NCERT Exemplar Solutions: Electric Current & Its effects

Multiple Choice Questions

Q1: When an electric current flows through a copper wire AB as shown in Figure,  the wire

(a) deflects a magnetic needle placed near it.
(b) becomes red hot.
(c) gives an electric shock.
(d) behaves like a fuse.

Ans: (a)
Explanation:

• A wire carrying an electric current produces a magnetic field around it.
• This magnetic field can deflect a nearby magnetic needle, so a needle placed close to a current-carrying wire will experience a force and get deflected.
• The strength of this magnetic field depends on the amount of current and the distance from the wire - larger currents produce a stronger effect.

Q2: Choose the statement which is not correct in the case of an electric fuse.
(a) Fuses are inserted in electric circuits of all buildings.
(b) There is a maximum limit on the current which can safely flow through the electric circuits.
(c) There is a minimum limit on the current which can safely flow in the electric circuits.
(d) If a proper fuse is inserted in a circuit it will blow off if current exceeds the safe limit.

Ans: (c)
Explanation:

• Fuses protect circuits by melting when the current exceeds a safe or specified maximum limit, thereby breaking the circuit and preventing damage or fire.
• There is no minimum limit of current that a fuse is required to detect - small currents simply flow without harming the circuit.
• Therefore statement (c) is not correct; statements (a), (b) and (d) correctly describe how fuses are used and why they blow when current is too high.

Q3:  Three bulbs A, B, C are connected in a circuit as shown in Figure 14.2. When the switch is 'ON'
(a) bulb C will glow first.
(b) bulb B and C will glow simultaneously and bulb A will glow after some time.
(c) all the bulbs A, B and C will glow at the same time.
(d) the bulbs will glow in the order A, B and C.

Ans: (c)
Explanation:

• When the switch is turned ON, the circuit becomes complete and electric current flows through all parts of the circuit.
• Because the electrical signals travel very fast in wires, all the bulbs receive the current essentially at the same time and therefore glow simultaneously.

Q4: When a switch is in OFF position,
(i) circuit starting from the positive terminal of the cell stops at the switch.
(ii) the circuit is open.
(iii) no current flows through it.
(iv) current flows after some time. Choose the combination of the correct answer from the following.
(a) all are correct
(b) (ii) and (iii) are correct
(c) only (iv) is correct
(d) only (i) and (ii) are correct

Ans: (b)
Explanation:

When the switch is in the OFF position:

• The circuit is open, so the continuous path needed for current is broken.
• Because the path is broken, no current flows through the circuit.

Q5: Which of the following precautions need not be taken while using electric gadgets/appliances/circuit?
(a) We should never touch a lighted electric bulb connected to the mains.
(b) We should never experiment with the electric supply from the mains or a generator or an inverter.
(c) We should never use just any wire or strip of metal in place of a fuse.
(d) We should never turn the switch in ON position.

Ans: (d)
Explanation:

Statement (d) is incorrect as a safety precaution. Turning a switch to the ON position is necessary to operate an appliance. The correct precautions are:

• Avoid touching electric bulbs or live parts connected to the mains to prevent electric shock.
• Never tamper with mains supply or try makeshift repairs using random wires - this is dangerous.
• Use the correct fuse rating; do not replace a fuse with any random metal strip because it will not protect the circuit.

Very Short Answer Questions

Q6: Which property of a conducting wire is utilised in making electric fuse?

Ans: Fuse wire has a low melting point and relatively high resistance, so it heats up and melts when excessive current flows.

Q7: Name the device used these days in place of electric fuses in electrical circuits.

Ans: Miniature Circuit Breaker (MCB).

Q8: Fill in the blanks:
(i) Our body is a ________________ of electricity.
(ii) An electric cell produces electricity from the __________ ___________ in it.
(iii) In an electric circuit a fuse is a _________ _______ to prevent possible fire.
(iv) A combination of two or more cells is called a _________.

Ans:
(i) Our body is a conductor of electricity.
(ii) An electric cell produces electricity from the chemicals stored in it.
(iii) In an electric circuit, a fuse is a safety device to prevent possible fire.
(iv) A combination of two or more cells is called a battery.

