Magnetic Fields

Question 1

Define one tesla.

Answer 1

Magnetic flux density that produces a force of 1 N on a 1 m wire carrying 1 A at right angles to the field.

Question 2

State Fleming’s Left Hand Rule.

Answer 2

Thumb = Force
First finger = Field
Second finger = Current

Question 3

When is the force on a current-carrying wire maximum?

Answer 3

When the wire is perpendicular to the magnetic field.

Question 4

When does a charged particle experience no force in a magnetic field?

Answer 4

When it moves parallel to the field lines.

Question 5

In what direction is the magnetic force on a moving charge?

Answer 5

Perpendicular to both velocity and magnetic field.

Question 6

What path does a charged particle follow in a uniform magnetic field?

Answer 6

Circular.

Question 6

Why does the particle move in a circle?

Answer 6

Magnetic force provides centripetal force.

Question 7

What is a cyclotron?

Answer 7

A device that accelerates charged particles using electric and magnetic fields.

Question 8

What is the role of the magnetic field in a cyclotron?

Answer 8

To keep particles moving in a circular path.

Question 9

What is the role of the electric field in a cyclotron?

Answer 9

To accelerate the particles across the gap.

Question 10

How does the radius of motion change in a cyclotron?

Answer 10

It increases as speed increases.

Question 11

When is flux linkage maximum?

Answer 11

When the coil is perpendicular to the magnetic field.

Question 12

State Faraday’s Law.

Answer 12

Induced emf is proportional to the rate of change of flux linkage.

Question 13

State Lenz’s Law.

Answer 13

Induced emf acts to oppose the change producing it.

Question 14

What happens when a straight conductor moves through a magnetic field?

Answer 14

An emf is induced across it.

Question 15

Why is an emf induced in a moving conductor?

Answer 15

It cuts magnetic field lines (flux changes).

Question 16

Define peak value.

Answer 16

Maximum value of current or voltage.

Question 17

Define peak-to-peak value.

Answer 17

Twice the peak value.

Question 18

What is mains voltage in the UK (rms)?

Answer 18

230 V

Question 19

What does an oscilloscope measure?

Answer 19

• AC/DC voltage
• Time period
• Frequency

Question 20

How is frequency found from oscilloscope trace?

Answer 20

f = 1/T

Question 21

What type of current is required for a transformer?

Answer 21

Alternating current.

Question 22

In a step-up transformer, which coil has more turns?

Answer 22

Secondary coil.

Question 22

Why must the current be alternating?

Answer 22

To produce a changing magnetic field.

Question 23

In a step-down transformer, which coil has more turns?

Answer 23

Primary coil.

Question 24

What are eddy currents?

Answer 24

Currents induced in the iron core

Question 25

Why are eddy currents a problem?

Answer 25

They cause heating and energy loss

Question 26

How are eddy current losses reduced?

Answer 26

Using a laminated iron core.

Question 27

Why is electrical power transmitted at high voltage?

Answer 27

To reduce current and reduce power loss