Chapter 6.3 - Electromagnetism Flashcards Preview

Physics OCR A A2 > Chapter 6.3 - Electromagnetism > Flashcards

Flashcards in Chapter 6.3 - Electromagnetism Deck (30)
Loading flashcards...

Magnetic Field

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


What the closeness of field lines represents

The magnetic flux density


Direction of a magnetic field around a wire

Use right hand rule


A circle with a dot vs cross

dot - out of plane
cross - into plane



A long coil of current carrying wire


What causes a magnetic field

Moving charges or permanent magnets


Draw the field lines for a wire



Draw the field lines for a flat coil

well done


Draw the field lines for a long solenoid



When a force is induced by a magnetic field

When a current is perpendicular to a uniform field


Flemings left hand rule

First finger - field
seCond finger - current
thuMb - motion


Causes of a magnetic field (2)

- Moving charges
- Permanent magnets


Force on a current carrying conductor

F = BILsinθ


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


Force on a charged particle in a magnetic field

F = BQv


Motion of charged particles inside a uniform magnetic field

They will move in a circular path


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


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


Magnetic flux

Magnetic flux density * perpendicular area


Magnetic flux linkage

Magnetic flux * number of coils


Faraday's law

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


Gradient of a flux linkage - time graph

negative e.m.f.


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

negative change in flux linkage


Lenz's law

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


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


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.)


Components of an A.C. generator (4)

- Magnets
- Coil
- Slip rings
- Brushes


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.


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.


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