Physics P3 27/5/2010

[J.Darren]

A reed switch is operated by a magnetic contact, when a piece of metal is brought near, the two contracts are ... well, in contact. When this arrangement is connected to a coil, it is connected to a circuit, hence when the two contacts are in contact, it completes the circuit and switches on, say a buzzer.

Another form of relay would be the magnetic relay.

http://www.irrigationtutorials.com/control1.gif

As shown above, when the switch is closed, the contact of the relay and the current with a pump is brought in contact, thus completing the output circuit (the one with a pump), it is then switched on. There is another form of relay where the contact is normally closed, and detaches when the switch is closed.

Circuit breaker

http://static.howstuffworks.com/gif/circuit-breaker-diagram.gif

The current flows through both terminals, this is an example of a normally closed relay as the contacts are normally closed. When the current gets strong enough, the pull from the electromagnet would release the iron catch, which detaches the contact and prevents current from going through. The switch is essentially a reset button which, by pressing it, enables you to close the contact again.

Magnetic field around a wire - Right hand grip rule (The fingers are pointed in the same direction as the magnetic field whereas your thumb is pointing in the direction of the current)

- The field lines are circles
- The field closest to the wire is the strongest
- Increasing the current would increase the overall field strength

A question from the 08 (?) Paper 3 asks what happens if there are two such magnetic field which interacts with each other. In this case they get attracted to each other.

Magnetic fields from coils - Right hand grip rule (Working out the poles - The direction which the thumb is pointing is the North pole, whereas the direction of your fingers represents the direction of the current)

- There are magnetic poles at both ends of the coil
- Increasing the number of turns in the solenoid increases the strength of the magnetic field
- Increasing the current increases the strength of the magnetic field

[6394]

Thanks a looooooooooooooot kimo jesus
n j.darren
prayers 4 u =D
 
realllllllyyyyyyyyyy help ful !!!!
 

[J.Darren]

Magnetic force on a current - Fleming's left hand rule

Fleming's left hand rule

Thumb : Force

Index finger : Magnetic field

Field direction : North to south

The finger : Current

Current direction : Positive to negative

The wire moves across the magnetic field without being attracted to either poles.

- The produced force increases if a stronger magnet is used
- The produced force increases if the current is made stronger
- The produced force increases if the length of wire is increased

[Ivo]

Magnetic force on a current - Fleming's left hand rule

Fleming's left hand rule

Thumb : Force

Index finger : Magnetic field

Field direction : North to south

The finger : Current

Current direction : Positive to negative

The wire moves across the magnetic field without being attracted to either poles.

- The produced force increases if a stronger magnet is used
- The produced force increases if the current is made stronger
- The produced force increases if the length of wire is increased

Easy way to remember FLHR:

First Finger: FIELD

SeCond Finger: CURRENT

ThuMb: Motion/Thrust

[WARRIOR]

THANKS ALOT DAREN..YOU SAVED ALLOT OF MY TIME !!!! ILL ADD THOSE WHN ICOMPLTETE THE ECLECTRICTY NOTES 2NIGHT

[Ivo]

Kimo Jesus, here are the Waves notes.  If they're any good, then post them.  If not, just ignore

GENERAL WAVE PROPERTIES

Distinguish between transverse and longitudinal waves

If the end of a slinky is shaken from side to side a transverse wave travels down the spring. The spring is displaced perpendicular to the direction of travel of the pulse. If the spring is continually shaken from side to side then a transverse wave travels down the spring.  

A water wave is an example of a transverse wave.  Looking down on a ripple tank, the wavefronts are just the crests of the waves.  Electromagnetic waves are also transverse.

If the end of the slinky is shaken along the length of the spring then a longitudinal pulse travels down the slinky. As the pulse moves along, the spring is displaced along the direction of travel. Sound waves travel as longitudinal waves.

Longitudinal waves travel as a series of compressions and decompressions (rarefactions), but there is no overall movement of the material they travel through.  The atoms vibrate backwards and forwards, parallel to the direction of motion. Sound waves are longitudinal.

