6.3 Electromagnetism

Question 1

Define Magnetic Field

Answer 1

A region of space in which moving charged
particles are subject to a magnetic force.
This force is caused by the interaction of two
Magnetic Fields (there is a field around the moving
charged particles which interacts with the existing
Magnetic Field they are passing through)

Question 2

Define Magnetic Field Line

Answer 2

The path which a north pole would take when
placed in a Magnetic Field.
Field lines go from north to south.

Question 3

How can you map field lines around a
magnet?

Answer 3

You can place iron filings on a piece of paper and then
put the magnet on the paper and the filings will align to
the field.
You can also use a plotting compass and place it in
various positions around the magnet, mark the direction
of the needle at each point and connect them.

Question 4

How do you represent the strength of a
Magnetic Field on a diagram?

Answer 4

It is represented by how close together the field
lines are – the closer they are, the stronger the
field. (It is the density of the field lines, which is
why magnetic flux density and Magnetic Field
strength are interchangable).

Question 5

Define Magnetic Flux Density

Answer 5

The force per unit current per unit length on
a current-carrying conductor placed in a
Magnetic Field perpendicular to the field
lines. (Magnetic flux per unit area).

Question 6

What is the unit of Magnetic Flux
Density?

Answer 6

1 T = 1 N m-1 A-1

Question 7

Why does a compass point to the North
Pole of the Earth?

Answer 7

The Earth’s geographic north pole is actually the magnetic
south pole, so the north pole of the compass magnet (the
needle) lines up with the Earth’s field and points to the
magnetic south (field lines go
from north to south), which is
what we call the geographic
north.

Question 8

How do you work out the shape of the
field around a current-carrying wire?

Answer 8

The right-hand thumb rule: take your right hand
and make a thumbs-up shape. Point your thumb in
the direction of the (conventional) current and the
field goes around the wire in the direction of your
fingers (from palm to tip).

Question 9

How do you work out the shape of the
field around a solenoid?

Answer 9

Where the current is going anticlockwise around the coil is the north
pole. At the south pole, the current goes clockwise. The shape of the
field is then similar to a bar magnet. A good way to remember it is by
the shapes of the letters:

Question 10

Define the motor effect

Answer 10

When a current-carrying conductor is placed within
a Magnetic Field, it experiences a force
perpendicular to the flow of current and the field
lines which pushes it out of the field.

Question 11

How can you predict which direction the
force will push the conductor?
(motor effect)

Answer 11

Using Fleming’s left-hand rule:
* First finger: Field lines
* Second finger: Current
(conventional)
* Thumb: Motion

Question 12

Give the formula relating magnetic force,
flux density, current, length and angle
between the field and the conductor

Answer 12

F = BILsinθ
F = Magnetic force (N)
B = Magnetic flux density (T)
I = Current in the conductor (A)
L = Length of conductor in the field (m)
θ = Angle between the field lines and the conductor
(º or rad)

Question 13

Describe an experiment to measure flux
density

Answer 13

1. Place a horseshoe magnet on a digital balance and zero it
2. Connect rigid piece of straight wire to DC supply, variable resistor and
   ammeter (in series)
3. Align the wire so the force on it acts upwards (so there will be a
   downward force on the magnet – Newton’s 3rd law)
4. Measure the length of the wire in the field
5. Record extra mass on the balance and use this to calculate force (F =
   mg)
6. Plot a graph of current against mass – gradient gives BL/g
   a. Since L and g are both known, B can be calculated

Question 14

What is the formula for magnetic force
on a moving charge at 90º to the field
lines?

Answer 14

F = BQv
F = Force (N)
B = Magnetic flux density (T)
Q = Charge of particle (C)
v = Velocity of particle (ms-1)

Question 15

How is F = BQv derived?

Answer 15

From F = BIL (for magnetic force on a conductor at
90º to field lines).
Use I = Q/t and L = vt (distance = speed x time).
F = BQvt/t
The t cancels out, leaving F = BQv

Question 16

Why do charged particles move in a
circular orbit in a Magnetic Field?

Answer 16

Force is always perpendicular to the velocity of the
particle, so they end up being forced in a circular
orbit. The particles undergo centripetal
acceleration, with the centripetal force being the
magnetic force

Question 17

How can you derive the formula for the
radius of the circular orbit?

Answer 17

Equating the formula for centripetal force and the formula for
magnetic force (since they are the same thing in this context),
you get mv2
/r = BQv.
Rearrange this and you get:
r = mv / BQ

Question 18

Using r = mv / BQ, explain how changing
the mass, velocity, flux density and
charge affects the radius of the orbit

Answer 18

Increasing mass or velocity will increase the
radius.
Increasing flux density or charge will decrease the
radius.

Question 19

What is the purpose of a velocity
selector?

Answer 19

They isolate particles of a specific velocity. This is
useful for things like mass spectrometry.

Question 20

How does a velocity selector work?

Answer 20

There are electric plates above and below so the electric force acts
upwards, and there is a Magnetic Field passing through sideways so the
magnetic force acts downwards. In order for the particles to pass
through undeflected, the electric and Magnetic Fields must be balanced
so BQv = EQ.
From this, you can derive v = E/B.
If the velocity is too fast or too slow,
the particle will be deflected and
not pass through

Question 21

Define Magnetic Flux

Answer 21

The product of the magnetic flux density and the
area perpendicular to the field lines. Magnetic flux
is represented by the Greek letter Phi, ɸ

Question 22

What is the unit for Magnetic Flux?

