Physics AQA GCSE Magnetism and Electromagnetism

what is the region around the magnet called

magnetic field

what are permanent magnets

they produce their own magnetic field

what are induced magnets

become a magnet when placed in a magnetic field

what way do the arrows on field lines run

north to south

what is the flux density

the density of the field lines - it indicates the strength of the field at that point (the closer the lines the higher the flux density = stronger the field and greater the force)

what does the compass needle align with and what direction does it point

the Earth's magnetic field and points to the north

how can a magnetic compass be used to plot the field around a bar magnet

place the bar magnet on a piece of paper

place the compass at one end of the magnet

mark on the paper where the point of the compass needle is

move the compass so the tail of the needle is at the point that has just been marked

mark on the paper a new point where the needle is

repeat and connect the marks until the full field is plotted

what happens when a current flows in a conducting wire

a magnetic field is produced around the wire

what does the direction of the field lines depend on

the direction of the current and can be found with the right hand grip method

demonstrate the right hand grip method

grip the wire in your right hand, with the thumb pointing in the direction of the current

the fingers curled around the wire will point in the direction that the field lines should be drawn

what does the strength of the field depend on

the size of the current and the distance from the wire

how is a solenoid formed

when a wire is looped into a cylindrical coil

how can you increase the strength of the magnetic field

shape the wire into a solenoid - will increase the strength because it conc. a longer piece of wire into a smaller area, the loop shape means the magnetic field lines are all in the same direction

add an iron core

how can the north pole of a solenoid be found with the right hand grip method

hold the solenoid in your right hand with you fingers following the direction the current flows

your thumb will point to the north pole of the solenoid

how does the electromagnetic device of an electric bell work

when the with is pushed, the electromagnetic is magnetised

the electromagnet attracts the armature

the hammer strikes the gong and breaks the circuit

the armature springs back, completing the circuit again and demagnetising the electromagnet

the cycle repeats for as long as the button remains pushed

what is the motor effect

when a current carrying conductor is placed in a magnetic field it experiences a forced (the motor effect)

what is the motor effect caused by

the field created by the current interacting with the magnetic field

how can the force be increased

by increasing either the size of the current, the length of the conductor or the flux density

what is the equation for force

force = magnetic flux density (T) x current x length of the wire within the field

what happens if the current or magnetic field is reversed

the direction of the force is reversed

how can the direction of the force be found using Fleming's left hand rule

hold the left hand so that the thumb, first finger and second finger are all at right angles to one another

align the first finer, so that it points in the direction of the magnetic field from north to south

rotate the wrist so that the second finer points along the wire in the direction that the current is flowing

the thumb will be pointing in the direction of the force - the direction the conductor would move

what are the conditions for the left hand rule to work

can be used to find the direction of the field / current / movement as long as the other two are known

a current carrying coil in a magnetic field will...

rotate

why?

the current going up one side of the coil I in the opposite direction to the current coming back down the other side, so one side moves up and the other move down

basis of an electric moron - continuously rotating coil

how do loudspeakers and headphones produce sound waves

they use the variations in an alternating current to produce sound waves

loudspeakers

as the current travels through the coil, it experiences a force due to the magnetic field, this makes the speaker cone move

because the current is alternating, the direction of the force alternates and speaker cone oscillates

increasing the electrical power will produce a larger force - the cone will oscillate with a higher amplitude and produce louder sound waves

the frequency of the sound produced matches the frequency of the alternating current

the size of the speaker affects how quickly or slowly it can oscillate and how big these oscillations will be

a large cone is better for low pitched sounds and a small cone is better for high pitched sounds

what is the generator effect

when conductors and magnetic fields interact, a potential difference is induced (the generator effect)

how can this potential difference be induced

an electrical conductor moving relative to a magnetic field

a change in magnetic field around a conductor

what will increasing the speed of the movement or magnetic field increase the size of

the induced potential difference

what will reversing the direction of the movement or magnetic field do

reverse the direction of any induced current

what does an induced current generate

a magnetic field that opposes the original change or the change in magnetic field

wha is the generator effect used in

alternators to generate an alternating current

dynamos to generate a direct current

what happens as the magnet rotates

the direction of the field through which the coil passes alternates - this induces an alternating potential and an alternating current

what happens in a dynamo

the coil rotates and the magnet is fixed. as the coil rotates it generates a potential difference in one direction

what does the use of the split-ring commutator mean

once the coil has passed the upright position, the connections are reversed - as a result the direction of the current output is always in the same direction

in both cases, the induced potential difference is greatest when the magnet field and the coil are parallel, so the magnetic field is being 'cut' by the rotating coil at the fastest rate

in both cases, the induced potential difference is greatest when the magnet field and the coil are parallel, so the magnetic field is being 'cut' by the rotating coil at the fastest rate

for alternators, what is the frequency of the alternating current

the speed of rotation

a moving coil microphone uses the generator effect to convert sound waves into electrical signals

the sound waves hit the microphone

the changes in air pressure related to the sound wave cause the microphone diaphragm to oscillate

the microphone diaphragm and coil vibrate at the same frequented as the incoming sound wave - the bigger the amplitude of the sound, the bigger the amplitude of the vibration

this induces a potential difference on the amplitude of the incoming wave

what does a basic transformer consist of

a primary and secondary coil wrapped around a soft iron core

what is the function of a transformer

an alternating current flows through the primary coil, which is effectively a solenoid

this alternating current induces an alternating magnetic field

the alternating magnetic field in the iron core induces an alternating potential difference in the secondary coil

if the secondary coil is part of a complete circuit, an alternating current flows in the secondary coil

what would happen if transformers were 100% efficient

the power output would be the same as the power input

what is the equation for power

voltage x cutest (when the potential difference is increased, the current is reduced and vice versa

voltage equation

voltage (primary number of turns (primary)

over = over

voltage (secondary number of turns (secondary)

voltage (secondary) x current (secondary) = voltage (primary) x current (primary)