Class 7 Science Chapter 14 Notes

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Class 7 Science Chapter 14 Notes

Chapter Name: - Electric Current and its Effects

Class 7 Science Chapter 14 Notes

Section - A (Keywords)

Explain the following Keywords

Battery – A device used as a source of power. In it, the chemical energy is converted into electrical energy. It has two terminal named positive and negative terminal.

Also, a battery is the arrangement of two or more cells, such that the positive terminal of one cell is connected to the negative terminal of the other cell.

Circuit diagram – The representation of the electric circuit with the help of symbols and lines is called a circuit diagram.

Electric components – Electric wire, battery, cell, switch, bulb, etc are known as the electric components.

Electric bell – A bell that has an electromagnet in it and works with electricity.

Electromagnet – A magnet that has a magnetic effect only when the electric current is passed through it.

Fuse – Fuse is made of low melting materials to prevent damage to the electric appliances.

Heating effect of current – The electric current heat up the wire it passes through, this is known as the heating effect of the current.

Magnetic effect of current – A metal wire behaves like a magnet when the electric current is passed through it.

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Class 7 Science Chapter 14 Notes

Section - B (Text Questions - Answer)

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.

(i) Motion of your hands while running – Oscillatory motion

(ii) Motion of a horse pulling a cart on a straight road – Motion along a straight line

(iii) Motion of a child in a merry-go-round – Circular motion

(iv) Motion of a child on a see-saw – Oscillatory motion

(v) Motion of the hammer of an electric bell – Oscillatory motion

(vi) Motion of a train on a straight bridge – Motion along a straight line

Question 2

Draw the circuit diagram to represent the circuit shown in Fig. 14.21.

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(i) The basic unit of time is second – Correct

(ii) Every object moves with a constant speed – Not correct

(iii) Distances between two cities are measured in kilometers – Correct

(iv) The time period of a given pendulum is not constant – Not correct

(v) The speed of a train is expressed in m/h – Not correct

Question 3

Fig.14.22 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.

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The time taken by the pendulum to complete one oscillation is known as time period of the pendulum.

Given that,

Total time taken = 32s

Number of oscillations = 20

Therefore,

Time period = Total time taken / Number of oscillations

= 32/20

= 1.6 seconds

Question 4

The bulb in the circuit shown in Fig.14.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

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Given That,

Distance covered = 240 km

Time taken = 4 h

Therefore,

Speed = Distance covered / Time taken

= 240/ 4

= 60 Km / h

Question 5

Name any two effects of electric current.

Given that,

Distance covered by car = Initial reading of the odometer – Final reading of the odometer.

= 57336.0 km – 57321.0 km

= 15.0 km

Time taken between = Initial reading of the clock – Final reading of the clock

= 08:30 AM to 08:50 AM

= 20 minutes = 20/60 hour = 1/3 hour

Therefore, speed in km/min

Distance covered = 15 km

Time taken = 20 min

Speed = 15/ 20 = 0.75 km/min

Speed in km/h

Distance covered = 15 km

Time taken = 1/3 hour

Speed = 15 x 3 km

= 45km/h

Question 6

When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.

Given that,

Speed of the bicycle = 2 m/s

Time taken = 15 minutes

= 15 x 60 seconds

= 900 seconds

Therefore,

distance = speed x time

= 2 x 900

= 1800 m = 1.8 km

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Class 7 Science Chapter 14 Notes

Question 7

Will the compass needle show deflection when the switch in the circuit shown by Fig.14.24 is closed?

Show the shape of the distance-time graph for the motion in the following cases-min
The distance-time graph

 

Explanation

Case i): – In case of a constant motion the car will cover the same distance in the same unit of time. Therefore the graph obtained will be a straight line at an angle.

Case ii): – In case of a car parked on a side road will show no motion. It means it is not covering any distance. Therefore the graph will show an increase in time but constant distance.

Question 8

Fill in the blanks: (a) Longer line in the symbol for a cell represents its ________________ 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) Longer line in the symbol for a cell represents its positive terminal.

(b) The combination of two or more cells is called a battery.

(c) When current is switched ‘on’ in a room heater, it becomes hot.

(d) The safety device based on the heating effect of electric current is called a 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) To make a battery of two cells, the negative terminal of one cell is connected to the negative

terminal of the other cell. (F)

(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T)

(c) An electromagnet does not attract a piece of iron. (F)

(d) An electric bell has an electromagnet. (T)

Question 10

Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.

(ii) 25 km

Solution:

Case I:

Speed of the car = 40 km/h

Time = 15 min = 15/60 hour

Distance covered by the car = Speed x Time = 40 x 15/60 = 10 km

Case II:

Speed of the car = 60 km/h

Time = 15 min = 15/60 hour

Distance covered by the car = Speed x Time = 60 x 15/60 = 15 km

Total distance covered by the car = 10 km + 15 km = 25 km

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.

From figures 13.1 and 13.2, we conclude that the blue coloured car is the fastest car. The distance covered by the fastest car in the photograph is nearly 1 cm.

Therefore, the actual distance covered by the car on ground = 1 × 100 m = 100 m

Time taken = 10 seconds

As, Speed = distance / Time

⇒ 100m / 10s

⇒ 10 m/s

Question 12

Zubeda made an electric circuit using a cell holder, 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.

Fig. 13.15 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster-min

The x-axis represents the time and the y-axis represents distance.

The distance covered by the vehicle A is more compared to the distance covered by the vehicle B in the same unit of time. Hence, vehicle A is moving faster than vehicle B.

Question 13

In the circuit shown in Fig. 14.25 (i) Would any of the bulb glow when the switch is in the 'OFF' position? (ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the 'ON' position?

Which of the following distance-time graphs shows a truck moving with speed which is not constant-min

Number (iii) distance-time graph of the truck is moving with the speed which is not constant.

Explanation:

Number (i) and (iv) distance-time graph shows a constant motion.

Number (ii) distance-time graph shows the truck is at rest as the time is increasing but the distance is the same.

Only Number (iii) distance-time graph shows different distance covered in the different interval of time. Hence the truck is moving with the speed which is not constant in this case.

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Class 7 Science Chapter 13 Notes

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