6.3 Electromagnatism 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
Solenoid
A long coil of current carrying wire
What causes a magnetic field
Moving charges or permanent magnets
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
