P4.2 Flashcards
What is the motor effect
When a current carrying wire, in a magnetic field, experiences a force
Size of force formula
Calculating Direction of force
Flemmings left Hand rule
What is a motor
An electric motor uses the motor effect to transfer electrical energy to kinetic energy.
What is a motor made from
An electric motor (in effect) consists of a loop/length of wire, with a current, in a magnetic field.
How does a motor work
- An electric motor (in effect) consists of a loop/length of wire, with a current, in a magnetic field.
- The wire will experience a force, given by F = BIL
- As current flows in opposite directions in theopposite sides of the coil, forces act in opposite directions on opposite sides of the coil, causing the coil to rotate.
- Once coil rotates fully up, the coil actually experiences NO FORCE ON ITS SIDES(as the current no longer runs perpendicular to B-field lines,) = The reason why the coil continues to move past this position is due to its momentum
- If the current stayed running in the same direction - the coil will turn anti-clockwise instead and keep flip-flopping
- Therefore, every half turn of the coil, the current direction must be reversed to ensure the coil continues to turn in the same direction.
How to increase speed of motor
must increase size of force acting, F=BIL, so…- - - increase current in coil
- increase strength of B-field/magnet
- have more loops/longer length of wire
- force is greatest at 90° and zero at 0°.
What is electromagnetic induction / generator effect
When a conductor cuts the magnetic field lines of a magnet, a potential difference is induced across the ends of the conductor
How does electromagnetic induction work
When a conductor (wire) cuts magnetic field lines, electrons are moved to one side of the conductor, creating a p.d.
If this conductor (wire) is part of a complete circuit, a current flows.
How to increase size of induced potential difference
- Increase number of coils
- Use a stronger magnet
- Move the wire quicker through the magnetic field
What is Lenz’s law
The direction of the induced p.d. is such that itopposes the change producing it.
Lenz’s Law ensures that the law of conservation of energy isn’t violated.
Example of Lenzes law in Action
If a magnet approaches a coil of wire with its north pole, 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 north pole of the magnet and north pole of coil will then repel, opposing the motion inducing the p.d.
How does an alternator work
- A rectangular coil is made to spin in a uniform magnetic field.
- The coil is connected to a meter by metal brushes that press on two metal commutator rings.- SLIP RINGS
- As the coil turns, each side of it cuts the field lines from below, then above and so on.
- The induced potential difference is therefore repeatedly changing direction (and so therefore is the induced current).
How to increase size of p.d in an alternator
- Rotating coil faster
2. Using a stronger magnet3. Using a coil with a larger surface area/more turns
The faster the coil rotates in an alternator
… the greater the rate of cutting field lines… the greater the peak value of induced p.d.… AND the greater the frequency of the induced p.d.
What is a dynamo
direct current generator.
How does a dynamo work
- A rectangular coil is made to spin in a uniform magnetic field.
- The coil is connected to a meter by a SPLIT RING COMMUTATOR
- reconnects the coil the other way round every half turn of the coil
- The induced potential difference is therefore in the same direction
Compare and alternator and a dynamo
Both use coils and magnets to induce a p.d. (and current).
Alternators use commutator slip-rings to give rise to an a.c. current.
Dynamos use split-ring commutators to give rise to a d.c. current.
What are transformers
are devices that use an alternating p.d. (current) in a primary circuit to induce an alternating p.d. (current) in a secondary circuit.
do transformers use AC or DC current
ONLY ONLY ONLY USE AC IN PRIMARY CIRUIT
How to transformers work
- P.D. (current) continually reverses direction in primary coil
- Magnetic field lines around primary coilcontinually expand and collapse
- Secondary coil experiences a constantly changing magnetic field
- P.D. (current) continually induced in secondary coil (in alternating directions)
What is a step up transformer
Increase the p.d.…(higher voltage induced in secondary circuit)
… because secondary circuit has more turns.
What is a step down transformer
Decrease the p.d.…(lower voltage induced in secondary circuit)
… because secondary circuit has fewer turns.
Transformer equation
The core of a transformer must be…
LAMINATED - to stop edi currents forming
How does a microphone work
- A sound wave hits a diaphragm and causes it to move in and out.
- A coil attached to the diaphragm now moves in and out.
- The coil moves inside a permanent magnet’s magnetic field and so cuts field lines.
- A p.d. (current) is now induced in the coil, producing an electrical signal that is fed to a speaker.
How does a loudspeaker work
- A changing p.d. in a coil of wire produces a changing current.
- The wire is inside a permanent magnet’s magnetic field and so experiences a changing magnetic force (size and direction).
- A diaphragm attached to the coil moves in and out.
- A sound wave is generated of the same frequency at which the diaphragm vibrates (moves in and out).
Is a loudspeaker like a motor or generator
Motor
Is a microphone like a motor or generator
Generator
What is the motor effect due to
two interacting magnetic fields
One is produced around the wire due to the current flowing through it
The second is the magnetic field into which the wire is placed, for example, between two magnets
For movement in motor effects
Two magnetic fields need to be parallel
Why does increasing the speed increase the induced p.d
Increasing the speed will increase the rate at which the magnetic field lines are cut
Why does increasing the number of turns on the coils increase the induced p.d
because each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines
What reverses the current every half turn in motor
split ring commutator
How does split ring commutator work
swaps the contacts of the coil
What are split rings made of
brass or copper
What are brushes made of in motor
Graphite - low friction + maintain good electrical contact
The key features of an electromagnet are:
it can be switched on or off by switching the current on or off
its strength can be varied by varying the magnitude of the current in the coil
its polarity can be reversed by reversing the direction of the current.
For electromagnetic induction to happen….
MUST HAVE RELATIVE MOTION
Instead of rotating in a magnetic field for electromagnetic induction
Connect to ac current
For ac electromagnetic induction how to increase
the ac frequency in coil A is increased – the rate of change of the field through coil B increases
- the ac amplitude in coil A is increased – the rate of change of the field through coil B increases*.
When an aircraft flies due north, its aluminium wings cut through the vertical part of the Earth’s magnetic field and a small voltage is induced across its wing tips. On the return journey, the aircraft is travelling due south at a higher speed.
How does the voltage induced across its wing tips on the return journey compare with that on the outward journey? Explain your answer.
A higher voltage, but the polarity has reversed (i.e. east-west changes to west-east). The induced voltage is generated when a conductor (the aluminium wings) cuts through a magnetic field (the vertical part of the Earth’s magnetic field). The magnitude of the induced voltage is proportional to the rate at which the magnetic field lines are cut; this has increased, so the voltage is greater. The polarity depends on the direction of cutting; this has reversed, so the polarity also reverses
Increasing the frequency of rotation of the coil has two effects:
it increases the frequency of the output ac voltage because the direction of cutting of the field lines changes more rapidly
it increases the amplitude of the output ac voltage because the rate at which the field lines are cut increases.
Understand this induced voltage
Now I’m going to increase frequency of rotation - draw new graph
Generators are used in nuclear power stations to generate electricity for the national grid. Which of the following energy transfers is carried out by the generator?
1) nuclear energy → electrical energy
2) chemical energy → electrical energy
3) kinetic energy → electrical energy
4) thermal energy → electrical energy
kinetic energy → electrical energy
Advantage of iron core in transformer
Iron is a magnetically soft material.
This means it can be magnetised and demagnetised easily and quickly.
This allows the magnetic field in the core to change both in magnitude and direction as the ac current in the primary coil changes.
Why aren’t real transformers 100% efficient
the resistance in the wires on the coils
heating effects in the core as it magnetises and demagnetises
currents induced in the core (eddy currents) by the changing magnetic field.
