Magnetism and Electromagnetism Flashcards
what metals can be magnetised?
iron, nickel, cobalt, steel
what is an induced magnet?
a magnet produced when a magnetic material is placed in a magnetic field
what is a magnetic field?
a region around a magnet where a magnetic material will experience a force
why are field lines forced closer together at the poles of a bar magnet?
the magnetic field is strongest here - highest flux density
what do the lines on magnetic field lines show?
direction of magnetic field - lines of magnetic flux
what is the unit of magnetic flux density?
tesla, T
what is magnetic flux density?
Strength of a magnet
what does the strength of a magnetic field depend on?
the distance from the magnet
what is the direction of a magnetic field at any point given by?
the direction of the force that would act on another north pole placed at that point
what is the direction of the magnetic field line from?
the north (seeking) pole to the south (seeking) pole
why does the earth have a magnetic field?
it has an iron core
where is the magnetic north pole on the earth?
at the geographical south pole
what happens when a current flows through a conductor?
a magnetic field is generated around it
what is a solenoid?
a coil of wire
what is the direction of field around a wire, with current flowing through it, shown by?
right hand grip rule
what is the symbol for current into page?
cross in a circle
what is the symbol for current out of page?
dot in a circle
what can the strength of an electromagnetic field be increased by?
- adding more turns on the coil
- increasing the current passing through it
- using an iron core
what are the advantages of electromagnets over normal magnets?
- variable strength
- can be turned on and off
- can reverse direction of magnetic field
how does an electric bell work using electromagnets?
- switch is closed to complete the circuit
- current flows through the solenoid
- solenoid becomes magnetic
- iron striker is attracted to the electromagnet and strikes the bell
- as striker moves towards the bell, the contact is broken
- current stops flowing through the coil, which loses its magnetism
- the spring returns the striker to its original position
- circuit complete so current flows again
- the bell will continue to ring as long as the switch is held closed
how does an relay switch work using electromagnets?
- relay contains 2 separate circuits: a low voltage one containing an electromagnet which is safe to be switched on and off and a high voltage one where the switch has been replaced with 2 metal contacts, one of which is connected to a spring which keeps the contacts apart and an iron block next to the spring
- low voltage circuit is switched on
- a current now flows around the circuit
- there is now a magnetic field around the electromagnet
- the magnetic field around the electromagnet attracts the iron block in the high voltage circuit, causing the contacts to close, switching on the high voltage circuit
what happens if a wire carries a current at right angles to a magnetic field?
it experiences a force
what does the size of the force produced by a wire carrying a current at right angles to a magnetic field depend on?
how the field and wire are angled:
- the force is greatest when the field direction and wire are at right angles
- the force is 0 when the direction of the field and wire are parallel
what are the parts of Fleming’s Left Hand Rule?
- thumb = motion or direction of force
- First Finger = magnetic field direction
- seCond finger = direction of current
how is a current formed by induction?
a p.d. is induced when a coil of wire cuts through the magnetic field of a magnet
how can the size of the p.d. (and current), formed in electromagnetic induction, be increased?
- use a stronger magnet
- move the magnet faster
- add more turns on the wire coil
- increase the area of the coil
how do DC motors work?
- a coil of wire is placed inside the field of a permanent magnet
- the coil will turn when a current flows
- there are opposite forces on each side, causing the coil to spin by turning electrical energy into kinetic
why is a split ring commutator used in DC motors?
- because the coil is vertical in between the magnets, the forces on it no longer make it turn, but pull it up and down on each side of the coil
- the split ring commutator breaks the flow of current when the coil is vertical and reverse the direction of current every half turn to ensure continuous rotation
what is the motor effect?
a current carrying wire in a magnetic field will experience a force (must be ppd to experience full force)
explain how an electric motor works
- coil of wire carrying a current is placed inside the field of a permanent magnet
- the force acting on one side of the coil will act upwards, and the other downwards causing the coil to spin (clockwise)
- once it has spun 180 degrees, the forces acting on the coil have swapped sides so will spin in the opposite direction (anticlockwise)
- left like this, the coil would flip back and forth, not spin 360 degrees
- so the current needs to change direction every 180 degrees (half turn) so coil will rotate in the same direction (full spin) - use a split ring commutator which moves as the wire moves to keep terminals the same
how do loudspeakers work?
- there is a varying current in the circuit which passes through a coil of wire
- the coil experiences a force (magnetic field) and reversing the current reverses the force (magnetic field)
- the magnetic field attracts to and then repels from the permanent magnet at the same frequency as the ac, meaning the coil vibrates at a specific frequency
- the vibrating coil causes the cone to vibrate
- the vibrating cone causes the air molecules to move
- the movement of air molecules produces pressure variations in the air needed for a soundwave
- air molecules bunch together forming compressions and spread out forming rarefactions
how does changing the frequency of the ac in a loudspeaker change the sound?
it changes the frequency (pitch) of the sound produced
how does changing the voltage (and therefore current) of the ac in a loudspeaker change the sound?
it changes the volume of the sound produced by changing the size of the force acting on the coil of wire
HOW DO MICROPHONES WORK?
- pressure variations in sound waves cause the flexible diaphragm to vibrate
- the vibrations of the diaphragm cause vibrations in the coil
- the coil moves relative to a permanent magnet, so a p.d. is induced in the coil
- the coil is part of a complete circuit so the induced p.d. causes a current to flow around the circuit
- the changing size and direction of the induced current matches the vibrations of the coil
- the electrical signals generated match the pressure variations in the sound waves
what is a transformer?
a device used to increase or decrease the p.d. of electricity
what is a transformer made of?
two coils of wire wrapped around an iron core
how does a transformer work?
- a primary voltage drives an alternating current through the primary coil
- the primary coil current produces a magnetic field, which changes as the current changes
- the iron core increases the strength of the magnetic field
- the changing magnetic field induces a changing potential difference in the secondary coil
- the induced potential difference produces an alternating current in the external circuit
do step up transformers have more turns on the primary or secondary coil?
secondary
do step down transformers have more turns on the primary or secondary coil?
primary
what does Nₛ / Nₚ = Vₛ / Vₚ stand for?
number of turns in secondary coil / number of runs in primary coil = voltage in secondary coil / voltage in primary core
why are transformers assumed to have 100% efficiency?
energy can’t be created or destroyed, only transferred so when electrical energy from primary coil is transferred to secondary, it can’t increase in energy
what is the generator effect?
the creation of p.d. therefore an induced current (flowing electrons) when a wire experiences a changing magnetic field
how does a faster relative motion between magnetic and wire coil increase p.d. in the generator effect?
it increases the magnetic field change, and p.d = change in magnetic field / change in time
