# NCERT Solutions for Class 7 Science Chapter 14 Electric Current and its Effects

These NCERT Solutions for Class 7 Science Chapter 14 Electric Current and its Effects Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

## Electric Current and its Effects NCERT Solutions for Class 7 Science Chapter 14

### Class 7 Science Chapter 14 Electric Current and its Effects Textbook Exercise Questions and Answers

Question 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.

Question 2.
Draw the circuit diagram to represent the circuit shown in figure given below.

Question 3.
The given figure 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.

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

The positive terminals of both the cells are together and that is why the bulb is not glowing. The cells should be kept in a way that the positive terminal of one cell is near the negative terminal of another cell, to make the bulb glow.

Question 5.
Name any two effects of electric current.
The effects of electric current are:

1. Heating effect
2. Magnetic effect

Question 6.
When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
The needle gets deflected when it is kept near a current carrying wire due to the magnetic effect of the current. The needle of the compass is made up of a thin magnet and when it comes in contact with the current carrying wire, which also acts like a magnet, then the like poles repel each other and opposite poles attract each other. Therefore the needle gets deflected.

Question 7.
Will the compass needle show deflection when the switch in the circuit shown below is closed?

The compass needle does not get deflected as there is no battery or any source of current.

Question 8.
Fill in the blanks:
a. Longer line in the symbol for a cell represents it’s ………………… terminal.
b. The combination of two or more cells is called a …………………
c. When current is switched ‘on’ in a room heater, it …………………
d. The safety device based on the heating effect of electric current is called a …………………
a. positive,
b. battery,
c. becomes red hot and emits heat
d. fuse

Question 9.
Mark ‘T’ if the statement is true and ‘F’ if it is 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. (T/F)
b. When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
c. An electromagnet does not attract a piece of iron. (T/F)
d. An electric bell has an electromagnet. (T/F)
a. F,
b. T,
c. F,
d. T

Question 10.
Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Electromagnets can only attract magnetic materials. Plastic bag is a non-magnetic material and will not be attracted by an electromagnet. Hence, an electromagnet cannot be used for separating plastic bags from a garbage heap.

Question 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, we would not agree to replace the fuse by a wire. Wires in the fuses are of specific melting points. So we should always use ISI marked fuses in our houses to prevent short circuits.

Question 12.
Zubeda made an electric circuit using a cell holder shown in the given figure, 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.

The cells should be connected properly. The positive terminal of one cell should be in series with the negative terminal of the other cell. The switch should be closed properly and a bulb which is fused should not be used. Then, the bulb will certainly glow.

Question 13.
In the circuit shown in the figure given below

a. Would any of the bulbs glow when the switch is in the ‘OFF’ position?
b. What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
a. No, when the switch is in ‘OFF’ position, the current does not flow through the circuit. Hence, none of the bulbs will glow,
b. Bulbs will glow simultaneously. When the switch is moved to the ‘ON’ position, then all the bulbs will glow at once. This is because they all are connected to the same battery and switch.

NCERT Extended Learning Activities and Projects

Question 1.
Set up the circuit shown in fig 14.17 (of NCERT textbook) 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 terminals of the cell. Again switch ‘ON’ the current. Note the direction in which the needle gets deflected. Think of an explanation.
Hint:
The direction of magnetic poles get reversed in a conductor, when the direction of current is reversed. Hence, the needle will show deflection in opposite direction in the second case.

Question 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.
Hint:
The electromagnet with 80 turns will attract the maximum number of pins, followed by that of 60 turns, then 40 turns and finally 20 turns which will attract the least number of pins. This is because the strength of an electromagnet increases by increasing the number of turns in the coil.

Question 3.
Using an electromagnet, you can make a working model of a railway signal as shown in figure given below.

