Science STD 7 Chapter 10: Electric Current and its Effects - Exercises

Science STD 7 Chapter 10: Electric Current and its Effects - Exercises

1. 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

The symbols for the given components are as follows:

  • Connecting wires: A straight line.
  • Switch in the 'OFF' position: An open switch symbol.
  • Bulb: A circle with an 'X' inside.
  • Cell: A long, thin line and a short, thick parallel line.
  • Switch in the 'ON' position: A closed switch symbol.
  • Battery: A combination of two or more cell symbols, with the positive terminal of one connected to the negative terminal of the next.

2. Draw the circuit diagram to represent the circuit shown in Fig.10.21.

The circuit diagram would show a battery, a switch, and a bulb connected in series with wires, represented by their respective symbols. The switch would be in the 'OFF' position, and the bulb would not be glowing.

[Image of a circuit diagram showing a battery, an open switch, and a bulb]

3. Fig.10.22 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.

To form a battery of four cells, the positive terminal of one cell must be connected to the negative terminal of the next. This would be shown by drawing lines connecting the terminals accordingly. The final connection would be between the negative terminal of the first cell and the positive terminal of the last cell.

[Image of a diagram showing four cells connected in series to form a battery]

4. The bulb in the circuit shown in Fig.10.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

The bulb does not glow because the terminals of the battery are connected incorrectly. To make the bulb glow, the positive terminal of the cell needs to be connected to one terminal of the bulb, and the negative terminal of the cell needs to be connected to the other terminal of the bulb, completing the circuit.

[Image of a corrected circuit diagram showing a glowing bulb]

5. Name any two effects of electric current.

Two effects of electric current are:

  1. Heating effect of electric current: When a current passes through a wire, the wire gets heated.
  2. Magnetic effect of electric current: When a current passes through a wire, the wire behaves like a magnet.

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

When an electric current flows through a wire, it produces a magnetic field around it. A compass needle is a small magnet. When it is brought near the current-carrying wire, it gets deflected due to the magnetic effect of the electric current, as like poles repel and opposite poles attract.

7. Will the compass needle show deflection when the switch in the circuit shown by Fig.10.24 is closed?

No, the compass needle will not show deflection. This is because the circuit is not complete. The current cannot flow from the battery to the compass needle. The wire is not connected to a complete circuit, therefore there is no electric current to produce a magnetic field that would deflect the needle.

8. Fill in the blanks:

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

9. Mark ‘T’ if the statement is true and ‘F’ if it is false:

  1. (a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)

    F

  2. (b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)

    T

  3. (c) An electromagnet does not attract a piece of iron. (T/F)

    F

  4. (d) An electric bell has an electromagnet. (T/F)

    T

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

No, an electromagnet cannot be used to separate plastic bags from a garbage heap. Electromagnets are only capable of attracting magnetic materials like iron and steel. Plastic is not a magnetic material, so an electromagnet would have no effect on it.

11. 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, you should not agree to this. A fuse is a safety device designed to melt and break the circuit when a current exceeds a safe limit, thus preventing fires and damage to appliances. A normal wire would not melt and break as easily, so it would not serve the safety purpose of a fuse and could lead to a short circuit or fire.

12. Zubeda made an electric circuit using a cell holder shown in Fig. 10.4, 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 defects in Zubeda's circuit could be:

  • The cell might have been placed incorrectly in the cell holder (e.g., negative terminal connected to negative).
  • The bulb could be fused, meaning its filament is broken.
  • The connecting wires might be loose or broken, creating an open circuit.
  • The switch might be faulty or improperly connected.

13. In the circuit shown in Fig. 10.25 (i) Would any of the bulb glow when the switch is in the ‘OFF’ position? (ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?

(i) When the switch is in the 'OFF' position, none of the bulbs will glow because the circuit is open and no current can flow.

(ii) When the switch is moved to the 'ON' position, all three bulbs (A, B, and C) will glow simultaneously, as they are connected in a parallel circuit. The current will split and flow through each bulb at the same time.

Suggested Activities and Projects

1. Set up the circuit shown in Fig. 10.17 again. Move the key to ‘ON’ position and watch carefully in which direction the compass needle gets deflected. Switch ‘OFF’ the current. Now keeping rest of the circuit intact, reverse the connections at the terminal of the cell. Again switch ‘on’ the current. Note the direction in which the needle gets deflected. Think of an explanation.

Outline: This activity demonstrates that the direction of the magnetic field produced by a current-carrying wire is dependent on the direction of the current. When the connections of the cell are reversed, the direction of the current flow also reverses. As a result, the compass needle will deflect in the opposite direction, confirming the relationship between current direction and magnetic field direction.

2. Make four electromagnets with 20, 40, 60 and 80 turns. Connect them one by one to a battery of 2 cells. Bring the electromagnet near a box of pins. Count the number of pins attracted by it. Compare the strengths of the electromagnets.

Outline: This experiment shows that the strength of an electromagnet is directly proportional to the number of turns in its coil. The electromagnet with 80 turns will attract the most pins, indicating it is the strongest. The one with 20 turns will attract the fewest, showing it is the weakest. This illustrates a key principle of electromagnetism.

3. Using an electromagnet, you can make a working model of a railway signal as shown in Fig.10.26.

Outline: This project involves creating a working model of a railway signal. By passing current through an electromagnet, you can attract an iron piece attached to a signal arm. When the current is on, the signal arm is attracted and changes position. When the current is off, the magnetic force disappears, and the arm returns to its original position. This demonstrates a practical application of the magnetic effect of electric current.

4. Visit an electric shop. Request an electrician to show you the various types of fuses and MCB and to explain how they work.

Outline: This activity is a field trip to learn about safety devices in electrical circuits. An electrician would show you different types of fuses (e.g., cartridge fuses, wire fuses) and Miniature Circuit Breakers (MCBs). They would explain that a fuse works by melting when too much current passes through it, breaking the circuit. An MCB is a switch that automatically turns off when the current exceeds a safe limit and can be reset manually.