Lesson: Electric current and its Effects

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

(a)               Deflects a magnetic needle placed near it.

(b)               Becomes red hot.

(c)               Gives electric shock.

(d)               Behaves like a fuse.

a

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.

c

Three bulbs A, B, C are connected in a circuit as shown in the figure. 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.

c

When a switch is in OFF position,

(i)                 Circuit starting from the positive terminal of the cell stops at the switch.

(ii)              Circuit is open.

(iii)            No current flows through it.

(iv)             Current flows after some time.

Choose the combination of 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.

b

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.

d

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

Low melting point

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

Miniature Circuit Breaker or MCB.

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 _________.

(i) Conductor

(ii) Chemicals stored

(iii) Safety device

(iv) Battery

Unscramble the following words:

TBTAYER

SFEU

HTRCO

HICWTS

(i) Battery

(ii) Fuse

(iii) Torch

 (iv) Switch

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.

No she didn’t take the right step. The bulb would be hot and towel may burn. It will also result in unnecessary wastage of electric energy.

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

CFLs are preferred over electric bulbs as they do not waste electrical energy as heat and hence need less electrical energy for providing same amount of light.

Why is an electric fuse required in all electrical appliances?

Fuses are safety devices that are necessary to check the excessive flow of electric current in order to protect the appliances from short circuit and fire.

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

A geyser and a television set require different amount of electrical energy. Also, the rating of a fuse is different for different appliances and depends on the electrical energy they consume. Thus, the same fuse cannot be used in a geyser and a television.

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

Heating effect of current is used in room heaters and geysers.

Magnetic effect of current is used in cranes that lift magnetic materials, and in electric bells.

Why do we cover plug pin holes which are within the reach of children with cello tape or a plastic cover when not in use?

Plug pin holes within the reach of children are covered with cello tape or a plastic cover when not in use because a child may get an electric shock by putting his/her fingers into the socket, and it could be fatal.

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 electro magnet will attract more pins? Give reason.

Paheli’s electromagnet will attract more pins because it has more number of turns, which makes it a stronger electromagnet than the one Boojho made.

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 the 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.

Some of the questions can be:

(i) Why did the iron nail attract the pins?

(ii) Will tighter wrapping of the wire making any difference?

(iii) If more cells are used in the circuit what will be the result?

(iv) Will the results be different if a wooden stick is used instead of the iron nail?

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 the figure. The current flowing in both the circuits is the same.

(i)                       Will the heat produced in both the cases be equal? Explain.

(ii)                    Will the heat produced be the same if the wires taken by them are of equal lengths but of different thickness? Explain.

(i) The amount of heat produced in the two cases will be different, as the amount of heat produced depends upon the wire’s length.

(ii) The heat produced will not be the same as the amount heat produced also depends upon the wire’s thickness.

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

An electromagnetic bell consists of a coiled wire wound on a piece of iron. An iron strip with a hammer at one end is placed near the electromagnet with a contact screw close to the iron strip. When the strip comes in contact with the screw, the circuit is complete and electric current flows through the coil turning it into an electromagnet. The electromagnet then pulls the iron strip and the hammer at the end of the strip then strikes the gong, and the sound of a ringing bell occurs.

Draw the symbols of the following circuit components.

(i)              Electric cell

(ii)           Switch in off position

(iii)         Electric bulb

(iv)          Battery