Hint:
To make a model railway signal, you will need an iron nail, a thread, two pieces of plywood, a paper clip, a battery, a pencil and a length of copper wire. Fix the two pieces of plywood to each other. They . should be perpendicular to each other. Tie the nail through the thread and fix the thread to the vertical plyboard. Take a card and cut out a signal arm and fix it to the upper end of the jaw with the help of paper clip. Next, take the pencil and wind some copper wire around it, making a hollow coil. There should be extra copper wire at both the ends. Place the coil on the horizontal plywood and connect it to the battery. Insert an iron nail inside the coil loosely. Taking another drawing pin and a paper clip, create a circuit. When you complete the circuit, the nail will be attracted inwards into the coil. This is because it creates a magnetic field. You will find the signal arm moving up and down lifting the signal.

Question 4.
Visit an electric shop. Request a mechanic to show you the various types of fuses and MCB and to explain how they work.
Hint:
Do it yourself.

Activity 1

Objective: To demonstrate the heating effect of electric current.
Materials Required: Electric cell (1.5 volts), small torch bulb, switch, connecting wires with the insulation removed from its ends and adhesive tape.
Procedure:

• Connect the electric cell to the torch bulb along with the switch with the help of connecting wires and tapes.
• Keep the switch in the OFF position.
• Touch the bulb.
• Now put the switch in the on position.
• Allow the bulb to glow for a while.
• Touch the bulb carefully.

Observation: When the switch is in OFF position, the bulb is not warm but when it is in ON position, the bulb feel warm.
Conclusion: When the electric current flows in the wires, the bulb becomes warm. Hence, current has heating effects.

Electric bulb: When electric current passes through the filament of an electric bulb, it gets heated up to high temperatures due to heating effect of current. The heat generated is so high that the filament of the bulb starts glowing, giving off light. Hence, filaments are made of tungsten that has very high melting point.

CFLs and LEDs: Incandescent bulbs give off heat along with light which leads to wastage of electricity in the form of heat. Hence, fluorescent tube lights or Compact Fluorescent Lamps (CFLs) are used which are better electricity efficient. Nowadays, light emitting diode (LED) bulbs are used because they consume much less electricity the CFLs and incandescent bulbs for the same intensity of light. Also, CFLs contain mercury vapour which is toxic and needs to be disposed off carefully.

ISI mark: Bureau of Indian Standards, New Delhi assign a standard mark, called the ISI mark on electrical products which is an assurance of conformity of the specifications given on the product and its safety. We should hence use products with ISI mark.

Electric Fuse: Electric fuse is a safety device which is used in household wirings and in many appliances. Electric fuse has a body made up of ceramic and two points for attaching the fuse wire. The fuse wire is made of a special material which easily melts whenever there is a large current in the wiring. This breaks the circuit and helps in preventing damage to costly appliances and to the wiring.

Activity 2

Objective: To demonstrate the working of a fuse.
Materials Required: Two nails, a sheet of thermocol/a wooden block, steel wool, a four-celled battery, switch, 6 volt bulb and connecting wires.
Procedure:

• Fix two nails about 5 to 10 cm apart on a sheet of thermocol or a wooden block.
• Tie a thin strand of steel wool between the two nails.
• Connect the nails to the two terminals of a four-celled battery through a switch and a 6 volt bulb by using connecting wires.
• Now, pass the electric current through the circuit by moving the switch to ‘ON’ position.

Observations: After a few seconds, the thin strand of steel wool melts and breaks and the bulb goes off. Actually, thin strand of steel wool is like a fuse wire here.
Conclusion: When a large current passes through the thin strand of steel wool, a lot of heat is produced in it due to the heating effect of current. This heat melts the thin strand of steel wool and breaks it, cutting off the flow of current in the circuit. This is how a fuse works in the household electric wiring.

MCBs (Miniature Circuit Breaker): MCBs have been replacing electric fuse from wirings at most of the places. The electric fuse has a big practical problem. Whenever the wire fuses, one needs to replace the wire to resume electric supply. More often than not, this proves to be a cumbersome task. Miniature circuit breakers break the circuit automatically. One just needs to switch it on to resume the electric supply. Many models of MCBs have a built-in mechanism by which the electric supply is automatically resumed.

