Magnetic field- a region where a force is exerted on magnetic materials. Represented by field lines going from north to south. The closer the lines, the stronger the field
If you put a current-carrying wire into a magnetic field, the fields will interact. The field lines from the magnet shorten and straighten to move to a lower energy level, creating a force to catapult the wire out of the field
The direction of the force is perpendicular to both the current direction and the magnetic field
Magnetic field strength- the force on one metre of wire carrying a current of one amp at right angles to the magnetic field. Also called flux density. Measured in teslas, T
If a current-carrying loop is placed in a magnetic field, the forces on the side arms will make it rotate. By using a split-ring commutator, the current can be reversed each time the loop becomes vertical (half-turn)
This allows the loop to rotate steadily- becomes a motor.
Induction motors operate by altering the magnetic field around a coil of wire that is free to move, which induces a current in the wire, causing it to rotate
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Electromagnetic Induction
Magnetic flux density- strength of magnetic field per unit area
When you move a coil in a magnetic field, the size of the emf induced depends on the magnetic flux passing through the coil, and the number of turns
If a conducting rod moves through a magnetic field, its electrons will experience a force and will accumulate on one end of the rod. This induces e.m.f. (electromotive force) across the ends.
E.m.f. is induced wherever there is relative motion between a conductor and a magnet. It is produced whenever lines of flux are cut.
Flux cutting always induces e.m.f. but will only induce current if the circuit is completed
Faraday's Law- The induced emf is directly proportional to the rate of change of flux linkage
Lenz's Law- The induced emf is always in such a direction as to oppose the change that caused it.
This agrees with the conservation of energy
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Transformers and Alternators 1
Transformers- make use of electromagnetic induction to change the size of voltage for an alternating current. An alternating current in the primary coil produces flux.
The magnetic field is passed through the iron core to the secondary coil, inducing a voltage of the same frequency.
Step-up transformers increase voltage by the secondary coil having more turns, and step-down transformers decrease voltage by having more turns on the primary coil
Permeance- the amount of flux induced in a material for a given number of turns that surround it. The higher the permeance, the greater the amount of flux induced
Permeance (like conductance) is inversely proportional to the length, and proportional to cross-sectional area
A good transformer has high permeance, so is short, fat and made from a high permeability material eg. iron
You also want the conductance of the copper coils to be as high as possible to limit energy loss. Need right number of turns with shortest piece of wire possible
Even though there is an air 'gap' in the iron core, flux still flows. As air has a very low permeability compared to iron, so the amount of flux in the circuit would be dramatically lower than without the air gap
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Transformers and Alternators 2
Transformers are not 100% inefficient. There are small losses of power, mostly in the form of heat
Heat is produced by eddy currents in the iron core- current induced by the changing magnetic flux in the core. The effect is reduced by laminating the core with layers of insulation
Heat is also generated by resistance in the coils- minimised by using thick copper wire with a low resistance
Electricity from power stations is sent round the national grid at a low current, as loses due to resistance in the cables are proportional to I2. Low current = high voltage
Generators- convert KE into electrical energy, They induce an electric current by rotating a coil in a magnetic field
An alternator looks like a motor, but with slip rings and brushes instead of a split-ring commutator. The output voltage and current change direction with every half-turn, producing alternating current
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