S3 Chapter 24 (Part 2)

S3 Chapter 24 (Part 2)

Multiple Choice

A student carries out an experiment to see the effect of a magnetic field on a wire carrying a current. The wire moves upwards as shown.

What should the student do to make the wire move downwards?

• change the direction of the current

• move the poles of the magnet closer together

• send a smaller current through the wire

• use a stronger magnet

Multiple Choice

When the electric current in wire XY is in the direction shown, there is an upward force on the wire.

If the north and south poles of the magnet exchange positions, in which direction will the force on the wire act?

• downwards

• upwards

• to the left

• to the right

Multiple Choice

Emf generated by a running motor, because it consists of a coil turning in a magnetic field; it opposes the voltage powering the motor

• back emf

• cosmic rays

• helical motion

• induced emf

Multiple Choice

WHAT IS BACK EMF?

• THE VOLTAGE OPPOSES THE SUPPLY VOLTAGE

• THE VOLTAGE DOES NOT OPPOSE THE SUPPLY VOLTAGE

• SUPPLY VOLTAGE

Multiple Choice

DC MOTOR CONVERTS ------- ENERGY INTO ----------ENERGY

• MECHANICAL, ELECTRICAL

• ELECTRICAL, MECHANICAL

• KINETIC, POTENTIAL

Open Ended

A dc motor with an armature  resistance of 0.75 ohm is connected to a 12 V supply. The motor rotates freely at 600 rpm and the current in the armature is 0.8 A. When a load is applied to the motor, its speed is 200 rpm. Calculate 

(i) the current in the motor at the instant the motor is switched on 

(ii) the back emf when the motor rotates freely.

Open Ended

A motor is connected to a voltage supply. When the motor is not loaded, it rotates freely.

(a) When the motor is loaded, its speed of rotation decreases, the armature current increases. Explain why.

(b) explain the consequence if the armature of the motor is prevented from rotating.

Open Ended

What is the phenomena known as in which an emf is induced in a conductor when the current in the conductor changes?

Open Ended

What is the unit of L?

Open Ended

The current in a coil of self-inductance 0.30 mH changes at a rate of 60 As^-1 , find the e.m.f. induced in the coil.

Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field.

Multiple Choice

Which of the following statement is correct?

• 感应电流是一种涡流 Induced current is a kind of eddy current.

• 涡流是一种感应电流 Eddy current is a kind of induced current.

Multiple Choice

Identify the law which is used to find out the direction of eddy currents.

• Lenz’s law

• Faraday’s law

• Ampere circuital law

• Maxwell’s law

Open Ended

Based on the application of eddy current in the induction cooker, what is the reason of using high frequency alternating current in the cooker?

Multiple Choice

Why is the iron core of a transformer made laminated instead of being in one solid piece?

• To reduce the magnetic field link losses

• To reduce the loss due to heating of coils

• To increase the loss due to heating of coils

• To reduce the losses due to eddy currents.

Multiple Choice

How can electromagnetic damping be increased?

• Increasing the magnetic density

• Reduce viscosity

• Winding the coil on the aluminum frame

• Increase the temperature of the coil