Electric Current and Circuits | Science | Chapter 14 | Class 7th | Questions and Answers |

 

JANDKNCERT | Science | Class 7th |

Electric Current and Circuits | Science | Chapter 14 | Class 7th | Questions and Answers |

 

Chapter 14: Electric Current and Circuits

EXERCISES

A. Fill in the blanks

1. A drawing of an electrical circuit with standard symbols is called a/an ______ (Circuit diagram/ electric diagram).

2. Hot plates, electric toasters, and electric irons get hot when switched on because of the ______ effect of current. (Magnetic/heating).

3. A fuse is a ______ device (safety/heating).

4. A wire twisted in the form of a circle is called a/an ______ (coil/electromagnet).

5. An ______ consists of a soft iron corer with an insulated wire wound around it (electromagnet/electric magnet).

Ans.

1. A drawing of an electrical circuit with standard symbols is called a/an, circuit diagram (Circuit diagram/ electric diagram).

2. Hot plates, electric toasters, and electric irons get hot when switched on because of the heating effect of current. (Magnetic/heating).

3. A fuse is a safety device (safety/heating).

4. A wire twisted in the form of a circle is called a/an, coil (coil/electromagnet).

5. An electromagnet consists of a soft iron corer with an insulated wire wound around it (electromagnet/electric magnet).

 

B. Choose the correct answer

1. A circuit diagram is

(a) a picture of a circuit

(b) a drawing of a circuit with standard pictures for the different electrical components.

(c) a diagram of an electrical circuit with standard symbols for different electrical components.

(d) a difficult representation of an electrical circuit.

Ans. (c) a diagram of an electrical circuit with standard symbols for different electrical components.

 

2. When an electric current, passes through a wire, the wire gets hot this is called

(a) Joule heating

(b) Conduction

(c) Electricity

(d) Thermal heating

Ans. (a) Joule heating

 

3. An electric fuse is

(a) a safety device

(b) an aeroplane

(c) used to produce an electric current

(d) used to heat a room

Ans. (a) a safety device

 

4. When we bring a magnetic compass near a current-carrying wire,

(a) it deflects the magnetic needle of the compass

(b) it makes the magnetic needlepoint North

(c) it makes the magnetic needlepoint South

(d) It has no effect on a magnetic needle

Ans. (a) it deflects the magnetic needle of the compass

 

5. A coil refers to

(a) an electrical wire

(b) a fuse

(c) a current-carrying conductor

(d) a wire twisted in the form of a circle

Ans. (d) a wire twisted in the form of a circle

 

6. An electromagnet acts like a magnet

(a) when a current is passed through the coil

(b) all the time

(c) because it has a magnetic core

(d) only if a current does not pass through the coil

Ans. (a) when a current is passed through the coil

 

7. In an electric bell, we have

(a) an electromagnet

(b) a hammer

(c) an interrupter

(d) all of these

Ans. (d) all of these

 

8. In an electric bell, which of these gets attracted to the electromagnet

(a) the hammer

(b) the thong

(c) the soft iron strips

(d) the screw

Ans. (c) the soft iron strips

 

9. Which of these does not use the heating effect of current?

(a) electric toaster

(b) electric fan

(c) electric iron

(d) room heater

Ans. (b) electric fan

 

10. An electric fuse wire melts if the amount of current flowing through it is

(a) more than a minimum amount

(b) less than a minimum amount

(c) more than a maximum amount

(d) less than a maximum amount

Ans. (c) more than a maximum amount

 

C. Answer the following questions

Q1. What is an electrical circuit?

Ans. The unbroken path through which electric current can flow is called an electric current.

 

Q2. Draw an electrical circuit with an electrical cell, a bulb, and an ‘ON’ switch.

Ans. An electrical circuit, with an electrical cell, a bulb, and an ‘ON’ switch is as shown in the following diagram.

Q3. Why does an electric bulb get hot if it is ep ‘ON’ for a little time?

Ans. When an electric current passes through a wire or an appliance it gets heated up due to the ohmic or joule heating effect. Hence an electric bulb gets hot when kept ‘ON’ for a while.

 

Q4. Name three appliances that use the heating effect of electric current.

