P4 Flashcards
Explain the action of the microphone in converting the pressure vibrations in sound waves into variations in current in electrical circuits, and the reverse effect as used in loudspeakers and headphones
MICROPHONE:
- sound wave is pressure wave
- as sound wave hits diaphragm of microphone, areas of high pressure, compressions, push the diaphragm in and areas of low pressure, rarefactions pull it out
- as the diaphragm ,moves in and out so does the coil so a p.d is induced across the ends of the wire
- this is the ‘electrical signal’ that a mic produces
LOUDSPEAKER:
- connect ends of wire to a changing p.d of a suitable freq to hear sound
- changing p.d produces a changing current that produces a force on a coil of wire because it is in a magnetic field. the cone will move in/out. it will not spin
- can used left hand rule to work out whether coil is moving in or out
Describe the attraction and repulsion between unlike and like poles for permanent magnets
like+unlike = attraction
Describe the characteristics of the magnetic field of a magnet, showing how strength and direction change from one point to another
goes from N to S
closer the field lines are to the magnet, the stronger the force
Explain how the behaviour of of a magnetic (dipping) compass is related to evidence that the core of the earth must be magnetic
if u hang up a magnet, it will line up so that 1 end of it points towards the magnetic north pole
magnetic north pole is not the same as the North Pole on earth (the point where the earth spins)
a compass also points towards the magnetic north pole
earth acts like it has a large bar magnet at its centre with the same magnetic field of a bar magnet
maybe caused by convection currents in the molten iron core of the Earth
many compasses are weighted so they lie horizontally
the dip is the angle between the field lines and a line horizontal to the surface of the earth. it is 90 degrees at the north and south magnetic poles, and zero at the magnetic equator
Describe how to show that a current can cause a magnetic effect and describe the directions of the magnetic field around a conducting wire
- place wire in centre of paper with plotting compass
- attach current from voltage pack using 2 wires at each end of wire
- current travels from - to + so if current is travelling up towards you then the field lines will go anticlockwise
* * CURVE YOUR RIGHT HAND AROUND THE WIRE IF CURRENT IS TOWARDS YOU **
Recall that the strength of the field depends on the current and the distance from the conductor
a bigger current = a stronger field
nearer the wire = a stronger field
strength of magnetic field is measured in teslas (T)
earth’s magnetic field is about 0.01 mT
Explain how a solenoid can enhance the magnetic effect
solenoid = many parallel loops forming a coil
a loop of wire carries its own current therefore its own magnetic field in the centre at a straight line
many fields produce a much stronger field and you can make it even stronger by putting magnetic material inside the core
Describe how a magnet and a current-carrying conductor exert a force on one another
you can combine the field due to a wire with the field due to a permanent magnet
when you put 2 fields together that are in the same direction they add up but if they are in opposite directions they will cancel out
RIGHT HAND CORKSCREW RULE WHEN THE ARROWS OF BOTH MAGNETIC FIELD LINES MATCH, ITLL WILL GO IN THE OPPOSITE DIRECTION
THAT IS THE MOTOR EFFECT.
Explain the right hand corkscrew
USE RIGHT HAND point thumb in direction of current curve fingers show direction of field then draw the magnetic fields of the magnets and the direction of the field and work out which ones cancel and where the motion/force goes motion is always the weaker one
Apply the equation that links the force on the conductor to the magnetic flux density, the current and the length of conductor to calculate the forces involved
force on conductor( at right angles to a magnetic field) carrying a current(N) = magentic flux density (T) x current (A) x length (m)
Explain how the force exerted from a magnet and a current-carrying conductor is used to cause rotation in electric motors
1 loop has 2 forces (because it changes into 2 directions). this is related to fleming’s left rule and right corkscrew
- wire turns a quarter (e.g from horizontal to vertical)
- if current stayed in same direction, wire will stop turning since both forces and opposite and equal since the current is the same
- when the loop is like that, we design to stop current using a split ring commutator which supplies the wire with current that spins with the wire
- as it continues just a little due to momentum, ends of looped wire touch the commutator and the current continues in the same direction relative to the magnet in the same direction and same force
- THE MOTOR EFFECT IS YOU PUT CURRENT AND FIELD IN AND YOU GET MOTION/FORCE
Recall that a change in the magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change
DYNAMO EFFECT: YOU PUT IN FIELD AND MOTION AND GET FORCE
GENERATES CURRENT FROM MOTION
its just the idea that there is an imaginary force that opposes the motion
Explain how this effect is used in an alternator to generate a.c, and in a dynamo to generate d.c
DIRECT CURRENT: use a split ring
ALTERNATING CURRENT: use slip ring
1. short end of wire and long end are fixed on 2 different rings
2. as loop turns in magneitc field, motion induces current and as it turns, current changes direction in loop
Explain how the effect of an alternating current in one circuit, in inducing a current in another, is used in transformers
PURPOSE IS TO CHANGE P.D
1. A.C induces changing magnetic field in primary coil
2. Magnetic field is passed through the whole metal
3. Change in magentic force in metal induces A.C in 2nd coil
CHANGE IN MF IS REVERSIBLE TO CHANGE IN A.C
More coils= higher p.d= more efficient
What happens when you combine the field due to a wire with the field due to a permanent magnet?
