Class 7 - Physics - Electric Current and its Effects

Question1.

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?

Answer:

  1. Connecting wires

Physics Class 7 Electric Currents Connecting Wires

2.   Switch in “ON” position

Physics Class 7 Electric Currents Switch ON

3.    Electric Bulb

Physics Class 7 Electric Current Bulb

4.    Electric Cell

Physics Class 7 Electric Current Electric Cell

5.  Switch in “OFF” position

Physics Class 7 ELectric Current Switch OFF

6.   Battery

Physics Class 7 Electric Currents Battery

Question2.

Draw the circuit diagram to represent the circuit shown in Fig?

Physics Class 7 Electric Circuits Electric Circuit 5

Answer:

In the above circuit one end of the bulb is connected to the battery and another end to the safety pin.

But the other end of the battery is not connected to the safety pin. Therefore there will be no flow of current as the switch is in “OFF” position.

This shows the circuit is not complete.

Circuit diagram of the above electric circuit will be:

Physics Class 7 Electric Circuits Electric Circuit 4

 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.

Physics Class 7 Electric Circuits InCorrect Connections

Answer:

In order to make battery, the positive terminal (+) of one cell should be connected to the (-) terminal of another cell and so on.

Physics Class 7 Electric Currents Battery With Correct Connections

Question 4.

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

Physics Class 7 ElectricCircuits CIrcuit 3

Answer:

The positive terminal (+) of one cell should always be connected to negative terminal (-) of another cell.

But in case of above circuit both the positive terminals are connected to each other. This is the reason battery is not able to generate electricity.

Therefore the change which should be made to the circuit will be that the positive terminal (+) should be connected to negative terminal (-) of the cell and vice-versa.

Physics Class 7 Electric Circuits Circuit 2

Question 5.

Name any two effects of electric current.

Answer:

The two effects of electric current are:-

  1. Heating effect
  2. Magnetic effect

Heating Effect of current: When an electric current passes through a wire, the wire gets heated. This is known as the heating effect of current.

The heating effect is used in many heating applications like electric heater, electric iron, light bulb etc.

Magnetic Effect of current: When an electric current flows through a wire, it generates magnetic field around it.

This effect is known as magnetic effect of current. Magnetic Effect of electric current has many applications like power lift, electric bell, electric fan etc.

Physics Class 7 Electric Currents Magnetic & Electric Effects

Question 6.

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

 Answer:

The current flowing through the conducting wire creates magnetic field. When a compass is kept nearby this wire,

it will experience two magnetic fields (one magnetic field due to wire and another magnetic field due to compass).

As a result when they interact with each other there will be deflection seen in the magnetic needle.

When the current is switched off, there is no magnetic field produced by the wire, magnetic needle does not deflect from its north-south position.

Physics Class 7 ElectricCircuits Magnetic Needle

Question 7.

Will the compass needle show deflection when the switch in the circuit shown by Figure is closed?

Physics Class 7 Electric Currents Circuit With Compass

Answer:

There is no source of electric current i.e. (battery or cell). The circuit is an open circuit.

There will be no current flowing it, as a result, there will be no magnetic field produced by the wires.

Therefore there is no deflection shown by the compass needle.

                                          

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 ___________.

Answer:

  • Longer line in the symbol for a cell represents its Positive­­­­

Physics Class 7 Electric Currents Electric Circuit With Battery1

  • The combination of two or more cells is called a Battery.

   Physics Class 7 Electric Currents Circuit With Battery     

Explanation: -

When a positive terminal of one cell is connected to a negative terminal of the second cell. Such a combination of two or more cells is called battery.

  • When current is switched ‘on’ in a room heater, it ­­­­­­­­produces heat.

Explanation: -

When the electric current passes through the wire in the electric heater it becomes hot. This effect is known as heating effect of the electric current.

Physics Class 7 ElectricCurrents Electric Heater 

  • The safety device based on the heating effect of electric current is called a fuse.

Explanation: -

A fuse is a safety device which limits current in circuit as a result it prevents damages to electrical circuits and possible fires.

Fuses used in electrical appliances.

Physics Class 7 ElectricCurrents 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)

Answer:

(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (False)

Explanation: -

In order to make a battery the negative terminal of one cell is connected to the positive terminal of the other cell.

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

Explanation: -

The wire of the fuse is made of special material whose melting point is very low.

So if large current flows through the circuit the wire melts, and breaks the circuit.

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

Explanation: -

A current carrying coil of an insulated wire wrapped around a piece of iron is called an electromagnet.

It behaves as a magnet when current passes through it which results in attracting the piece of iron.

Physics Class 7 Electric Currents Electromagnet

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

Explanation: -

In an electric bell piece of iron is wound over a coil. When current passes through the coil it behaves as an electromagnet.

Physics Class 7 Electric Current Electric Bell

Electric Bell

Question 10.

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

Answer:

No, plastic bags are non-magnetic materials so they will be not attracted by the electromagnet. Electromagnets will only attract magnetic materials.

Therefore electromagnet cannot be used for separating plastic bags from a garbage heap.

Physics Class 7 ElectricCurrent Uses of Electromagnets

 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.

Answer:

No. I don’t agree with the electrician. Fuse is a safety device that is a safeguard to all of our electrical appliances.

They are made up of special material which melt quickly and break when large electric currents passed through it.

If the electrician does not use proper fuse wire, instead he uses any ordinary electric wire as fuse; it increases the risk of overheating of wires due to flow of excessive current.

It may lead to short circuit in electric equipment and these appliances may catch fire. It is advised to use standard fuse wire or MCBs to prevent electrical accidents.

Question 12.

Zubeda made an electric circuit using a cell holder shown in Fig (4), 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.

Physics Class 7 Electric Currents Circuit With Battery

Answer:

Following are the possible reasons for the defects in the circuit:-

  1. Cells are not connected properly.
  2. Rubber band which is used in the cell holder is not tight enough to keep two cells in contact with each other.
  3. Bulb can be fused.
  4. Switch is not functioning properly.

Question 13.

 In the circuit shown in Figure.

Physics Class 7 ElectricCurrents Circuit 1

 (i)  Would any of the bulbs 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?

Answer:

  • When the switch is in ‘OFF’ position, electric circuit is not closed as a result none of the bulbs will glow.
  • When the switch is in ‘ON’ position, circuit is complete and electric current will flow. As a result all the bulbs will glow.

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