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

Tesla (T)
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 7

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

Answer 7

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 8

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

Answer 8

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 9

What does the Magnetic Field around a flat, circular coil look like

Answer 9

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/ga. Since L and g are both known, B can be calculated

Question 14

What does the previous experiment setup look like?

Answer 14

Question 15

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

Answer 15

F = BQv

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

Question 16

How is F = BQv derived?

Answer 16

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 17

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

Answer 17

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 18

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

Answer 18

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 19

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

Answer 19

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

Question 20

What is the purpose of a velocity selector?

Answer 20

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

Question 21

How does a velocity selector work?

Answer 21

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 22

Define Magnetic Flux

Answer 22

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 23

What is the unit for Magnetic Flux?

Answer 23

Weber (Wb), where 1Wb = 1Tm²

Question 24

What is the formula for Magnetic Flux?

Answer 24

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 25

Define Magnetic Flux linkage

Answer 25

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 26

State Lenz’s Law

Answer 26

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

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

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 do emf and flux linkage vary in a simple A.C. generator? (graphs)

Answer 34

EMF is ±maximum when the coil is parallel to the field lines, whereas Nϕ is ±maximum when the coil is 90º to the field lines.

Question 35

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

Answer 35

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 36

What is the purpose of a transformer?

Answer 36

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

Question 37

Describe the structure of a simple transformer

Answer 37

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 38

How does a transformer work?

Answer 38

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 39

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

Answer 39

Vₛ/ Vₚ = Nₛ/Nₚ

V = potential difference
N = number of turns
ₚ = primary coil
ₛ = secondary coil

Question 40

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

Answer 40

If efficiency = 100%, power in the primary coil = power in the secondary coil. This means IₚVₚ = IₛVₛ. Rearranging this gives Iₚ/Iₛ = Vₛ/Vₚ (which is also the same as Nₛ/Nₚ)

Question 41

What role do transformers play in the National Grid?

Answer 41

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 42

Describe an experiment to investigate the relationship Vₛ/ Vₚ = Nₛ/Nₚ

Answer 42

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 43

How can you investigate the efficiency of a transformer?

Answer 43

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 IₚVₚ/IₛVₛ x100%) to calculate the efficiency of the transformer