Question

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Answer 1

✯ An electric motor is a device which converts electrical energy into mechanical energy.

Principle:- The principle behind the electric motor is based on Fleming’s left hand rule.

Working:- When a current is allowed to flow through the coil PQRS the coil starts rotating anti-clockwise. This happens because a downward force acts on length PQ and at the same time, an upward force acts on length RS. As a result, the coil rotates anti-clockwise.

Current in the length PQ flows from P to Q and the magnetic field acts from left to right, normal to length PQ. Therefore, according to Fleming’s left hand rule, a downward force acts on the length PQ. Similarly at an upward force acts on the length RS. These two forces cause the coil to rotate anti-clockwise.

When coil complete half rotation, the position of PQ and RS interchange. The half-ring D comes in contact with brush A and half-ring C comes in contact with brush B. therefore the direction of current in the coil PQRS gets reversed. Now the current flows through the coil in the direction SRQP. The direction of current through the coil PQRS reverse after every half rotation. As a result, the coil rotates in same direction. The split rings help to reverse the direction of current in the circuit. These are called the commutator.

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✯ Given:-

A luminous object is placed 20cm from the surface of a convex mirror is set so that the virtual image formed in the two mirror coincide. If the plane mirror is at a distance 12cm from the object

To find:-

The focal length of the convex mirror.

Solution:-

As we know that when plane mirror is placed at a distance of 12 cm from the object, the image formed by plane mirror will be at a distance of 12 cm from the plane mirror.

AS is the virtual image formed by two image coincides, the distance between object and image in both cases is 12+12=24cm.

Given object distance in convex mirror is 20 cm from the convex mirror.

24−20=4

So the image formed will be 4cm away from the convex mirror, as shown in the figure.

Therefore, Object distance (u)=−20cm

Image distance (v)=4cm

By Applying the mirror formula,

$\sf \frac{1}{f} = \frac{1}{v} + \frac{1}{u}$

$\sf \frac{1}{f} = \frac{1}{4} + \frac{1}{(-20)}$

$\sf \frac{1}{f} = \frac{1}{4} - \frac{1}{20}$

$\sf \frac{1}{f} = \frac{5}{20} - \frac{1}{20}$

$\sf \frac{1}{f} = \frac{5-1}{20}$

$\sf \frac{1}{f} = \frac{\cancel{4}}{\cancel{20}}$

$\sf \frac{1}{f} = \frac{1}{5}$

$\sf f = 5$

Hence, f=5 is the required focal length of the convex mirror.

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Reflex action:- A reflex action is an involuntary and nearly instantaneous movement in response to a stimulus. A reflex action is made possible by neural pathways (reflex arcs) which can act on an impulse before that impulse reaches the brain.

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