Ans. Electric toasters, electric irons, and electric ovens use heating effect of electric current.

 

Q5. What is an electric fuse?

Ans. An electric fuse a safety device that prevents a large amount of current, flowing into any device. It consists of a thin wire made of a material that melts easily when the current exceeds its maximum capacity.

 

Q6. Draw an electric fuse and explain briefly how it works?

Ans. Following are the diagrams of some of the electric fuses:

Working and Principle of Fuse: An electric fuse consists of a thin wire which is made of a material that melts easily when heated. It allows a certain maximum amount of current. When the current exceeds the maximum amount of the current, the wire heats and causes it to melt and break the circuit, thereby preventing further flow of current.

 

Q7. How can show that an electric current has a magnetic effect?

Ans. With the help of a compass needle, we can show the deflection of the needle due to the magnetic effect of an electric current.

          Take two insulated pieces of wire and connect two ends with positive and negative terminals of the battery respectively. Bring a small magnetic compass near the wire and touch the free ends. A deflection in the needle of a compass is seen. This shows that an electric current has a magnetic effect.

 

Q8. What is an electromagnet?

Ans. An electromagnet is a magnet that is made by wrapping a wire around a soft iron piece. When the electric current is passed through the wire, the iron piece behaves like a magnet, that attracts iron pieces.

 

Q9. Name two factors on which the strength of an electromagnet depends.

Ans. The strength of an electromagnet depends on the number of turns of the wire around the core and the amount of current passing through it.

 

Q10. Explain the working of an electric bell using diagrams.

Ans. Working of an Electric Bell:

Step 1: When the bell is switched on, the electric current flows to the electromagnet.

Step 2: The electromagnet attracts the soft iron strip. The hammer attached to the strip then hits the gong, causing a ring.

Step 3: When the soft iron strip gets attached to the electromagnet, it no longer touches the screw (interrupter), and hence the circuit is broken.

Think and Answer

1. We have seen that the heating effect of electric current has been put to use in many appliances like toasters, heaters, etc. This kind of heating can also be a waste in many places. Can you think of one example?

(Hint: Think of appliances, electrical wires, etc. Where there is excessive heating.)

Ans. One example, of excess heating and its wastage, is an electric geyser. These appliances are continuously used in homes and therefore are the cause of electric wastage.

 

2. You know that the strength of an electromagnet depends on the amount of current flowing in the coil. Do you think the magnetic strength will increase/decrease if the amount of current in the coil is increased?

(Hint: Think what happens if the current is totally switched off. Then think about what will happen if we slowly increase the current.)

Ans. We know that no current flows when the current is totally switched off, hence there will no, magnetic effect. The magnetic effect increases when we increase the amount of the current in the coil.

 

3. Do you think all electrical fuses should have the same maximum current?

(Hint: Different appliances require different amounts of current.)

Ans. No, all electrical fuses should not have the same maximum current. These fuses must have the current as per the capacity of their load.

 

Additional Questions

Page No. 208

 

 

(I) Match the following

 

Column A	Column B
1. Circuit diagram	(a) Heating of a conductor due to an electric current
2. Joule heating	(b) Fuse
3. Safety device	(c) Fuse wire
4. Melts easily	(d) A drawing of an electrical circuit with standard symbols

 

 

Ans.

 

Column A	Column B
1. Circuit diagram	(d) A drawing of an electrical circuit with standard symbols.
2. Joule heating	(a) Heating of a conductor due to an electric current.
3. Safety device	(b) Fuse
4. Melts easily	(c) Fuse wire

 

 

(II) Draw the standard symbol used to represent each of the following components in a circuit diagram

 

1. Electric cell

2. An open switch

3. A closed switch

4. A bulb

5. A wire

Ans. See Q2

 

(III) Fill in the blanks

 

1. A magnet has two poles, namely the ______ pole and the ______ pole.

2. Like poles of a magnet _______ each other and unlike poles _______ each other.

3. A magnet, when suspended freely, aligns itself in the _______ direction.

 

Ans.

1. A magnet has two poles, namely the North pole and the South pole.

2. Like poles of a magnet repel each other and unlike poles attract each other.

3. A magnet, when suspended freely, aligns itself in the N-S direction.