Q9: Unscramble the following words:
(i) TBTAYER
(ii) SFEU
(iii) HTRCO
(iv) HICWTS

Ans:
Battery
Fuse
Torch
Switch

Q10: Paheli does not have a night lamp in her room. She covered the bulb of her room with a towel in the night to get dim light. Has she taken the right step? Give one reason to justify your answer.

Ans: No. Paheli's action is unsafe.

• The towel can catch fire because the bulb produces heat; covering it increases the risk of ignition.
• It also wastes energy to keep the bulb on when a proper dim lamp would be safer and more efficient.

Q11: Why are compact fluorescent lamps (CFLs) preferred over electric bulbs?

Ans: CFLs use less electrical energy and produce less heat than ordinary incandescent bulbs, so they are more energy-efficient.

Q12: Why is an electric fuse required in all electrical appliances?

Ans: A fuse is required as a safety device. It melts when the current becomes too high, thereby breaking the circuit and protecting the appliance and wiring from damage or fire caused by excessive current.

Short Answer Questions

Q13: Can we use the same fuse in a geyser and a television set? Explain.

Ans: No. A geyser and a television draw very different amounts of current. Each appliance needs a fuse with a rating suited to its normal current. Using a fuse with too low a rating will cause it to blow often; using one with too high a rating will fail to protect the appliance and wiring.

Q14: Name two electric devices for each where (i) heating effect of current is used and (ii) magnetic effect of current is used.

Ans:
Heating effect - Geyser, room heater.

Magnetic effect - Electric bell, cranes (electromagnets used to lift magnetic material).

Q15: Boojho made an electromagnet by winding 50 turns of wire over an iron screw. Paheli also made an electromagnet by winding 100 turns over a similar iron screw. Which electromagnet will attract more pins? Give reason.

Ans: Paheli's electromagnet will attract more pins because it has a larger number of turns of wire. With the same current and similar iron core, more turns produce a stronger magnetic effect, so the electromagnet is stronger and can attract more pins.

Long Answer Questions

Q16: Your teacher has shown you the following activity.

Activity: Teacher has wound a long insulated piece of wire around an iron nail in the form of a coil. Free ends of the wire are connected to a cell through a switch as shown in Figure. The current is switched on and some pins are placed near the ends of the nail.
Write down any three questions that come to your mind about this activity.

Ans:
(i) Why does the nail attract the pins?
(ii) What will happen if we add more cells (increase the battery) in the circuit?
(iii) What will happen if we use a non-metallic object (for example, a straw) instead of the nail?

Q17: Paheli took a wire of length 10 cm. Boojho took a wire of 5 cm of the same material and thickness. Both of them connected the wires as shown in the circuit given in Figure. The current flowing in both circuits is the same.

1. Will the heat produced in both the cases be equal? Explain.
2. Will, the heat produced be the same if the wires taken by them are of equal lengths but of different thickness? Explain.

Ans:
(i) No. When the same current flows, a longer wire produces more heat because it has greater electrical resistance. More resistance means more electrical energy is converted into heat in the longer wire.
(ii) No. If wires have equal length but different thickness (thicker and thinner), the thicker wire has lower resistance and will produce less heat for the same current, while the thinner wire has higher resistance and will produce more heat.

Q18: How does the magnetic effect of electric current help in the working of an electric bell? Explain with the help of a diagram.

Ans: An electric bell works by turning electrical energy into mechanical motion using the magnetic effect of current:

• The bell has a coil of wire wound around an iron piece that acts as an electromagnet when current flows through the coil.
• An iron strip with a hammer at its end is placed so that the electromagnet can attract it. A contact screw completes the circuit when the strip touches it.
• When the circuit is closed, current flows through the coil and the coil becomes magnetised. The electromagnet then pulls the iron strip, causing the hammer to strike the bell and make a sound.
• As the strip moves away, the contact is broken so the current stops; the electromagnet loses magnetism and a spring pulls the strip back to its original position, restoring the contact.
• This make-and-break action repeats rapidly, causing the hammer to strike the bell many times and produce a continuous ringing sound.

Q19: Draw the symbols of the following circuit components.
(i) electric cell
(ii) switch in off position
(iii) electric bulb
(iv) battery

Ans: