6.3 Electromagnatism

Question 1

Magnetic Field

Answer 1

A region where a force will act on a magnetic or charged material

Question 2

What the closeness of field lines represents

Answer 2

The magnetic flux density

Question 3

Direction of a magnetic field around a wire

Answer 3

Use right hand rule

Question 4

A circle with a dot vs cross

Answer 4

dot - out of plane

cross - into plane

Question 5

Solenoid

Answer 5

A long coil of current carrying wire

Question 6

What causes a magnetic field

Answer 6

Moving charges or permanent magnets

Question 7

When a force is induced by a magnetic field

Answer 7

When a current is perpendicular to a uniform field

Question 8

Flemings left hand rule

Answer 8

First finger - field
seCond finger - current
thuMb - motion

Question 9

Causes of a magnetic field (2)

Answer 9

• Moving charges

- Permanent magnets

Question 10

Force on a current carrying conductor

Answer 10

F = BILsinθ

Question 11

Experiment to determine magnetic flux density

Answer 11

Set up a wire between two permanent magnets and place the setup on some digital scales. Set the scales to 0. The circuit should contain a variable resistor and ammeter. Turn on the circuit and a downwards force will be produced causing a reading on the scales, note this down along with the current. Vary the resistance and repeat this. Plot a graph of F(=mg) against I and the gradient will be Bl so divide by l (length of magnets) to obtain the flux density

Question 12

Force on a charged particle in a magnetic field

Answer 12

F = BQv

Question 13

Motion of charged particles inside a uniform magnetic field

Answer 13

They will move in a circular path

Question 14

How to calculate the radius of the circular path of a charged particle in a magnetic field

Answer 14

Equate centripetal force to the force from the magnetic field

Question 15

How velocity selectors work

Answer 15

There is a magnetic field acting in an opposite direction to an electric field and charged particles are projected perpendicular to the direction of the fields. There is a small gap directly ahead and therefore to make it through the gap the force of the magnetic field must equal the force of the electric field.
EQ = BQv
E = Bv
v = E/B
Therefore only particles with a velocity of E/B will make it through

Question 16

Magnetic flux

Answer 16

Magnetic flux density * perpendicular area

Question 17

Magnetic flux linkage

Answer 17

Magnetic flux * number of coils

Question 18

Faraday’s law

Answer 18

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

Question 19

Gradient of a flux linkage - time graph

Answer 19

negative e.m.f.

Question 20

Area under a e.m.f. - time graph

Answer 20

negative change in flux linkage

Question 21

Lenz’s law

Answer 21

Induced e.m.f. will be in a direction to oppose the change that caused it

Question 22

So whats the big idea with why you can move straight wires in a field and get emf but when you move a coil you dont

Answer 22

Perhaps because its round so the emf would go both ways and cancel itself out who knows

Question 23

Experiment to investigate link between e.m.f. and flux linkage

Answer 23

Place a search coil between two magnets. The search coil should be connected to a data recorder which will record the induced e.m.f. with a very small time interval. Move the coil out of the field and look at the graph. The area under the e.m.f-time graph should be equal to the magnetic flux linkage. So you can then find B from this (youll need to measure area and number of coils e.t.c.)

Question 24

Components of an A.C. generator (4)

Answer 24

• Magnets
• Coil
• Slip rings
• Brushes

Question 25

How an A.C. generator works

Answer 25

A coil is forced to rotate inside a magnetic field. The relative change in flux linkage induces an e.m.f.

Question 26

How transformers work

Answer 26

The alternating current in the primary coil causes the magnetic field in the iron core to change. This change in the magnetic field induces an e.m.f. in the secondary coil.

Question 27

Experiment to investigate transformer

Answer 27

Set up a transformer with a voltmeter on both coils. Run a current through the primary coil and record both voltages, repeat for different currents. n1/n2 should equal v1/v2