Chapter 6.3 - Electromagnetism

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

Draw the field lines for a wire

Answer 7

gj

Question 8

Draw the field lines for a flat coil

Answer 8

well done

Question 9

Draw the field lines for a long solenoid

Answer 9

congrats

Question 10

When a force is induced by a magnetic field

Answer 10

When a current is perpendicular to a uniform field

Question 11

Flemings left hand rule

Answer 11

First finger - field
seCond finger - current
thuMb - motion

Question 12

Causes of a magnetic field (2)

Answer 12

• Moving charges

- Permanent magnets

Question 13

Force on a current carrying conductor

Answer 13

F = BILsinθ

Question 14

Experiment to determine magnetic flux density

Answer 14

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 15

Force on a charged particle in a magnetic field

Answer 15

F = BQv

Question 16

Motion of charged particles inside a uniform magnetic field

Answer 16

They will move in a circular path

Question 17

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

Answer 17

Equate centripetal force to the force from the magnetic field

Question 18

How velocity selectors work

Answer 18

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 19

Magnetic flux

Answer 19

Magnetic flux density * perpendicular area

Question 20

Magnetic flux linkage

Answer 20

Magnetic flux * number of coils

Question 21

Faraday’s law

Answer 21

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

Question 22

Gradient of a flux linkage - time graph

Answer 22

negative e.m.f.

Question 23

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

Answer 23

negative change in flux linkage

Question 24

Lenz’s law

Answer 24

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

Question 25

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 25

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

Question 26

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

Answer 26

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 27

Components of an A.C. generator (4)

Answer 27

• Magnets
• Coil
• Slip rings
• Brushes

Question 28

How an A.C. generator works

Answer 28

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

Question 29

How transformers work

Answer 29

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 30

Experiment to investigate transformer

Answer 30

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