NCERT Textbook Exercises

Q1. Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery.

(Ref: Textbook Table 14.1)

Connecting Wire: Straight Line.
Switch OFF: Two dots with open arm.
Bulb: Circle with loop inside.
Cell: One long line (+), one short thick line (-).
Switch ON: Two dots with closed arm (connected).
Battery: Combination of cells (one after another).

Q2. Draw the circuit diagram to represent the circuit shown in Fig. 14.21.

The diagram will have a cell, a bulb, a switch, and connecting wires connected in a closed loop.

Q3. Fig. 14.22 shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.

Connect Negative terminal of first cell to Positive terminal of second cell. Negative of second to Positive of third, and so on.

Q4. The bulb in the circuit shown in Fig. 14.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

Problem: The cells are connected incorrectly (Negative terminal connected to Negative terminal).

Solution: Connect the Positive terminal of one cell to the Negative terminal of the other cell.

Q5. Name any two effects of electric current.

Heating Effect: Wire gets hot when current passes through it (e.g., Electric Iron).
Magnetic Effect: Wire behaves like a magnet when current passes through it (e.g., Electromagnet).

Q6. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.

This is due to the Magnetic Effect of Electric Current. When current passes through a wire, it behaves like a magnet. This magnetic field interacts with the magnetic needle of the compass, causing it to deflect.

Q7. Will the compass needle show deflection when the switch in the circuit shown by Fig. 14.24 is closed?

No, because there is no source of electric current (like a cell or battery) in the given circuit. Current will not flow, so no magnetic effect will be produced.

Q8. Fill in the blanks:

(a) Longer line in the symbol for a cell represents its positive terminal.
(b) The combination of two or more cells is called a battery.
(c) When current is switched ‘on’ in a room heater, it produces heat (becomes red hot).
(d) The safety device based on the heating effect of electric current is called a fuse.

Q9. Mark 'True' or 'False':

(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (False)
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (True)
(c) An electromagnet does not attract a piece of iron. (False)
(d) An electric bell has an electromagnet. (True)

Q10. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.

No. Plastic is a non-magnetic material. An electromagnet attracts only magnetic materials (like iron). So, it cannot attract or separate plastic bags.

Q11. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.

No, I would not agree. A fuse wire is made of a special material with a low melting point. If we replace it with an ordinary wire (which has a high melting point), it will not melt when excessive current flows. This can damage electrical appliances or cause a fire.

Q12. Zubeda made an electric circuit using a cell holder... and a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.

Possible reasons:

Connecting wires may be loose or broken.
Cells may be connected incorrectly (+ to + or - to -).
The bulb may be fused.
The cells may be dead (used up).

Detailed Chapter Topics

1. Electric Components and Symbols

Cell: Longer line (+) and Shorter thick line (-).
Battery: Combination of cells. (+ of one to - of other).
Switch: Device to complete (ON) or break (OFF) the circuit.
Wire: Straight line.
Bulb: Filament glows when current flows.

2. Heating Effect of Electric Current

When current passes through a wire, it gets hot.
Used in: Iron, Heater, Geyser, Toaster, Hair Dryer.
Element: Coil of wire (like Nichrome) which produces heat.
Fuse: Safety device. Wire melts and breaks circuit if current is too high. Prevents fire.
MCB (Miniature Circuit Breaker): Switch that turns off automatically when current exceeds limit.

3. Magnetic Effect of Electric Current

Discovered by Hans Christian Oersted.
Current carrying wire behaves like a magnet.
Deflects magnetic compass needle.
Electromagnet: Coil of insulated wire wrapped around iron piece. Magnet only when current flows.
Used in: Electric Bell, Cranes (to lift heavy iron loads), Toys.

4. Electric Bell

Works on magnetic effect of current.
Contains an electromagnet.
Hammer strikes gong when electromagnet pulls the iron strip. Circuit breaks. Hammer goes back. Cycle repeats.

Key Facts and Definitions

1. Important Terms

Circuit Diagram: Representation of an electric circuit using symbols.
Filament: Thin wire in bulb (Tungsten) that glows. If it breaks, bulb gets fused.
CFLs: Compact Fluorescent Lamps. Consume less electricity than bulbs.
ISI Mark: Ensure safety of appliances. Always look for it.

2. Quick Facts

Nichrome wire is used as heating element.
Copper wire is used for connections (low resistance, less heat).
MCBs are increasingly being used in place of fuses.
An electromagnet can be very strong and lift very heavy loads.

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