NCERT Exemplar Problems Class 7 Science – Electric Current and Its Effects
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 electric shock
(d) behaves like a fuse
(c) It deflects a magnetic needle placed near it because a magnetic field is created when the current passes through it.
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) In case of electric fuse, there is only a maximum limit on the current which can safely flow in the electric circuits.
Three bulbs A, B, C are connected in a circuit as shown in 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) All the bulbs will glow simultaneously because there is no time lag in the flow of current as soon as the circuit is complete.
When a switch is in OFF position,
(i) circuit starting from positive terminal of the cell stops at the switch
(ii) circuit is open
(iii) no current flows through it
(iv) current flows after sometime
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) (i) and (ii) are correct
(b) When the switch is in OFF position, circuit is incomplete. It is said to be open and therefore, no current flows through it.
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 do 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) For using electric gadgets/ appliances/ circuit, we have to turn the switch in ON position. Therefore, precaution given in option (d) need not be considered.
Very Short Answer Type Questions
Which property of a conducting wire is utilised in making electric fuse?
Electric fuse wire is made from some special material which has low melting point. When large electric current is passed through a circuit, it melts quickly and breaks the circuit and prevents damages to electrical circuit.
Name device used these days in place of electric fuses in electrical circuits.
The device used these days in place of electric fuse is MCB (Miniature Circuit Breaker). This is a switch which automatically turns off when current in a circuit exceeds the safe limit.
Fill in the blanks:
(a) Our body is a………… of electricity.
(b) An electric cell produces electricity from the……….. in it.
(c) In an electric circuit, a fuse is a…………… to prevent possible fire.
(d) A combination of two or more cells is called a……………..
(a) Conductor Our body is a good conductor of electricity as it allows the electric current to pass through it.
(b)Chemicals stored, an electric cell is a device capable of changing chemical energy into electrical energy (electricity).
(c)Safety device If by accident the current exceeds the safe limit, in a circuit, the fuse will blow off and break the circuit and will prevent damage to circuit and possible fire.
(d)Battery A combination of two or more cells, where the positive terminal of one cell is connected to the negative terminal of the next cell, is called a battery.
Unscramble the following words:
(a) BATTERY It is a combination of two or more cells.
(b) FUSE It is a safety device in an electric circuit to prevent possible fire.
(c) TORCH It is a device portable lamp which uses battery for its operation.
(d) SWITCH It is an electric component which can make or break an electric circuit.
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 has not taken the right step. Because due to the excessive heat of bulb, the towel may burn and it will also result in the wastage of electrical energy.
Why are Compact Fluorescent Lamps (CFLs) preferred over electric bulbs?
CFLs are preferred over electric bulbs because electric bulbs use more power of electricity and losses electrical energy in the form of heat. However, CFLs do not waste electrical energy as heat.
Why is an electric fuse required in all electrical appliance?
Electric fuse is required in all electrical appliances to prevent damage from excessive current flow and during short circuit.
Short Answer Type Questions
Can we use the same fuse in a geyser and a television set? Explain.
No, we cannot use same fuse in a geyser and in a television set because the fuse used in every appliance has some limit to withstand the current flowing through it. So, different appliances have fuses of different ratings.
Name two electric devices for each where
(a) heating effect of current is used and
(b) magnetic effect of current is used.
(a) Heating effect of current is used in electric heater and geyser.
(b) Magnetic effect of current is used in electric bell and cranes to lift heavy magnetic materials from one place to other.
Why do we cover plug pinholes which are within the reach of children with cellotape or a plastic cover when not in use?
We do cover plug pinholes which are within the reach of children with cellotape or plastic cover to avoid electric shocks. If unconsciously, a child puts his finger into the electric socket, he/she may get a shock which 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 electromagnet will attract more pins? Give reason.
Since, the magnetic effect directly depends on the number of turns of wire on electromagnet. So, electromagnet of Paheli will attract more pins as it has more number of turns of wire on it and thus, it is a stronger electromagnet.
Your teacher has shown you the following activity.
Activity: Teacher has wound a long insulated piece of wire around and 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. 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 which may arise in our mind are as follows:
(i) Why does the iron nail attract the pins placed near its ends?
(ii) On disconnecting the circuit, why pins get detached from the iron nail?
(ii) What will happen if we use some other material like wood or straw in place of 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 with wires as shown in the circuit given in figure. The current flowing in both the circuits is the same.
(a) Will the heat produced in both the cases be equal? Explain.
(b) Will the heat produced be the same, if the wires taken by them are of equal lengths but of different thickness? Explain.
(a) No, the amount of heat produced in both the wires will be different because amount of heat produced in a wire on passing electric current depends upon the length of wire and here length is different for both‘the wires.
(b) No, the amount of heat produced in the wires of same lengths but different thickness cannot be the same because amount of heat produced in a wire depends upon the thickness of the wire.
How does the magnetic effect of electric current help in the working of an electric bell? Explain with the help of a diagram.
As depicted in the diagram, when switch is in ON position, electric current is passed through the coil or solenoid. By the phenomenon of magnetic effect of current, the coil or solenoid gets magnetised which attracts nearby soft iron armature and the hammer strikes the gong to produce a sound.
Thus, electric bell starts ringing and as soon as switch is OFF, no current flows through the coil and its magnetisation stops. So, bell stops ringing.
Draw the symbols of the following circuit components.
(a) electric cell
(b) switch in OFF position
(c) electric bulb