Magnetic Fields and Electromagnetism

Question 1

What is a magnetic field?

Answer 1

A region of space which will cause a magnetic pole to experience a force

Question 2

What direction does the magnetic field travel?

Answer 2

North to south

Question 3

What are magnetic field lines called?

Answer 3

Magnetic flux

Question 4

What is the strength of the magnetic field called?

Answer 4

Magnetic flux density

Question 5

What is the unit for magnetic flux density?

Answer 5

Tesla

Question 6

If the magnetic field lines are close together, is the field weak or strong?

Answer 6

Strong

Question 7

What is the unit for magnetic flux?

Answer 7

Weber

Question 8

What is the motor effect?

Answer 8

If a wire is placed in a magnetic field and a current is paced through it, the wire will feel a force

Question 9

When is the motor effect at its greatest?

Answer 9

When the wire and the field are perpendicular

Question 10

Do electric currents passing through a wire cause a magnetic field?

Answer 10

Yes, the magnetic field is produced around the wire

Question 11

Is magnetic flux density a scalar or vector quantity?

Answer 11

Vector

Question 12

Why does an electron moving in a magnetic field travel in a circle?

Answer 12

The force is perpendicular to the magnetic field and the direction in which the electron is moving. Acts as a centripetal force

Question 13

What is Faraday’s law?

Answer 13

The induced e.m.f is proportional to the rate of change of flux linkage

Question 14

What is Lenz’s law?

Answer 14

The direction of an induced e.m.f is such as to oppose the change creating it

Question 15

What can Flemming’s right-hand rule be used for?

Answer 15

To find the direction of the induced e.m.f

Question 16

Explain what electromagnetic induction is

Answer 16

If you move a magnet near a wire, the electrons in that wire will feel a force tending them to move through the wire. This is an e.m.f

Question 17

What is a solenoid?

Answer 17

A long spring-like coil

Question 18

What do transformers do?

Answer 18

Increase or decrease the voltage of A.C power supplies

Question 19

Will a transformer work with a D.C supply? Why?

Answer 19

No. Because a D.C will not produce an alternating magnetic field

Question 20

What is the transformer equation for an ideal transformer?

Answer 20

Vs/Vp = Ns/Np

Question 21

How can you show a transformer is 100% efficient?

Answer 21

IsVs = IpVp

Question 22

If there are more turns on the secondary coil, will it be a step-up or step-down transformer?

Answer 22

Step-up

Question 23

Why does the motor turn when a current is passed through it?

Answer 23

The current travels in opposite directions on opposite sides of the coil. This causes a force in opposite directions to twist the coil (Flemmings left hand rule)

Question 24

Why is it necessary for the molecules to be ionised in a mass-spectrometer?

Answer 24

Only charged particles can be accelerated to produce a beam

Question 25

State the condition under which this equation applies. F=BIL

Answer 25

The magnetic field must be at right angles to the current

Question 26

When will induced e.m.f be a maximum? (2)

Answer 26

When the coil is parallel to the M field

Greatest rate of change of flux is when the angle = 90

Question 27

2 electric cables connect to a p.d and there is a coil. How does this work? (4)

Answer 27

Cables produce an alternating M field
Coil rotates, cuts the M field
Induces e.m.f
Induces current

Question 28

What is a neutral point?

Answer 28

Where the fields from 2 magnets cancel out and the resultant field strength is zero

Question 29

What is magnetic flux density?

Answer 29

The quantity of flux passing through a unit area, at each point in the field

Question 30

How can you increase the size of the induced e.m.f across a coil? (4)

Answer 30

Use a stronger magnet
Use a coil with more turns
Use a coil with a greater area
Move the magnet faster

Question 31

What is Lenz’s law a result of?

Answer 31

Conservation of energy

Question 32

What is an advantage and disadvantage of using electro magnetics rather than permanent magnetics for building a motor?

Answer 32

Advantage: the motor power can be altered for different applications
Disadvantage: it is more complex

Question 33

When will Flemings right hand rule be used?

Answer 33

To find the direction of an induced e.m.f

Question 34

Explain the shape of the graph of a magnet falling through a coil (5)

Answer 34

The e.m.f increases to a level then decreases
When the middle of the magnet falls though the middle of the coil no lines of flux is being cut -> zero e.m.f
Second peak is greater due to the magnetic accelerating
The areas of each peak are equal
First peak has longer time period

Question 35

When a magnet falls through a coil, when does maximum e.m.f occur? Why?

Answer 35

When the magnetic leaves the coil, because the magnetic accelerates as it falls -> fastest rate of flux being cut

Question 36

Explain why a coil turns and why it continues to rotate?

Answer 36

Vertical forces on each side of the coil
Force on one side acts up, rotates clockwise
Force on other side acts down, rotates anti-clockwise
Split ring commutator switches current direction

Question 37

Explain the role of a magnetic field in a cyclotron

Answer 37

Force on particles is at right angles to the motion

Force acts as a centripetal, keeps it in circular motion