Answer 22

Weber (Wb), where 1Wb = 1Tm2

Question 23

What is the formula for Magnetic Flux?

Answer 23

ɸ = BAcosθ
ɸ = Magnetic flux (Wb)
B = Magnetic flux density (T)
A = Area perpendicular to the field (in a coil this is the
cross-sectional area) (m2
)
θ = angle between the normal to the coil and the field
lines (º or rad)

Question 24

Define Magnetic Flux linkage

Answer 24

The magnetic flux of an entire coil of wire. This is
the product of the magnetic flux and the number of
turns on the coil.
Flux linkage is also measured in Wb, and it is
represented as Nɸ.

Question 25

State Lenz’s Law

Answer 25

Induced emf is always in a direction so as to
oppose the change that caused it.

Question 26

Explain Lenz’s Law in terms of energy

Answer 26

Lenz’s law follows the principle of the conservation of
energy. If the induced emf was in a direction that aided the
change which caused it, it would be creating electrical
energy from nowhere.
For example, if the north pole of a bar magnet was pushed
into a solenoid and that end became a south pole, it would
then pull the magnet into the coil faster and field would get
stronger, pulling the magnet in faster still, etc.

Question 27

Explain Lenz’s Law in terms of energy

Answer 27

Lenz’s law follows the principle of the conservation of
energy. If the induced emf was in a direction that aided the
change which caused it, it would be creating electrical
energy from nowhere.
For example, if the north pole of a bar magnet was pushed
into a solenoid and that end became a south pole, it would
then pull the magnet into the coil faster and field would get
stronger, pulling the magnet in faster still, etc.

Question 28

State Faraday’s Law

Answer 28

The induced emf in a circuit is proportional to the
rate of change of flux linkage throughout the
circuit.

Question 29

What is the formula that links Faraday’s
Law and Lenz’s Law?

Answer 29

emf = - delta Nɸ/delta t

Question 30

What is a search coil?

Answer 30

A flat coil of insulated wire connected to a
galvanometer (a sensitive ammeter). It can be
used to determine magnetic flux density from the
current induced in the coil when it is withdrawn
from a Magnetic Field.

Question 31

How can you measure magnetic flux
using a search coil?

Answer 31

1. Place the coil in a Magnetic Field of known strength and pull
   it out again. Since Imax
   ∝ B, you can calculate the constant
   of proportionality, k (therefore calibrating the coil).
2. Place the coil in the field that is to be measured and
   withdraw it. Use the value for k and the current to calculate
   the flux density.
3. Calculate magnetic flux from the flux density using
   ɸ = BAcosθ

Question 32

What is the structure of a simple A.C.
generator?

Answer 32

A rectangular coil
which spins in a
uniform Magnetic
Field.

Question 33

How does a simple A.C. generator work?

Answer 33

The flux linkage in the coil changes continuously,
inducing an alternating current in the coil.
(A and θ change as the coil turns and
Nɸ = BANcosθ)

Question 34

How can the peak emf of an A.C.
generator be increased?

Answer 34

1. Increase the speed of rotation
2. Increase the magnetic flux density of the field
3. Increase the cross-sectional area of the coil
4. Increase the number of turns on the coil

Question 35

What is the purpose of a transformer?

Answer 35

Transformers change the peak value of an
alternating PD to a different value. Step-up
transformers increase it, step-down transformers
decrease it.

Question 36

Describe the structure of a simple
transformer

Answer 36

Two coils – primary coil and secondary coil –
wrapped around the two sides of a laminated iron
ring. For a step-up, there are more turns on the
secondary coil.
For a step-down, there are
more turns on the primary
coil

Question 37

How does a transformer work?

Answer 37

An alternating current is run through the primary
coil, which induces an alternating Magnetic Field in
the iron core. This, in turn, induces an alternating
emf in the secondary coil.

Question 38

Give the formula that relates the number
of turns with the potential difference of
each coil

Answer 38

V (s) / V (p)
= N (s) / N (p)

Question 39

For an ideal transformer (100% efficient),
give the formula relating potential
difference and current in both coils

Answer 39

If efficiency = 100%, power in the primary coil =
power in the secondary coil.
)

Question 40

What role do transformers play in the
National Grid?

Answer 40

Step-up transformers are used to increase the
voltage (and decrease current) before the
electricity travels long distances. This is to reduce
energy lost as heat due to resistance in the wires
as the electricity passes through.

Question 41

Describe an experiment to investigate
the relationship Vs
/ V
p
= N
s
/ N
p

Answer 41

Vary the number of turns on each of the coils and
measure the peak potential difference of each one every
time. Then use the values to prove the relationship
between potential difference and number of turns. It is
better to use an oscilloscope than a voltmeter for an
alternating pd since it is easier to see the peak value.

Question 42

How can you investigate the efficiency of
a transformer?

Answer 42

Measure the current in each coil with an ammeter
and a variable resistor. The variable resistor is used
to vary the current at a constant pd.
Use the formula Pout / Pin x100% (or Ip
V
p
/ I
s
V
s
x100%) to calculate the efficiency of the transformer.