A force is produced on the wire
When you put two fields together they ______ but if they are in opposite directions they ______
add up, cancel out
What is the formula to calculate the size of a force?
force on a conductor (perpendicular to magnetic field) carrying current (N)
= magnetic flux density (T) x Current (A) x Length (m)
How do you make a simple motor?
Make a loop out of a coil of wire, and place it into a magnetic field. When the wire is connected to a battery, current will flow. One side of the wire will go upwards, and the other side will go down.
What is wrong with simple motors?
They do not spin very well. This is because the direction of the forces change every time the coil passes the vertical position, causing the coil to spin in the other direction as well.
How does an electric motor work?
A split-ring commutator enables the current to change to different halves of the coil as it spins. This means that every half turn, the current can reverse direction, causing the forces to remain in the same direction for the whole cycle.
How can you change the speed of the motor?
By changing the magnitude of the current in the coil
The strength if the magnetic field
The number of coils of wire
The length of the coil
What is electromagnetic induction?
When you produce and induced potential difference along the ends of a wire in a changing magnetic field so that it cuts field lines.
There must be relative motion between the wire and the magnets, which causes potential difference to be induced.
What does the magnitude of induced potential difference depend on?
The rate at which field lines are cut
How do you increase the induced potential difference?
Moving the wire faster (increasing the relative motion between wire and magnet)
Using a stronger magnetic field
Using more wire (more coils - more field lines cut per second)
How do you know the direction of an induced potential difference?
The magnetic field produced is in the opposite direction to the field that produces the potential difference
What type of magnetic field is needed to induce a potential difference?
A changing magnetic field - such as moving a magnet in and out of a coil
What is alternating potential difference?
A potential difference that changes direction
What is an alternator?
An alternating current generator
What happens to the induced potential difference when a magnetic field is changing?
The induced potential difference changes in the same way
How is a changing magnetic field produced in an alternator?
The coil of wire spins between the poles of a magnet - which is equivalent to moving the magnet in and out
How are the brushes in an alternator connected?
The brushes are not attached to the slip rings, they brush against the slip rings so the voltmeter is always connected to the ends of the coil but the coil does not become tangled
What happens in a commercial generator in a power station?
There is a rotating electromagnet inside a coil of wire, and the generator spins at a frequency of 50Hz.
What is a dynamo?
A direct current generator
What type of potential difference is produced by a dynamo?
The potential difference drives a current that does not change direction, because the coil is connected to a split ring commutator. The potential difference does change in magnitude however.
How can be output of alternators and dynamos be increased?
By using a stronger magnetic field
Using more turns on the coil
By spinning the coil faster
What does a transformer do?
It can be used to increase or decrease a potential difference, which is induced when the magnetic field lines cutting a coil change
How can you change the field lines that cut the coil?
Moving the magnet or the coil
Using another coil and turning the current in it on and off
Using another coil that has an alternating current in it
How does a transformer work?
If you make a loop of iron with two coils you have made a transformer. The magnetic fiel is trapped inside the iron core
An alternating p.d. across the primary coil produces an alternating current in the primary coil
An alternating current in the primary coil produced a constantly changing magnetic field in the iron core
This induces a changing p.d. in the secondary coil
What is the relation between number of coils and potential difference in a transformer?
potential difference across primary coil (V) / potential difference across secondary coil (V) = number of turns in primary coil / number of turns in secondary coil
What does a step up transformer do?
Increases voltage and decreases current
What does a step down transformer do?
Decreases voltage and increases current
What is the difference between a motor and a generator?
- In a generator, movement produces potential difference in a wire
- In a motor, a current (that flows because of p.d. across the ends of the wire) produces movement
What does a dynamic microphone work like?
A generator
How does a dynamic microphone work?
As a sour wave hits the diaphragm of a microphone, areas of high pressure, compressions, push the diaphragm in, and areas of low pressure, rare fractions, pull it out.
As the diaphragm moves in and out so does the coil, so a p.d. is induced across the wires ends. This is the electrical signal that a microphone produces.
How does a carbon microphone work?
