An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

Answer

The direction of the magnetic field can be determined using the Fleming’s Left hand rule. The direction of the magnetic field will be perpendicular to the direction of current and the direction of deflection, i.e., either upward or downward. The direction of the current is from the front wall to the back wall because negatively charged electrons move from the back wall to the front wall. The directed of the magnetic force is rightward. Hence, using Fleming’s left hand rule it can be concluded

Question - 32 : - Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?

Answer - 32 : - An electric motor is a device that converts electrical energy to mechanical energy. It works on the principle of magnetic effect of current. The figure listed below shows a simple electric motor.

When current is made to flow through the coil MNST by closing the switch, the coil starts to rotate in the anticlockwise direction. This is due to the downward force acting on the length MN and simultaneously an upward force acting along the length ST. As a result of which the coil rotates in the anticlockwise direction. Current in the length MN flows from M to N and the magnetic fields act from left to right normal to the length MN. According to Fleming’s Left Hand rule, a downward force acts along the length MN. Similarly, the current along the length ST flows from S to T and the magnetic field acts from left to right. Therefore, an upward force acts along the length ST. These two forces together cause the coil to rotate anti-clockwise. After half a rotation, the position of MN and ST interchange. The half ring C come in contact with brush B and the half ring D comes in contact with rush C. Hence the direction of current in the coil MNST gets reversed.

Question - 33 : - Name some devices in which electric motors are used.

Answer - 33 : -

A few devices in which electric motors are used are:

• Electric fans

• Water pumps

• Mixers

• Washing machines

Question - 34 : - A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is

Answer - 34 : - (i) pushed into the coil,

(ii) withdrawn from inside the coil,

(iii) held stationary inside the coil?

Answer

(i) When a bar magnet is pushed into the coil, current is induced in the coil momentarily as a result the galvanometer deflects in a particular direction momentarily.

(ii) When the bar magnet is withdrawn from inside the coil, current is induced momentarily but in the opposite direction and the galvanometer deflects in the opposite direction momentarily.

(iii) When the bar magnet is held stationary inside the coil, no current will be induced as a result there will be no deflection in the galvanometer.

Question - 35 : - Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

Answer - 35 : - When the current in coil A changes, the magnetic field associated with it also changes. As a result the magnetic field around coil B undergoes change. The change in the magnetic field of coil B induces current in it.

Question - 36 : - State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current,

Answer - 36 : - (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and

(iii) current induced in a coil due to its rotation in a magnetic field.

Answer

(i) The rule used to determine the direction of the magnetic field produced around a straight conductor-carrying current is the Maxwell’s right hand thumb rule.

(ii) The rule used to determine the force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it is the Fleming’s left hand rule.

(iii) The rule used to determine the current induced in a coil due to its rotation in a magnetic field is the Fleming’s right-hand rule.

Question - 37 : - Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Answer - 37 : - The electric generator coverts the mechanical energy into the electrical energy. The working principle of the electric generator is the electromagnetic induction. It generates electricity by rotating a coil in the magnetic field. The figure below shows the construction of a simple AC generator.

In the diagram,

A and B are brushes,

C and D are slip rings

X is the axle

G is the galvanometer

When the axle X is rotated clockwise, MN moves upwards while ST moves downward. The movement of MN and ST in the magnetic field results in the production of electric current due to electromagnetic induction. MN moves upwards and the magnetic fields act from left to right. Therefore, according to Fleming’s right hand rule, the direction of the induced current will be from M to N along the length MN. Similarly, the direction of the induced current will be from S to T along the length ST. The direction of the current in the coil is MNST. Hence, galvanometer shows a deflection in a particular direction.

After half a rotation, length MN starts moving downwards while the length ST starts moving upwards. Now, the direction of the induced current reverses to TSNM. Since the direction of the induced current reverses every half rotation, the current induced is known as alternating current.

Function of Brushes

Brushes are kept pressed on to two slip rings separately. Outer ends of brushes are connected to the galvanometer. Thus, brushes help in transferring current from coil to the external circuit.

Question - 38 : - When does an electric short circuit occur?

Answer - 38 : -

Listed below are two instances of when a short-circuit can occur:

1) When too many appliances are connected to a single socket or when high power rating appliances are connected to a light circuit, the resistance of the circuit becomes low as a result the current flowing through the circuit becomes very high. This condition results in a short-circuit.

2) When live wires whose insulation have worn off come in contact with each other, the current flowing in the circuit increases abruptly which results in a short circuit.

Question - 39 : - What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Answer - 39 : -

The metallic body of electric appliances is earthed by means of earth wire. Any leakage of electric wire is transferred to the ground by means of earth wire. This prevents the user of the electric appliance from getting electric shocks. This is the reason why it is important for the metallic appliances to be earthed.

Free - Previous Years Question Papers

Chapter 13 Magnetic Effects of Electric Current Solutions

Subscribe Now!