P15-electromagnetism Flashcards
What is a magnetic field?
It is the region around a magnet where the effects can be felt.
What are magnetic field lines?
They are the lines of force. They are more concentrated at the poles than elsewhere as the force is the most concentrated at the poles.
What is a plotting compass?
It is a compass placed in the magnetic field, aligning itself along the field line pointing away from the N pole of the magnet and towards the S pole.
What way does the magnetic field go for magnets and the earth?
For the magnets it goes North to South, but for the earth it goes South to North.
What is induced magnetism?
It is where an unmagnetized magnetic material can be magnetised by placing it in a magnetic field. e.g. a chain of paperclips on the end of a magnet.
How does current impact the magnetic field?
When an electric current passes along a wire, a magnetic field is set up around it. Reversing the direction of the current reverses the direction of the magnetic field. The magnetic field lines are perpendicular to the plane.
What is a solenoid?
It is a long coil of wire, where a strong magnetic field can be produced.
What is inside a solenoid?
The magnetic field is stronger than a straight wire. The field lines are parallel to the axis of the solenoid and are in uniform. The magnetic field strength is also uniform.
What is outside the solenoid?
The magnetic field lines bend around from one end to the other. Furthermore, the magnetic field outside is like the field of a bar magnet except each field line is a complete loop as it goes inside.
What is an electromagnet(solenoid)?
It is a solenoid that has an iron core with insulated wire wrapped round. When a current is passed along, it creates a magnetic field. When the current switches off, the bar loses most magnetism.
What are 4 things electromagnets are used in?
Scrapyard cranes
A circuit breakers
An electric bell
The relay
How can the size of the force in an electromagnet be increased by?
Increasing the current, increase number of turns in the coil while keeping the length the same, decrease the length but keep the coils the same and adding an iron core.
What is the motor effect?
Where a current carrying wire in the presence of a magnetic field will experience a force.
What does the size of the force in the motor effect depend on?
The size of the force depends on the angle between the wire and the magnetic field lines. The force is greatest when the wire is perpendicular to the magnetic field, an zero when the wire is parallel to the magnetic field lines.
What is Fleming’s left hand rule?
First Finger=Field
SeCond finger=Current
ThuMb=Movement
What is the magnetic flux density and its formula?
The measure of the strength of the magnetic field. Force=Magnetic flux density, B(tesla) x current, I(amps) x length, l
What is an electric motor?
It is designed to use the motor effect by having two wires connected with different directions of current, so it rotates.
What does a split-ring commutator do?
It reverses the direction of current in a motors coil every half term so it continues in the same direction.
What is electromagnetic induction?
It is when a potential difference is created across ends of the wire when it crosses through the magnetic field lines.
What is the generator effect?
It is where you move a wire through a magnetic field, which induces a potential difference in the wire, and if the wire is connected in a full circuit it causes a current as electrons would be able to flow around the circuit.
What is an alternator?
A coil of wire that rotates in a magnetic field. It is connected to 2 metal rings called commutators. It produces an alternating P.D and an alternating current.
How does a moving coil microphone work?
It is a coil connected to a sheet of plastic called a diaphragm on one end and a permanent magnet on the other end. When sound waves hit the diaphragm it causes it to move the wire back and forth on the magnet, inducing a potential difference which is changing.
What is a step up and step down transformer?
Step up are used to increase the size of a potential difference. Step down are used to decrease the size of a potential difference.
What is a transformer made of?
It is made of an iron core with a primary coil and a secondary coil.
Why does the secondary coil get the AC supply as well?
As the alternating current passing through the primary coil generates an alternating magnetic field in the iron core. The lines of the alternating field pass through the secondary coil so the magnetic field in the secondary coils induces the alternating PD.
What is the transformer equation?
PD across primary coil/PD against secondary coil=number of turns on primary coil/number of turns on secondary coil
What is the transformer efficiency equation?
Primary potential difference x primary current = secondary potential difference x secondary current
Why is high potential difference desired?
This is to get rid of the heating effect, with less power being wasted making it more efficient.
What is the basis of electromagnetism?
Where a current produces their own magnetic field.
What happens if you change the current of a solenoid?
The magnetic field increases in strength if the current is increased and reverses its direction if the current is reversed.
How does the motor effect work?
A current carrying wire has a magnetic field is put into the magnetic field of two other magnets. These magnetic fields interact and results in a force on the wire pushing it out of the field.
How can you control the movement of an electric motor?
You can control the speed of it by changing the current and reverse the direction by reversing the current.
How can we increase rotations in a motor?
Increase the current, add more turns to the coil(wire) and use stronger magnets.
What happens when the wire in a generator stops moving?
The potential difference disappears.
What happens in a generator when you change the direction of the wire?
The direction of the potential difference swaps.
How else can you use the generator effect?
By moving the magnets around the wire instead of moving the wire around the magnets as there is still a change in magnetic field.
What happens to the generator effect if you move the wire back and forth?
There is no induced potential difference or current as the wire is not experiencing a change in magnetic field.
How can we change the size of the induced potential difference from the generator effect?
Use stronger magnets, move the wire or magnets more quickly and shape the wire into a coil so it has more turns.
What do commutators do?
They allow the current to pass out of the coil.
When is the maximum P.D in a AC generator?
When the wire is going directly through the magnetic field at the fastest possible rate, so it is moving perpendicular to the magnetic field.
When is the minimum P.D in a AC generator?
When the coil is vertical as the P.D falls to 0, due to it being parallel to the direction of the magnetic field so is not cutting through any magnetic field lines
What happens to P.D as the coil rotates in an AC generator?
It continues but it has a reversed direction as the two sides of the coil are moving opposite to how they were before.
How to increase the size of an AC?
Increase the strength of the magnetic field, increase the number of turns of the coil, increase the area of the coil and increase the rotation speed of the coil.
What is a dynamo?
It has a split ring commutator, which is split into two and connects onto the both ends of the coil. As the coil turns, the split ring commutator does not turn.
What does a dynamo cause?
The direction of the potential difference and current does not reverse when the coil rotates.
What happens to the P.D of a dynamo?
It has two peaks for each rotation of the coil as each side of the coil passes through the magnetic field twice during each rotation.
What happens with the P.D from the microphone?
The changing pattern of P.D is passed through an amplifier onto a moving coil loudspeaker.
What does a moving coil loudspeaker consist of?
It consists of a cone connected to a coil which is joined up with a AC supply. The cone is also connected to a permanent magnet.
How does a moving coil loudspeaker work?
As the coil has a current passed through, it generates a magnetic field, which interacts with the magnetic field from the magnet which either attract or repel each other, causing the cone to move. When the current changes direction, the direction of the force on the cone reverses, generating sound waves.
What does changing the frequency of the AC supply on a loudspeaker do?
It changes the frequency at which the cone vibrates, producing different pitches.
What does changing the current of the AC supply on a loudspeaker do?
It increases the amplitude of the vibration which changes the volume of the sound.
How does a transformer work? (part 1)
An alternating potential difference is applied across a primary coil, causing a current to flow, generating a magnetic field around the coil. However as it is alternating, the direction of the magnetic field and current will also alternate
How does a transformer work? (part 2)
The alternating magnetic field in the primary coil produces an alternating magnetic field in the iron core. This induces the potential difference in the secondary coil.
Why does the current flow in the secondary coil?
As it is part of a circuit, the potential difference causes current to flow.
How does a transformer work? (part 3)
This changes the P.D as there are different numbers of turns on the primary and secondary coil. More turns = higher P.D.