Magnetic Effect of Electric Current: We know that magnets can attract iron pieces. A compass needle is a small magnet that points in North-South, direction. This needle gets deflected whenever an magnet is brought close to it. Hans Christian Oersted first observed that when a compass needle is kept close to a current carrying wire, the needle gets deflected. This shows that current passing through a conductor produces magnetic effects.

Activity 3

Objective: To demonstrate the magnetic effect of current.
Materials Required: Insulted copper wire, a compass needle, electric cell, switch and bar magnet.
Procedure:

• Take a thick, insulated copper wire ‘AB’ and fix it on a table in such a way that this wire is in the North-South direction.
• A compass is placed under the wire AB.
• The two ends of the wire AB are connected to a cell through a switch by using connective wires.
• Now close the switch so that an electric current passes through the wire AB.

Observation: When we closed the switch then the current passes through the wire AB and the needle of the compass moves from its original direction.
Conclusion: When an electric current flows through a wire, it behaves like a magnet.

Electromagnets: An electromagnet is a temporary magnet, which behaves like a magnet only when electric current is passed through it. It consists of an insulated wire wound around a magnetic material like iron. Both ends of the wires behave like both poles of the magnet.
Uses of electromagnet: Electromagnets are used in telephones, telegraphs, electric motors, electric bells, generators, etc.

Activity 4

Objective: To make an electromagnet and to show that it attracts iron.
Materials Required: Long piece of copper wire, iron nail, electric cell and iron pins.
Procedure:

• Wind the copper wire around the iron nail.
• Connect the two ends of the wire to the two terminals of an electrical cell to form an electromagnet.
• Bring a small iron pin close to it and observe.
• Disconnect the wire ends from the two terminals of the electric cell and observe what happens.

Observation: When the two ends of the wire are connected to the cell, the current is flowing through the coil. As a result, the pin gets attracted to the electromagnet. When we disconnect the wire, the pin falls off.
Conclusion: The iron nail behaves like a magnet which attracts the pins towards it. It will happen when the current flows through the wire, but if we disconnect the circuit, the iron losses its magnetic property.

Electric Bell: Electric bell is composed of two rods of cast iron. Coil is wound around the iron rods. A metallic strip is placed parallel to the coils. The metallic strip is fitted with a hammer at one end. Another end of the strip is connected to the circuit. A gong is placed in a position so that it can be hit by the hammer.

When current flows in the circuit, the cast iron rods become electromagnet and attract the metallic strip. The metallic hammer hits the gong because of that. Once the metallic strip is pulled towards the electromagnet, it gets disconnected from the contact screw which has joined it to the circuit and there is a break in the circuit. A spring pulls the metallic strip back because no magnetism is left in the electromagnet. This cycle continues and the electric bell produces a ringing sound.

### Class 7 Science Chapter 14 Electric Current and its Effects Additional Important Questions and Answers

Question 1.
What are the components of a simple electric circuit?
Battery, wire, key and bulb are the components of a simple electric circuit.

Question 2.
Give the symbol of a battery.

Question 3.
Does a bulb glow when the circuit is open?
No, the bulb does not glow when the circuit is open.

Question 4.
What are the effects of electricity?
The flow of electric current causes the following effects in a conductor:

• Heating effect
• Chemical effect
• Magnetic effect

Question 5.
On what factors does the heat produced in a wire depend?
It depends on the material, length and thickness of the wire.

Question 6.
Differentiate between electric current and electric circuit.
Electric current is the flow of charges. But an electric circuit is the closed path in which electric current flows.

Question 7.
What is an electric cell?
It is device which produces electric charge because of some chemical reactions.

Question 8.
The electric bulb works on which property of electric current?
Heating effect of electric current.

Question 9.
What is the full form of MCB?
Miniature Circuit Breaker.

Question 10.
Write full form of CFLs.
Compact Fluorescent Lamps.

Question 11.
What do you mean by open circuit?
Open circuit means the switch is in ‘on’ position.

Question 12.
What do you mean by a closed circuit?
Closed circuit means switch is ‘in’ on position.

