Probe and Ponder
Q1. If we don't have an electric lamp while making an electric circuit with an electric cell, is there any other way through which we can find out if current is flowing in the circuit?
Answer:
Yes, we can find out if current is flowing by using the magnetic effect of current. For example, we can place a magnetic compass near the wire. If current flows, the compass needle will deflect, indicating current is flowing. Alternatively, we can use a heating effect—if the wire gets warm, it indicates current is flowing (though this is less sensitive).
Q2. Is it possible to make temporary magnets? How can these be made?
Answer:
Yes, temporary magnets can be made using electromagnets. When an electric current flows through a coil of wire wrapped around an iron core, the coil behaves like a magnet. This temporary magnet can be turned on or off by switching the current on or off. The strength of the magnet can be changed by changing the number of turns or the current.
Q3. We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?
Answer:
Heat is generated in electrical appliances due to the heating effect of electric current. When an electric current passes through a conductor (like a nichrome wire), it faces resistance, which converts some electrical energy into heat energy. This heat is used in devices like electric irons, heaters, kettles, and stoves.
Q4. How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
Answer:
A cell or battery is dead when it can no longer supply enough electric current to power a device. This can be tested by connecting it to a bulb or LED—if it doesn't glow or is very dim, the cell is dead. Not all cells and batteries can be recharged. Primary cells (like dry cells) are single-use and cannot be recharged, while secondary cells (like rechargeable batteries) can be recharged multiple times.
Activity 4.1
Q1. Collect a magnetic compass, an electric cell, a cell holder, two drawing pins, a safety pin, two nails, two pieces of connecting wires (one longer and one shorter), and two small pieces of cardboard. Using two drawing pins, a safety pin, and a cardboard piece, make a switch (as you made it earlier in the chapter 'Electricity: Circuits and their Components' in Curiosity, Grade 7). Place the cell in the cell holder. Fix two nails to a piece of cardboard as shown in Fig. 4.1a. Fix the middle portion of the longer wire stretched between the nails, such that it is slightly above the surface of the cardboard. Attach one end of that wire to the cell holder and another end to the switch. Connect the second wire between the cell holder and the switch. Place the magnetic compass beneath the wire between the two nails (Fig. 4.1a). While watching the compass needle, move the switch to 'ON' position to allow electric current to flow through the wire (Fig. 4.1b). What do you observe? Now again while watching the compass needle, move the switch to 'OFF' position. What do you observe this time? Move the switch between 'ON' and 'OFF' positions a few more times. Carefully observe how the compass needle behaves the each time. 
Answer:
When the switch is in the 'ON' position, electric current flows through the wire. The magnetic compass needle deflects from its original direction. When the switch is moved to the 'OFF' position, the current stops, and the needle returns to its original position. This shows that a current-carrying wire produces a magnetic field around it, which deflects the compass needle.
Activity 4.2
Q1. Take around 50 cm long length of a flexible insulated wire, an iron nail, an electric cell, and few iron paper clips. Tightly wrap the wire around the nail in the form of a coil, as shown in Fig. 4.2, and secure it with an adhesive tape. Connect the ends of the wire to the cell. Take care to not connect the wires to the cell for more than a few seconds; otherwise, the cell may weaken quickly. Bring the nail close to the iron paper clips and lift up. Do the clips hang to the ends of the nail? Disconnect the wire from the cell to stop the flow of electric current in the wire. Do the clips fall down? 
Answer:
When the wire is connected to the cell, electric current flows through the coil. The iron nail behaves like a magnet and attracts the paper clips. When the wire is disconnected from the cell, the current stops, the magnetic effect disappears, and the clips fall down. This shows that the nail acts as a temporary magnet (electromagnet) only when current flows through the coil.
Activity 4.3
Q1. Take around 100 cm long flexible insulated wire, a piece of chart paper, an iron nail, an electric cell, two magnetic compasses, and few iron/steel paper clips. Roll a piece of chart paper to make a cylinder of diameter roughly equal to the width of a pencil. Secure it with an adhesive tape. Tightly wind around 50 turns of the insulated wire on the cylinder to form a cylindrical coil as shown in Fig. 4.3a. Secure the wire with an adhesive tape. Place the compasses near the two ends of the cylindrical coil (Fig. 4.3b). Connect the two ends of the coil with the terminals of the cell as shown in Fig. 4.3c and observe the magnetic compasses. Do you find any deflection in the needles of the compasses? Disconnect the wire from the cell. Do the needles of the compasses come back to their original positions? Insert an iron nail in the paper cylinder (Fig. 4.3d) and repeat the steps. Is there any difference in the deflection of the compass needles? Place some iron paper clips near the two ends of the nail. Are the clips attracted to the ends of the nail? 