Are are carbon granules behind the diaphragm. As the sound waves hits the diaphragm it changes the resistance of the carbon. A current passing through the carbon granules increases or decreases as the resistance changes.
What does a loudspeaker work like?
A motor
How does a loudspeaker work?
If you connect the ends of the wire to a changing potential difference of a suitable of a suitable frequency you will hear a sound.
The changing p.d. produces a changing current which produces a force on a coil wire because it’s in a magnetic field. The cone does not spin but moves in and out. This produces a sound wave.
If you take a short section of the coil and apply Fleming’s left-hand rule you can work out whether the coil is moving in or out.
The movement of the coil depends on the size of the p.d. producing current in the coil
You can amplify the changing p.d. and send it to the loudspeakers if you to produce a loud sound
What are magnetic field lines?
a way of modelling a magnetic field, and there represent magnetic flux density
What is an induced magnet?
A material that has domains that only line up when they are in a magnetic field, causing them to act as temporary magnets
What did Hans Christian Oersted discover?
There is magnetic field around a wire that is carrying a current, and if the current is coming towards you the field lines are anticlockwise
What does the strength of a field around a wire (magnetic flux density) depend on?
The magnitude of the current
The distance from the wire
What is a solenoid?
Parallel loops of wire making a coil
What is the use of a solenoid?
A solenoid is like adding together many fields to produce a much stronger field
How can you make a solenoid even stronger?
Adding a magnetic material inside the core
What is a permanent magnet?
One which always has a magnetic field around it
What affects do permanent magnets have on objects?
Can sometimes attract or repel other permanent magnets, and sometimes even non-magnets (which can have magnetism induced in them when the permanent magnet is brought close
How does the Earth behave (magnetically)?
As if there is a bar magnet inside of it
What does the Earth have around it?
A magnetic field with a North and South Pole that exerts a small but noticeable force upon every magnetic material on Earth
How do we know from a field line diagram that the magnetic field of a bar magnet is stronger near the poles?
The field line are closer together
Why does the model of the Earths magnetic field as a bar magnet have the South Pole of the magnet near the North Pole of the earth?
The North Pole is named thus because the north poles of magnets point in its direction, which happens because north poles are attracted to south poles, thus the Earths North Pole is actually a magnetic South Pole
Describe the behaviour of compass as you move from the equator to the north magnetic pole
The compass’ north will originally point at the equator, and as you move towards the north pole, the needle will slowly turn, and by the time you reach the North Pole, you end up pointing exactly 90 degrees from its starting position
paperclip held upright on a thread, with a card above it and a bar magnet above that
Why does a paperclip fall when you insert a magnetic material between the card and the paperclip?
Field lines never cross, so when the two magnets are brought near each other, their field liens will bend around each other (interact), producing regions of high density of field lines (therefore strong field)
What is the motor effect?
When a current-carrying wire in a magnetic field experiences a force
What happens whenever a current flows in a wire?
A magnetic field is generated around it
What is a field?
A region of space where a force is exerted
What do currents, fields and forces have in common?
They all have a direction - vectors
How can you calculate the direction of the force on a wire?
Using Fleming’s left hand rule
When two parallel wires carry current, why is there a force on each wire?
When a current flows through a wire a magnetic field is generated around it - when the fields of the two wires interact and the wires experience an attractive force if the currents in the wires flow in the same direction and repulsive if the currents in the wires flow in opposite directions
Why do some motors use electromagnets instead of permanent magnets?
They produce stronger fields which produces a larger force
Do do bigger motors produce a larger current in the coil?
It will have more length of wire, in a stronger magnetic field, by F=BIL this will results in a larger force
It would be incorrect to say that a bigger motor would have larger current - current would have to be the same in both motors, as larger currents transfer electrical energy to heat more quickly via wire resistance
What happens when a wire cuts magnetic field lines?
Electrons are moved to one side of the conductor, and if the wire is part of a complete circuit, a current will flow.
This is the process of electromagnetic induction.
How do generators produce electricity?
By electromagnetic induction
What is Lenz’s law?
The direction if the induced p.d. opposes the change producing it
How does Lenz’s Law work?
If a magnet approaches a coil with its north poles the p.d. induced current in the coil of wire will give rise to a North Pole on the end the magnet is approaching.
The magnets north and the coils north Weill repel, opposing the motion inducing the p.d.
What happens in an alternator?
A rectangular coil is made to spin in a uniform magnetic field, and the coil is connected to a voltmeter by metal brushes that press on two metal commutators
As the coil turns each side cuts field lines, so the induced p.d. is therefore repeated changing direction (and so is the induced current)
How does doubling a dynamos speed of rotation change the potential difference induced?
The size of the peak p.d. induced will be double
the frequency at which the p.d. is generated will double