Definitions of speed, frequency, wavelength and amplitude

The speed v of a wave is simply the speed at which the pulse or wave travels. e.g. how far the crest of a water wave, or the compression of a sound wave, travels in one second.

The frequency, f, of a wave is the number of complete waves passing by per second. It is measured in cycles/sec or hertz (Hz)  

The wavelength, ?, of a wave is the distance between two adjacent crests of the wave.
The amplitude is the height of the wave measured from the undisturbed position (midpoint).

v = f ?

Wave speed (m/s) = frequency (Hz) × wavelength (m)

Reflection, refraction and diffraction

When drawing wave diagrams, make sure the wavelength doesn’t change on reflection or diffraction.  

In diffraction, the amount of spreading depends on the wavelength and the width of the gap. The same waves will diffract more at a narrower gap.  Longer wavelengths will diffract more at the same size gap.

In refraction, the wavelength decreases when the wave slows down – this is what causes the wave to change direction, e.g. for water waves passing into shallower water, or light travelling into glass.

FYI, for some reason I can't add the lambda sign for the equation!

[WARRIOR]

thanks man ! i appreciate your help . you and j darren ! thx alot

+ rep By the way !!

[J.Darren]

Turning effect on a coil - Fleming's left hand rule

The coil is placed between the poles of a magnetic field. The current flows in opposite directions along the two sides of the coil. According to Fleming's left hand rule. One side is pushed up and the other side is pushed down. With more turns in the coil, the turning effect is increased.

DC Motor

- The coil is made of insulated copper wire.

- The motor runs on Direct Current (Obviously !)

- A device called commutator changes the direction of the current when the coil is nearly vertical. The carbon brush enables the coils to overshoot each other. The forces changes direction and keep the coil turning.

- The coil keeps rotating clockwise, half a turn at a time. This is because that the direction of the forces are reversed when the coil is nearly vertical as it overshoots the commutator. Reversing the battery or the poles of the magnet would cause the coil to rotate anticlockwise.

- 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 current is increased

[J.Darren]

Electromagnetic Induction - Moving wire

When a wire is moved across a magnetic field, an electromotive force is induced in the wires. As the wire forms part of a complete circuit, the emf makes a current flow.

Faraday's law of electromagnetic induction

The emf induced in a conductor is proportional to the rate at which the magnetic field lines are cut by the conductor. By taking the measures below, the emf induced in a conductor (wire) would increase proportionally as the the magnetic field lines are being cut more frequently.

- The induced emf and current incrases by moving the wire faster.
- The induced emf and current incrases by using a stronger magnet.

Bear in mind that the magnetic field lines are used to represent the strength of a magnetic field as well as its direction. The closer together the lines, the stronger the magnetic field.

- The induced emf and current incrases by increasing the length of the wire in the magnetic field.
- The induced emf and current incrases by looping the wire.

Moving the wire in opposite direction or reversing the poles of the magnet will reverse the direction of the induced emf and current.

The current flow can be detected by a device called a galvanometer, its pointer moves to the left or right, depending on the direction of the current.

[J.Darren]

Electromagnetic Induction - Coil

- If a bar magnet is pushed into a coil, an electromotive force is induced in the coil.
- The field lines of the magnetic field caused by the bar magnet are being cut by the coils.
- As the coil forms part of a complete circuit, the emf makes a current flow.

- The induced emf and current incrases by increasing the number of turns in the coil.
- The induced emf and current incrases by stronger magnet.
- The induced emf and current incrases by moving the magnet faster.

- If the magnet is pulled out of the coil, the direction of the induced emf and current is reversed.
- If the S pole of the magnet, instead of the N pole, is pushed into the coil, it reverses the direction of the current.
- If the magnet is held still, no field line is cut, no emf and current are induced.

The current flow can be detected by a device called a galvanometer, its pointer moves to the left or right, depending on the direction of the current.

[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 !!!!!!!!!!!!!!!!!!!!!!!!!