Answer:
When current flows through the cylindrical coil, the magnetic compass needles deflect, indicating that the coil behaves like a magnet. When the current stops, the needles return to their original positions. When an iron nail is inserted into the coil, the deflection of the compass needles is much larger, showing that the iron core makes the electromagnet stronger. The nail also attracts the iron clips. This demonstrates that a current-carrying coil with an iron core is a strong electromagnet.
Activity 4.4
Q1. Take the electromagnet made in Activity 4.3 and a magnetic compass. Label the two ends of the coil as A and B. Place the magnetic compass near the end A of the coil as shown in Fig. 4.4a. Connect the coil to the cell and observe the compass. Note down which pole of the magnetic compass is attracted to end A. Repeat this procedure to find the polarity of end B as well (Fig. 4.4b). Did you find that the polarity of end B is opposite to the polarity of end A? 
Answer:
When the compass is placed near end A, if the north pole of the compass is attracted to end A, then end A is the south pole of the electromagnet, and end B is the north pole (and vice versa). This confirms that an electromagnet has two poles, North and South, just like a bar magnet.
Think Like a Scientist (Activity 4.3 Extension)
Q1. Repeat Activity 4.3 with—(i) 2 and 4 cells with the same coil, (ii) 2 cells but different number of turns of the coil. What do you observe?
Answer:
(i) Different cells with same coil: With more cells, the current is larger, and the magnetic field becomes stronger. The compass needle shows more deflection, and the coil attracts more clips.
(ii) Different number of turns with same cells: More turns of wire increase the magnetic field strength. The compass needle shows more deflection, and the coil attracts more clips.
Conclusion: The strength of an electromagnet increases with increasing current (more cells) and increasing number of turns of the coil.
Activity 4.5
Q1. Take a cardboard piece of about 10 cm length and 10 cm width, two nails, a nichrome wire of thickness about 0.3 mm (26-28 gauge) and length of 10 cm an electric cell, a cell holder, a switch, and connecting wires. Mount the nails on the cardboard about 5 cm apart. Tie the nichrome wire between these nails and make the connections as shown in Fig. 4.5 with the switch in OFF position. Touch the nichrome wire. What do you feel? Move the switch to ON position for about 30 s and then move it back to OFF. Touch the nichrome wire momentarily (Do not hold the nichrome wire). What difference do you feel? Repeat the last two steps to confirm the observation. 
Answer:
Before the current flows, the nichrome wire is at room temperature. After the switch is turned ON and current flows for 30 seconds, the nichrome wire feels warm. This happens because of the heating effect of electric current—the wire resists the flow of current, converting electrical energy into heat energy. Nichrome is used because it offers high resistance and generates more heat.
Think Like a Scientist (Activity 4.5 Extension)
Q1. Repeat Activity 4.5 with a battery of 2 cells. What do you notice? For the same duration, does the wire heat up more with one cell or two cells?
Answer:
With 2 cells, the current is larger, and the wire heats up more. The amount of heat generated depends on the magnitude of the current—more current means more heat. Thus, the wire heats up more with 2 cells than with a single cell.
Lemon Cell Activity
Q1. Take five or six juicy lemons, copper wires/strips (1-2 mm thick) and iron nails. Also take one LED and some connecting wires. Insert the copper wire and the iron nail in one of the lemons keeping them apart by a small distance as shown in Fig. 4.8a. Repeat the above step for all the remaining lemons. Join the copper wires and nails as shown in Fig. 4.8b. Connect the LED between the copper wire of the first lemon and the iron nail of the last lemon, using connecting wires. What do you observe? Does the LED glow? If the LED does not glow, reverse its connections. Does the LED glow now? 
Answer:
When the connections are made correctly, the LED glows. The lemon cell works because the lemon juice acts as an electrolyte, and the copper wire and iron nail act as electrodes. A chemical reaction between the electrodes and the electrolyte generates electric current. If the LED does not glow, reversing the connections can make it glow because LEDs only work when current flows in the correct direction (positive to positive, negative to negative).
Questions and Activities
Q1. Fill in the blanks: (i) The solution used in a Voltaic cell is called ________. (ii) A current carrying coil behaves like a ________.
Answer:
(i) The solution used in a Voltaic cell is called electrolyte.
(ii) A current carrying coil behaves like a magnet (electromagnet).
