Qualification > Reference Material
Physics P3 27/5/2010
J.Darren:
Lenz's law
- If a bar magnet is pushed into a coil, a current is induced in the coil.
- If a bar magnet is pulled out of a coil, a current is induced in the coil.
According to Lenz's law, an induced current flows in a direction such that it opposes the charge which produced it.
The Lenz's law is an example of the law of conservation of enerrgy as energy is spent when a current flows around the circuit, energy must be spent to induce the current in the first place. The energy spent in this case would be the energy required to overcome the opposing force of the magnetic pole of the coil against the bar magnet which has been pushed in.
My way of remembering Lenz's law : When you push a bar magnet into the coil, the current runs away from the magnet. However when the bar magnet is being pulled out of the coil, the current does not want the magnet to go and chases after it.
The direction of the magnetic poles can be predicted using the right hand grip rule. The direction which the thumb is pointing is the North pole, whereas the direction of your fingers represents the direction of the current.
J.Darren:
AC Generator
- Alternating current (AC) is generated (Obviously !).
- The coil is made of insulated copper wire.
- The coil is rotated by turning the shaft.
- A device called slip-rings is fixed to the coil and rotates with it. The carbon brushes keeps the coil connected to the current by rubbing against the slip rings.
- When the coil is rotated, it cuts magnetic field lines and generates emf. This makes the current flow.
- As the coil rotates, each side travels upwards and downwards. Similarly, the current flows forwards and backwards. In order words, the current is AC.
- The turning effect on the coil increases if a stronger magnet is used
- The turning effect on the coil increases if the area of the coil is increased
- The turning effect on the coil increases if the number of turns of the coil is increased
- The turning effect on the coil increases if the coil is rotated faster.
J.Darren:
Transformer
- AC voltages can be increased or decreased using a transformer.
- The transformer works by mutual induction.
- Alternating current flows throught the primary output.
- An alternating magnetic field is created in the core.
- The changing field induces an alternating voltage in the secondary coil.
In the case of an 100% transformer, the following equations applies :
output voltage / input voltage = turns in output coil / turns in input coil
input voltage x input current = output voltage x output current
- A step-up transformer has more turns in the secondary coil.
Examples include : Power stations - A step-up transformer increases the voltage to the level needed for overhead power cables.
- A step-down transformer has fewer turns in the secondary coil.
Examples include : Battery charges and computers - A step-down transformer reduces the voltage of the AC mains to the level needed for other cicuits.
If a transformer is connected to DC supply, it may damange it as the high current which flows through the input coil can result in overheat.
J.Darren:
Mutual Induction
Imagine that we have an electromagnet (iron core) which is being wrapped by coil and is connected to a power pack, which also has a switch. Right next to it we have a coil, connected to a galvanometer. As the switched is closed, an emf is induced in the other coil, but only for a split second. It is the same as pushing a magnet towards the coil very fast. As a steady current now goes through the electromagnet, since the magnetic field is not changing, no emf is induced. When the electromagnet is being switched off, an emf is induced in the other coil in the opposite direction, but only for a split second. It is the same as pulling a magnet away from the coil very fast.
- The induced emf at switch on and switch off is increased if the number of turns in the other coil is increased.
- The induced emf at switch on and switch off is increased if the core of the electromagnet goes right theough the other coil (Parallel to each other).
WARRIOR:
WOAH
ELECTRICTY + GENERAL PHYS + ATOMIC pHYS ( thanks TO J.DAREN !) + THERMAL PHYS ARE DOOOOOOONEEEEEEEEEEEEEEEEEEEEEEEEEEE
ONLY WAVES LEFT !!!!!!!!!!!!!!!!!!!!!!!!!
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