6.3 Electromagnetism (Magnetic Fields and Forces)

Question 1

What is a magnetic field?

Answer 1

Region of space in which moving charged particles are subject to a magnetic force.

Question 2

What kind of field does moving charges create?

Answer 2

A magnetic field.

Question 3

What results in a magnetic force?

Answer 3

The interaction of two magnetic fields.

Question 4

A current carrying wire will generate a circular magnetic field. Why?

Answer 4

Magnetic fields are created by moving charges. In this case, the moving charges are the electrons flowing through the wire.

Question 5

How strong is the field produced by a current carrying wire?

Answer 5

Strong enough to deflect a small magnet (eg a compass needle)

Question 6

What is a simple way of demonstrating the magnetic field produced by a current carrying wire?

Answer 6

Move a compass around the wire. If the current and resulting magnetic field are large enough, the compass will be seen to point in the direction of the field.

Question 7

Permanent magnet

Answer 7

An object made from a magnetized material that creates its own persistent magnetic field.

Question 8

How is the magnetic field produced in a permanent magnet?

Answer 8

The electrons in the material move in an ordered way to produce an overall magnetic field around the material.

Question 9

How can we demonstrate the magnetic field of bar magnets?

Answer 9

Using iron filings. An induced magnetic field is created in the iron. The induced north pole points in the direction of the magnetic field.

Question 10

What is a solenoid?

Answer 10

An electromagnet, made from a coil of wire. It behaves as a bar magnet when an electric current passes through it.

Question 11

What does increasing the number of turns in a solenoid (or coil of wire) do to its magnetic field?

Answer 11

It enhances the magnetic field produced by the coil.

Question 12

In which direction do magnetic field lines point?

Answer 12

From the north pole to the south pole.

Question 13

How are the terms magnetic flux density and magnetic field strength different?

Answer 13

They aren’t. Magnetic Flux Density and Magnetic Field Strength are interchangeable terms.

Question 14

What kind of magnet produces a uniform magnetic field?

Answer 14

Horseshoe magnet.

Question 15

Describe the field lines of a current carrying wire.

Answer 15

Concentric circles centred on the wire.

Question 16

How can we work out the direction of the field lines around a current carrying wire?

Answer 16

The right hand grip rule.
Thumb = Current
Fingers = Field Direction

Question 17

How does the right hand grip rule apply to a solenoid?

Answer 17

Thumb = Direction of field INSIDE solenoid. Points to north pole
Fingers = Current.

Question 18

What does a dot inside a circle represent?
What about an X inside a circle?

Answer 18

Dot: Current or a field line coming out of the paper
X: Current or field line going into the paper.

Question 19

A current carrying wire is placed into a magnetic field. It experiences a force. Why?

Answer 19

The wire’s magnetic field interacts with the external field.

Question 20

A current carrying wire is placed into a magnetic field. It experiences a force. What can be used to determine the direction of the force?

Answer 20

Flemings Left hand rule
- Thumb: Force
- Index: Field
- Middle: Current

Question 21

A current carrying wire is placed in a magnetic field, at an angle. At what angle would the force exerted on the wire be greatest?

Answer 21

90 degrees. This means the current is perpendicular to the magnetic field.

Question 22

Define Magnetic Flux Density

Answer 22

Force per unit current per unit length on a current-carrying wire perpendicular to the magnetic field lines.

Question 23

Magnetic flux density is magnetic flux per unit area. Magnetic flux has the units Weber (Wb). What are the units for magnetic flux density? (aside from the tesla)

Answer 23

Wbm-2

Question 24

Define 1 Tesla

Answer 24

The magnetic flux density required to generate a force of 1N on a wire carrying a current of 1A per metre perpendicular to the magnetic field.

Question 25

Experimental Technique: Determining B

Answer 25

1) Place a horseshoe magnet on a set of digical balance scales. Zero the scales.
2) Measure the length of a rigid piece of straight wire
3) Connect the wire to a DC power supply and an ammeter in series.
4) Align the wire so that the force produced is upwards using flemings left hand rule. This ensures the force on the magnet is downwards by newtons third law.
5) Switch on the supply and adjust the voltage so that the current is 6A. Record the mass on the balance.
6) Repeat the steps for different current values (eg 5.8,5.6,..,4)
7) Weight = Magnetic Force, so mg = BIL.
8) Plot a graph of mass against current. Find the gradient.

Question 26

How can we find the force acting on a charged particle travelling perpendicular to a uniform magnetic field?

Answer 26

Using F = BQv

(from F = BIL, I = Q/t, v = L/t)

Question 27

How does the magnetic force on a charge change in a magnetic field, depending on whether the charge is positive or negative?

Answer 27

Negatively charged particles have velocity in the opposite direction to the current, and so the force is negative. We can see this with Fleming’s left hand rule (index = conventional current)

Question 28

For a free particle in a uniform magnetic field, what is the force perpendicular to?

Answer 28

Its velocity, and also the field.

Question 29

Does the speed of a free particle in a uniform magnetic field change? Why?

Answer 29

No. No work has been done on the particle.

Question 30

A particle enters a uniform magnetic field. It travels in a circle. How can we find the radius of this circle?

Answer 30

Equate the force acting on the particle due to the magnetic field with the formula for centripetal force.

F = mv2 / r
F = BQv
mv2 / r = BQv
r = mv/BQ

Question 31

What do velocity selectors do?

Answer 31

They isolate particles with a specific velocity, by using magnetic and electric fields.

Question 32

How does a velocity selector work?

Answer 32

A uniform electric field acts in the opposite direction to the magnetic field.
This is done by positioning two parallel plates with a voltage across them, at right angles to a uniform magnetic field from a horseshoe magnet.

Question 33

What equation allows us to determine the required velocity for a particle to travel in a straight line in a velocity selector?

Answer 33

F = EQ
F = BQv
EQ = BQv
E = Bv
v = E/B

Question 34

What happens to particles that do not travel at the required velocity in a velocity selector?

Answer 34

They curve and collide with the plates.