Question Opening prompt 1
Without an electric lamp in a circuit with a cell, how else could we find out whether current is flowing?
Steps only
Current can deflect a nearby compass or warm a wire.
Answer
Use the magnetic or heating effect of current.
CLASS 8 · SCIENCE · CHAPTER 4
Electricity: Magnetic and Heating Effects
Simple answers to the opening prompts, six chapter activities, investigation prompts, all eleven end questions, battery disposal note, and all three projects.
30 worked answersComplete chapter answers
30 worked answers · short steps first, explanations when you need them
Probe and Ponder
Textbook pages 48
Question Opening prompt 2
Can we make temporary magnets? How?
Steps only
Wrap a wire coil around an iron nail and pass current through it.
Answer
The nail acts as a temporary electromagnet while current flows.
Question Opening prompt 3
How is heat produced in electrical appliances?
Steps only
Resistance in a wire changes some electrical energy into heat.
Answer
A heating element warms up when current flows through it.
Question Opening prompt 4
How can we tell if a cell or battery is dead? Can every cell be recharged?
Steps only
Check the cell in a working circuit; recharge only batteries designed for it.
Answer
Not every cell is rechargeable.
Question Opening prompt 5
Share one question you have about electricity, magnets, or heating.
Steps only
Ask a clear question that can be tested safely.
Answer
Example: How does the number of coil turns affect an electromagnet?
Keep the Curiosity Alive
Textbook pages 58–61
Question 1
Fill in the blanks: (i) The solution used in a Voltaic cell is called ____. (ii) A current-carrying coil behaves like a ____.
Steps only
Recall the names for the cell liquid and the magnetic coil.
Answer
- (i) Electrolyte
- (ii) Magnet
Question 2
Say whether each statement is true or false: (i) Dry cells are less portable than Voltaic cells. (ii) A coil becomes an electromagnet only when electric current flows through it. (iii) An electromagnet with one cell attracts more paper clips than the same electromagnet with two cells.
Steps only
Check portability, the need for current, and the effect of adding a cell.
Answer
- (i) False
- (ii) True
- (iii) False
Question 3
Current flows through a nichrome wire for a short time. (i) The wire becomes warm. (ii) A compass placed below the wire is deflected. Choose: (a) only i (b) only ii (c) both (d) neither.
Steps only
A current-carrying wire has both heating and magnetic effects.
Answer
(c) Both statements are correct.
Question 4
Match: Voltaic cell; electric iron; nichrome wire; electromagnet with: best suited for an electric heater; works by the magnetic effect of current; works by the heating effect of current; chemical reactions.
Steps only
Match each item to the effect or function it uses.
Answer
Voltaic cell—chemical reactions; iron—heating; nichrome—heater element; electromagnet—magnetic effect.
Question 5
Nichrome wire is commonly used in electrical heating devices because it: (i) conducts electricity well (ii) generates more heat for a given current (iii) is cheaper than copper (iv) is an insulator.
Steps only
Higher resistance produces more heat when the current is the same.
Answer
(ii) Generates more heat for a given current.
Question 6
Why are electric heating devices often considered more convenient than firewood or charcoal, considering their effect on society?
Steps only
They are easy to control and do not create indoor smoke at the point of use.
Answer
Electric heating can make cooking or warming more convenient and reduce indoor smoke.
Question 7
In the circuit shown in Fig. 4.4a, (i) show the path of current (ii) explain why the compass needle moves (iii) predict what happens if the battery terminals are reversed.
Steps only
Current makes a magnetic field; reversing current reverses that field.
Answer
Draw current from + to –; the compass deflects the other way if the terminals are reversed.
Question Question-making activity
Prepare questions using prompts such as why, when, how long, where, and why not, based on the chapter.
Steps only
Use question words to ask about current, magnets, heat, and safety.
Answer
Answers vary; questions should connect to the chapter.
Question 8
Sumana leaves her electromagnet switch on. Later, the nail no longer picks up paper clips, though the wire is warm. Give possible reasons.
Steps only
A weak cell, reduced current, loose connection, or shifted coil can weaken the magnet.
Answer
Some current can still heat the wire while the electromagnet is too weak to lift clips.
Question 9
In Fig. 4.11, in which case will the LED glow when the switch is closed: copper and iron electrodes in lemon juice, or the same electrodes in pure water?
