7.5 Magnetic fields

Question 1

What happens when a current passes through a wire

Answer 1

a magnetic field is induced

Question 2

Define tesla

Answer 2

force of 1 N on 1 metre of wire carrying 1A of current perpendicular to a magnetic field

Question 3

magnetic flux density (B)

Answer 3

a measure of the strength of the field, T

Question 4

Fleming’s left hand rule for motors
represents what properties on what
fingers?

Answer 4

Thumb - Thrust/Force
First finger - Field (Magnetic)
Second finger - Current

Question 5

A charged particle moving through a field feels a
force when it is traveling along the field lines or
perpendicular to them?

Answer 5

Perpendicular to the field.

Question 6

Is the force applied to the particles applied perpendicular
to the particles motion or in one direction?

Answer 6

Perpendicular to its motion, causing it to
move in a circular motion.

Question 7

Which fields do cyclotrons use?
A. Electric field
B. Magnetic field
C. Gravitational field
D. Both Electric and Magnetic

Answer 7

D. An electric field and a magnetic field

Question 8

What are the uses for cyclotrons

Answer 8

producing ion beams for
radiotherapy, and radioactive tracers.

Question 9

What is a cyclotron

Answer 9

a type of particle
accelerator

Question 10

How does a Cyclotron work and what’s
the electric and magnetic fields purpose
in a cyclotron?

Answer 10

• A cyclotron is made up of 2 semicircular electrodes called “Dees” with a magnetic field applied perpendicular to the Dees and an alternating pd applied between the Dees.
• Each Dee is a metal electrodes with opposite charges, this creates an electric field in the gap between the two Dees. This is what accelerates the particles.
• The magnetic field causes the particles to move in a circular motion, which allows it to gain speed whilst minimising space. As they speed up the radius of their motion increases until it breaks free tangential to one of the Dees.

Question 11

What is flux linkage?

Answer 11

NΦ = The number of turns cutting the flux at one time

Question 12

What is Faraday’s law?

Answer 12

induced emf is directly proportional to the rate of change flux linkage

Question 13

What is Lenz’s law?

Answer 13

induced emf is always in such a direction as to oppose the change that caused it

Question 14

What would be the emf produced when a rotating coil is at a constant rate in a magnetic field?

Answer 14

ε = BANω sin(ωt)

Question 15

What happens when you move a straight conductor through a magnetic field?

Answer 15

The electrons experience a force pushing them to one end of the conductor creating an emf across the conductor. The rod obeys Faraday’s law, it is changing flux as it moves through the field hence an emf is induced.

Question 16

Describe how to use oscilloscopes

Answer 16

Oscilloscopes are used to displace AC waves, the x axis is
called the time base and shows how long it takes the wave to
move 1 division and the y axis shows how much PD is
needed to move the wave up 1 division. Using this we find the
peak voltage, time period and frequency.

Question 17

How does a transformer work?

Answer 17

A primary coil wrapped around an iron core with
an alternating p.d. creates an alternating
magnetic field, this magnetic field induces an
EMF in a secondary coil also wrapped around
the core. This creates a current in the secondary
coil.

Question 18

What kind of current is produced by a transformer
and why?

Answer 18

An alternating current.

An EMF is induced by a changing magnetic
field, hence the EMF induced is alternating
producing an alternating current.

Question 19

Step-up transformer

Answer 19

increases the input voltage by having more turns on the secondary
coil than the primary

Question 20

Step-down transformer

Answer 20

decreases the input voltage by having less turns on the
secondary coil.

Question 21

What are one of the main causes of energy loss in a transformer?

Answer 21

the production of eddy currents

Question 22

What are the eddy currents?

Answer 22

• When the changing flux passes through the iron core. Electrons in the iron core respond and induce an emf.
• As the core is a circuit, a current is generated in the iron core. The magnetic field generated by the current will oppose the core as per Lenz’s law

Question 23

How can eddy currents be reduced?

Answer 23

by using a laminated iron core, meaning that the core is made using layers of iron between layers of an insulator, because the eddy currents cannot pass through the insulator and so their amplitude is reduced.

Question 24

How else is energy lost and how can it be reduced?

Answer 24

• due to resistance in the coils which causes heating and if the core is
  not easily magnetised
• using thick wire, which has a low resistance.
  -using a magnetically soft iron core to allow easy magnetisation and
  demagnetisation.

Question 25

Why is it vital to reduce the current to a minimum value to prevent unnecessary energy losses?

Answer 25

When transferring electrical power, the power lost due to resistance is equal to I²R

Question 26

Why are transformers used?

Answer 26

By changing the number of coils, the transformers can be used to increase the voltage and reduce current when transporting power, with minimal energy loses. The voltage is then dropped again locally to ensure safe usage in households.

Question 27

Why are eddy currents a problem?

Answer 27

By Lenz’s law the emf created and its field opposes that of the primary coil. This causes energy loss via resistive heating of the iron core by the eddy currents, which reduces efficiency.