Question 13.
Which effect of electric current is utilised in an electric bell?
Magnetic effect.

Question 14.
Which effect of electric current is utilised in the working of an electric fuse?
An electric fuse works on the heating effect of current.

Question 15.
Name the alloy which is used to make the heating elements of electrical heating appliances like an electric iron.
Nichrome.

Question 16.
Write the name of the scientist who discovered the magnetic effect of electric current.
Hans Christian Oersted (H.C Oersted).

Question 17.
How can we reduce the wastage of electricity?
The wastage of electricity can be reduced by using CFLs and LEDs.

Question 18.
What is meant by a battery?
When two or more cells are joined together in such a way that the positive terminal of one cell is kept in contact with the negative terminal of other, we get a battery of cells.

Question 19.
Name two devices which are based on the magnetic effect of current.

1. Compass needle
2. Electric bell

Question 20.
What is a circuit diagram?
A circuit diagram is used to symbolically represent an electric circuit.

Question 1.
Define an electric circuit and explain it with the help of a diagram.
Any closed path along which electric current can flow is called an electric circuit. An electric circuit generally consists of the following components:

1. A source of electric current (cell or battery).
2. A conducting wire (say copper wire).
3. An electrical appliance (like a bulb).
4. A switch/key.

Question 2.
Name and define any two effects of electric current.
a. Heating Effect: When an electric current flows through a wire, the wire gets heated. It is the heating effect of current due to the resistance of wire to the flow of current. This effect has many applications like electric heater, light bulb, etc.

b. Magnetic effect: When an electric current flows through a wire, it produces magnetic effect around it that is detected by a magnetic compass. For examples: electric bell, motor, fan, etc., are based on this effect.

Question 3.
What are filaments of a bulb and a heater made up of?
In a bulb, there is a thin wire called the filament made up of tungsten. When
current passes through it, it gets heated up and glows. In a heater, there is a coil of wire called element which is made up of nichrome. When current passes through the element, it becomes red hot and gives out heat.

Question 4.
What is a switch?
Switch is used to close and open a circuit. When switch is ‘ON’, circuit is complete and current flows through it and when it is ‘OFF’, circuit breaks (air is an insulator) and flow of current stops.

Question 5.
What are the reasons for excessive currents in electrical circuits?
One reason for excessive currents in electrical circuits is the direct touching of wires. This may happen if the insulation on the wires has come off due to wear and tear. This may cause a short circuit. Another reason for excessive current can be the connection of many devices to a single socket. This may cause overload in the circuit.

Question 6.
What is the use of electrical fuse?
In all buildings, fuses are inserted in all electrical circuits. There is a maximum limit on the current which can safely flow through a circuit. If by accident, the current exceeds this safe limit, the wires may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off by melting away and break the circuit. A fuse is thus a safety device which prevents damages to electrical circuits and possible fires.

Question 7.
Why should we not touch the lighted bulb with bare hands?
We should never touch a lighted electric bulb connected to the mains with bare hands. It may be very hot and our hand may get burnt badly. In the same way, we should not experiment with the electric supply from the mains or a generator or an inverter. It may cause an electric shock.

Question 8.
What are MCBs? How do they work?
MCBs (Miniature Circuit Breaker) have been replacing electric fuse from wirings at most of the places. The electric fuse has a big practical problem. Whenever the wire fuses, one needs to replace the wire to resume electric supply. More often than not, this proves to be a cumbersome task. Miniature circuit breakers break the circuit automatically. One just needs to switch it on to resume the electric supply. Many models of MCBs have a built-in mechanism by which the electric supply is automatically resumed.

Question 9.
Write three differences between a bar magnet and an electromagnet.

 Bar magnet Electromagnet 1. It is a permanent magnet. 1. It is a temporary magnet. 2. Its power cannot be changed. 2. The power can be changed. 3. It has fixed poles. 3. The poles can be reversed.

Question 10.
Differentiate between an open and a closed circuit.
Closed circuit: The electric path which starts from the positive terminal of a cell or a battery ends at its negative terminal, without any break, is called a closed electric circuit. Current flows in this circuit and switch is in ‘on’ position.