Q2. Choose the correct option: (i) Dry cells are less portable compared to Voltaic cells. (True/False) (ii) A coil becomes an electromagnet only when electric current flows through it. (True/False) (iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
Answer:
(i) False. Dry cells are more portable than Voltaic cells because they have a paste electrolyte and are sealed.
(ii) True. A coil behaves as an electromagnet only when current flows through it.
(iii) False. An electromagnet with more cells (more current) is stronger and attracts more clips.
Q3. 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:
Answer: (c) Both (i) and (ii) are correct
When current flows through a nichrome wire, it produces both heating effect (wire becomes warm) and magnetic effect (compass needle deflects).
Q4. Match the items in Column A with those in Column B.
| Column A | Column B |
|---|---|
| (i) Voltaic cell | (a) Best suited for electric heater |
| (ii) Electric iron | (b) Works on magnetic effect of electric current |
| (iii) Nichrome wire | (c) Works on heating effect of electric current |
| (iv) Electromagnet | (d) Generates electricity by chemical reactions |
Answer:
(i) Voltaic cell → (d) Generates electricity by 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
Q5. 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:
Answer: (ii) generates more heat for a given current.
Nichrome has high resistance, which causes it to generate more heat when current passes through it. This makes it suitable for heating elements.
Q6. 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 heating devices are more convenient because:
- Cleaner: They do not produce smoke or ash, reducing air pollution.
- Safer: No risk of fire hazards from open flames.
- Easier to use: Just plug in and switch on; no need for fuel collection or storage.
- More efficient: They convert electrical energy directly into heat with less waste.
- Environmentally friendly: If powered by renewable energy, they reduce carbon emissions compared to burning wood or fossil fuels.
Q7. 
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) In Fig. 4.4a, the current flows from the positive terminal of the battery through the coil to the negative terminal. The arrow should be drawn in the direction of current flow in the circuit.
(ii) The compass needle moves because the current-carrying coil produces a magnetic field around it. The magnetic field deflects the compass needle from its original north-south orientation.
(iii) If the battery terminals are reversed, the direction of current reverses. The polarity of the electromagnet will also reverse (North and South poles swap), so the compass needle will deflect in the opposite direction.
Q8. 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:
The lifting electromagnet stopped lifting the clips because:
- Cell/battery got drained: The cell was connected for a long time, and the chemicals inside the cell were used up, so the cell could no longer supply enough current.
- Overheating: The wire became warm due to the heating effect. The resistance of the wire may have increased, reducing the current and weakening the magnetic effect.
Q9. In Fig. 4.12, in which case the LED will glow when the switch is closed? 
Answer:
Answer: Case (b)
In Case (b), the LED is correctly connected with its positive terminal (longer leg) to the positive side of the battery and negative to negative. In Case (a), the LED is reversed, so it will not glow.
Q10. 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 because a current-carrying coil produces a magnetic field even without an iron core. However, the deflection will be less than before because the iron core increases the strength of the magnetic field significantly.
Q11. We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.13. 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:
Answer: (iv) In all four circuits
All four coils are conductors of electricity. When current flows through any conductor, it produces a magnetic field around it, which will deflect a compass needle. The material of the coil does not prevent the magnetic effect, though the strength of the magnetic field may vary.
Discover, Design, and Debate
Q1. Make coils of turns 25, 50, 75, and 100. Connect them to the same cell one by one. Note the deflection in a magnetic compass placed in the same position in all the cases. Report your observations. Draw conclusion of the effect of number of turns of the coil on the strength of the electromagnet. Take two thin nichrome wires of equal length and different thickness (approximately one of these wire thickness to be double of the other, say 0.3 mm and 0.6 mm). Connect them one by one in a circuit which has a switch and a cell, and allow the current to flow for 30 s in each case. Momentarily touch these wires. Which wire heats up more? Now repeat the same activity with two nichrome wires of same diameter but of different lengths. Prepare a brief report of your activity. Try to make an electric cell using various fruits and vegetables. Also try with electrodes of different metals. Prepare a brief report.
Answer:
Activity 1 (Coils with different turns): Observations show that the compass deflection increases with the number of turns. Conclusion: The strength of an electromagnet increases with the number of turns of the coil.
Activity 2 (Nichrome wires): The thinner wire (0.3 mm) heats up more than the thicker wire (0.6 mm) because it has higher resistance. Similarly, the longer wire heats up more than the shorter wire (same thickness) because resistance is directly proportional to length.
Activity 3 (Fruit cells): Different fruits (like lemons, potatoes, oranges) can act as electrolytes. The voltage generated depends on the type of fruit and the metals used. Copper and zinc or copper and iron work well. A report can include the fruit used, electrodes, and whether the LED glowed.