battery terminals.

Answer:

• (i) Arrow from positive to negative terminal.
• (ii) Needle moves due to magnetic field.
• (iii) If battery terminals are reversed, deflection direction will also reverse.

Question 8

Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position (in introduction story). After some time, the iron nail no longer picks up the iron paper clips, but the wire wrapped around the iron nail is still warm. Why did the lifting electromagnet stop lifting the clips? Give possible reasons.

Answer: If the switch is not turned off, the cell gets weak over time, reducing the current. The iron nail no longer acts as a strong magnet, so it cannot lift clips, but heating continues due to current flow.

Ohm's Law & Magnetic Field Relation:
I = V / R
As the cell depletes, Voltage (V) ↓ ⇒ Current (I) ↓
Since Magnetic Field Strength (B) ∝ I,
a decrease in current leads to a weaker magnetic field, losing lifting power.

Question 9

[Figure 4.11: (a) Setup with Lemon juice, (b) Setup with Pure water ]

In Fig. 4.11, in which case the LED will glow when the switch is closed?

Answer:

• The LED will glow in case (a) because lemon juice acts as an effective electrolyte.
• Lemon juice is an electrolyte, which means it contains acids that allow electric current to flow through it by enabling chemical reactions between the copper and iron.
• When the switch is closed, these reactions generate electric current, and the LED glows, indicating current flow.

Question 10

Neha keeps the coil exactly the same as in Activity 4.4 but slides the iron nail out, leaving only the coiled wire. Will the coil still deflect the compass? If yes, will the deflection be more or less than before?

Answer: Yes, the coil will still deflect the compass but less than before. The iron core strengthens the magnet.

B_{with core} = μ_r × B_{without core}
Because the relative permeability (μ_r) of iron is much greater than air, removing the iron nail significantly reduces the magnetic field strength.

Question 11

[Figure 4.12: Four coils labeled (a) Iron, (b) Copper, (c) Aluminium, (d) Nichrome ]

We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.12. When current is passed through the coils, compass needles placed near the coils will show deflection:

• (i) Only in circuit (a)
• (ii) Only in circuits (a) and (b)
• (iii) Only in circuits (a), (b), and (c)
• (iv) In all four circuits

Answer: (iv) In all four circuits. All conductors (iron, copper, aluminium, nichrome) show magnetic effect with current.