Open circuit: The electric path which starts from the positive terminal of a cell or battery but is broken at some point, is called an open electric circuit. Current does not flow from such a circuit and the switch is in ‘off’ position.

Question 11.
Of what substance is the fuse wire made? Why?
The fuse wire is usually made from tin-plated copper wire. The tin-plated copper wire has low melting point due to which it can easily melt by the heating caused by the excessive flow of current in the circuit.

Question 12.
Why are CFLs preferred over incandescent bulbs?
An incandescent bulb is used for lighting but it also gives heat. This is not desirable. This results in the wastage of electricity. This wastage can be reduced by using compact fluorescent lights (CFLs) in place of the bulbs. CFLs are also used to reduce wastage of electricity. These can be fixed in ordinary bulb holders.

Question 1.
Explain with the help of a diagram, the construction and the working of an electric bell.
Electric bell is composed of two rods of cast iron. Coil is wound around the iron rods. A metallic strip is placed parallel to the coils. The metallic strip is fitted with a hammer at one end. Another end of the strip is connected to the circuit. A gong is placed in a position so that it can be hit by the hammer. When current flows in the circuit, the cast iron rods become electromagnet and attract the metallic strip. The metallic hammer hits the gong because of that. Once the metallic strip is pulled towards the electromagnet, it gets disconnected from the contact screw that has attached it to the circuit and there is a break in the circuit. A spring pulls the metallic strip back because no magnetism is left in the electromagnet. This cycle continues and the electric bell produces a ringing sound.

Question 2.
Define an electromagnet. Give various uses of electromagnets.
A core of magnetic material (such as iron) surrounded by a coil of wire behaves like a magnet when electric current flows through it. When the electric current is switched off, the coil generally loses its magnetism. Such coils are called electromagnets. Magnetic effect of electric current has been used in making powerful electromagnets. More number of turns makes more powerful magnet.

Uses of Electromagnets:
Electromagnets are used in telephones, telegraphs, electric motors, electric bells, generators, etc.

• Powerful electromagnets are used in cranes.
• Bullet trains use the principle of electromagnetism.
• Audio and video tapes use the principle of magnetism.
• The most popular use of the electromagnets is in electric motors.

Question 3.
a. Overloading: All electrical appliances and wires used in a circuit have definite rating. If the appliances connected to the circuit draw more current than rated, then it is called overloading.

b. Heating effect: When current is passed through a wire the current starts to flow from the positive to the negative terminal. This current produces heat energy. It is called heating effect of current.

Question 4.
What are the differences between an electromagnet and a permanent magnet?

 Electromagnet Permanent Magnet 1. The magnetic properties are displayed when current is passed through it. 1. Magnetic properties exist even when no current is there. 2. The strength is adjusted depending upon the amount of flow of current. 2. The strength depends upon the nature of the material used in its creation. 3. Removal of magnetic properties is temporary. 3. Once magnetic properties are lost, it becomes useless. 4. It is usually made up of soft materials. 4. It is usually made of hard materials. 5. The poles of this kind of magnet can be altered with the flow of current. 5. The poles of this kind of magnet cannot be changed and are fixed.

Question 5.
How does an electric room heater or an electric iron work?
The wires get hot when an electric current passes through them. An electric room heater or immersion heater, geysers, electric kettles, hair dryers, etc., contain a coil of wire. This coil of wire is called an element. When these appliances are switched ON after connecting to the electrical supply, their elements become red hot and give out heat. These appliances work on the heating effects of current, i.e., the resistance offered by the metal alloy to the flow of current which produces the heating effect.

Picture-Based Questions

Question 1.
Draw diagrams of: (a) closed circuit, (b) open circuit. In which of these does the bulb glow?
The bulb glows when the circuit is closed or switched ‘ON’.

Question 2.
a. Draw a labelled diagram to show how an electromagnet is made.
b. What is the purpose of soft iron core used in making an electromagnet?