Qualification > Reference Material
Physics P3 27/5/2010
Ivo:
Kimo Jesus, here are the Waves notes. If they're any good, then post them. If not, just ignore :P
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.
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