6.3 Electromagnatism Flashcards Preview

OCR A Physics A Level > 6.3 Electromagnatism > Flashcards

Flashcards in 6.3 Electromagnatism Deck (27):
1

Magnetic Field

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

2

What the closeness of field lines represents

The magnetic flux density

3

Direction of a magnetic field around a wire

Use right hand rule

4

A circle with a dot vs cross

dot - out of plane
cross - into plane

5

Solenoid

A long coil of current carrying wire

6

What causes a magnetic field

Moving charges or permanent magnets

7

When a force is induced by a magnetic field

When a current is perpendicular to a uniform field

8

Flemings left hand rule

First finger - field
seCond finger - current
thuMb - motion

9

Causes of a magnetic field (2)

- Moving charges
- Permanent magnets

10

Force on a current carrying conductor

F = BILsinθ

11

Experiment to determine magnetic flux density

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

12

Force on a charged particle in a magnetic field

F = BQv

13

Motion of charged particles inside a uniform magnetic field

They will move in a circular path

14

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

Equate centripetal force to the force from the magnetic field

15

How velocity selectors work

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

16

Magnetic flux

Magnetic flux density * perpendicular area

17

Magnetic flux linkage

Magnetic flux * number of coils

18

Faraday's law

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

19

Gradient of a flux linkage - time graph

negative e.m.f.

20

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

negative change in flux linkage

21

Lenz's law

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

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

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

23

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

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.)

24

Components of an A.C. generator (4)

- Magnets
- Coil
- Slip rings
- Brushes

25

How an A.C. generator works

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

26

How transformers work

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.

27

Experiment to investigate transformer

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