Steps only
Lemon juice is the electrolyte; pure water conducts poorly.
Answer
Case (a), with lemon juice, should make the LED glow.
Question 10
If the iron nail is removed from the current-carrying coil, will a nearby compass still deflect? Will it deflect more or less than before?
Steps only
The coil still makes a field, but the iron core makes it stronger.
Answer
Yes; the compass deflection will be less without the iron nail.
Question 11
Four similar coils of iron, copper, aluminium, and nichrome carry current. Near which coils will a compass deflect? (i) a only (ii) a and b (iii) a, b, and c (iv) all four.
Steps only
Every current-carrying coil creates a magnetic field.
Answer
(iv) All four coils.
Discover, Design, and Debate
Textbook pages 61
Question Project 1
Compare coils with 25, 50, 75, and 100 turns using the same cell. Record the compass deflection and conclude how turns affect an electromagnet.
Steps only
Keep other conditions fixed and compare compass deflection for each coil.
Answer
More turns usually make a stronger electromagnet.
Question Project 2
Compare equal-length nichrome wires of different thickness, then wires of the same thickness but different lengths, using the same cell for 30 seconds. Which heats more?
Steps only
Keep the setup and time the same, and compare measured temperature changes.
Answer
Record the observed result: resistance and current both affect heating. Never touch a connected or hot wire.
Question Project 3
Try to make electric cells using different fruits or vegetables and electrodes made from different metals. Prepare a brief report.
Steps only
Use fruit juice as electrolyte and two different metals as electrodes; compare readings fairly.
Answer
Record the fruit, metals, and meter reading. Do not eat the test fruit.
Activities and Chapter Notes
Textbook pages 48–58, 61
Question Activity 4.1
What happens to a compass placed under a wire when the switch is turned on and off?
Steps only
What happens to a compass placed under a wire when the switch is turned on and off?
Answer
When the switch is ON, the compass needle deflects.
Question Activity 4.2
What happens to the iron clips when a wire-wrapped nail is connected to a cell and then disconnected?
Steps only
What happens to the iron clips when a wire-wrapped nail is connected to a cell and then disconnected?
Answer
With the cell connected briefly, the nail attracts the iron clips.
Question Activity 4.3
What do the compasses and iron clips show when current flows through a coil, with and without an iron nail inside it?
Steps only
What do the compasses and iron clips show when current flows through a coil, with and without an iron nail inside it?
Answer
The compasses deflect when current flows through the coil.
Question Activity 4.4
How can a compass help identify the two ends of an electromagnet?
Steps only
How can a compass help identify the two ends of an electromagnet?
Answer
Place a compass near each end while current flows.
Question Think like a scientist: coil strength
What should happen when the same coil is tested with more cells, and when the number of turns is increased?
Steps only
What should happen when the same coil is tested with more cells, and when the number of turns is increased?
Answer
Using more cells usually gives a stronger magnetic field, so the compass deflects more and the coil can pick up more clips.
Question Think like a scientist: reverse current
What changes when the direction of current through an electromagnet is reversed?
Steps only
What changes when the direction of current through an electromagnet is reversed?
Answer
The north and south poles swap ends.
Question Activity 4.5
How does the nichrome wire feel before and after current flows through it for about 30 seconds?
Steps only
How does the nichrome wire feel before and after current flows through it for about 30 seconds?
Answer
The nichrome wire feels cool before the circuit is switched on and becomes warm after current flows.
Question Think like a scientist: two cells
For the same wire and time, does it heat more with one cell or two cells?
Steps only
For the same wire and time, does it heat more with one cell or two cells?
Answer
It usually heats more with two cells because the current is larger.
Question Activity 4.6
What may happen when several lemon cells are connected to an LED, and why might reversing the LED connections help?
Steps only
What may happen when several lemon cells are connected to an LED, and why might reversing the LED connections help?
Answer
Copper and iron in lemon juice can make a small electric cell.
Question Battery disposal note
Why should used batteries be taken to a proper recycling point instead of being thrown in ordinary rubbish?
Steps only
Why should used batteries be taken to a proper recycling point instead of being thrown in ordinary rubbish?
Answer
Used batteries may contain harmful chemicals or metals and can cause fires if damaged.