Magnetism and Electromagnetism Flashcards
1
Q
In what direction do magnetic field lines go
A
North to South
(away from N, towards S)
2
Q
Opposite poles…
A
…Attract
3
Q
Like poles…
A
…Repel
4
Q
What is a permanent magnet?
A
- always magnetic
- fixed poles
- produces its own magnetic field
5
Q
What is an induced magnet?
A
- materials that are magnetic but do not have fixed poles
- induced (made into temporary magnets) by stroking with a permanent magnet - when it is in a magnetic field
- they are only attracted by other magnets, they are not repelled
- they lose most or all of their magnetism when they are removed from the magnetic field
- eg nickel, iron, cobalt
6
Q
How are strong vs weak magnetic fields expressed in a diagram?
A
strong - lines close together, close to magnet
weak - field lines are far apart, far from magnet
- closer to the magnet = stronger field strength
7
Q
How do compasses work?
A
- The Earth’s core is magnetic so it creates a large magnetic field around the Earth
- Compasses have a magnet inside on a pivot. It aligns itself with the magnetic field lines from the Earth
- The compass needle points in the direction of the Earth’s magnetic field
- The geographic N is the magnetic S
- The geographic S is the magnetic N
8
Q
What happens when a current is passed through a wire?
A
A magnetic field is produced around it in a spiral.
Higher current=stronger magnetic field around it.
Greater distance from wire=weaker magnetic field.
9
Q
What is the right hand grip rule?
A
Thumb - direction of current
Rest of fingers - magnetic field direction
x - into the page
o - out of the page
10
Q
What is a solenoid?
A
- A long coil of insulated wire.
- When an electric current flows, the shape of the magnetic field is very similar to the field of a bar magnet.
- The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field
- The field inside a solenoid is strong and uniform.
11
Q
What is an electromagnet?
A
- A solenoid with an iron core (increases field strength as magnetic field lines pass through air more easily)
- When the current is switched off, the iron bar loses most of its magnetism
12
Q
How to increase strength of solenoid?
A
- add soft iron core in the middle
- increase current flowing through wire
- increase number of turns in the coil
13
Q
What are the uses of an electromagnet?
A
- cranes to attract and pick up magnetic material
- switches -> when turned on the solenoid will attract the switch/ when current is too large the switch is attracted to solenoid and breaks the circuit until it is manually reset.
- electric bell - When the electromagnet has pulled the striker towards the gong, the current stops to let the striker move back away from the gong again. The current restarts and the movement of the striker continues. The circuit is constantly made and broken
14
Q
What is the motor effect?
A
A current carrying witre experiences a force in a magnetic field.
This can make it rotate.
15
Q
How can the size of force created in the motor effect be increased?
A
- increasing the current
- using a stronger magnet
- putting the wire perpendicular to the magnetic field. If it is parallel, it experiences 0N of force.
16
Q
What is Fleming’s left hand rule?
A
thumb=force
index=field direction
middle=current (hold perpendicular to index)
(FBI)
align fingers with the 2 directions that are given in the question to find out the last variable.
17
Q
What happens in an electric motor?
A
- Circuit (rectangular coil) between two opposite poles
- current is passed through
- rotates due to the motor effect - b/c a force acts on each side of the coil, each in opposite directions
- the coil is connected to the battery by two metal/graphite brushes
- these press onto a split-ring commutator fixed to the coil
- the split-ring commutator reverses the current round the coil evry half-turn of the coil
- This means it is pushed in the same direction every half-turn
18
Q
What is the generator effect?
A
- if there is relative movement between a conductor and a magnetic field OR the magnetic field strength/direction changes, a p.d. is induced in the conductor
- If it is a complete circuit then a current will flow
- Current carrying wires produce another magnetic field - it will oppose the change inducing it, going in the opposite direction
19
Q
How does an alternator work?
A
- wire turns (same as in a motor) - due to a force on each wire in opposite directions causing it to rotate
- As wire turns, it cuts the magnetic field lines and so it experiences a change in magnetic field (relative motion)
- Due to the generator effect, potential difference is created
- This p.d creates a current in the opposite direction to what it was initially
- cycle continues
- The graph is a sine curve
- produces AC current
20
Q
The maximum potential difference or current on an alternator can be increased by:
A
- increasing the rate of rotation
- increasing the strength of the magnetic field
- increasing the number of turns on the coil
21
Q
How does a dynamo work?
A
- same process as an alternator
- contains a split ring commutator instead of slip rings
- changes the coil connections every half turn.
- As the induced potential difference is about to change direction, the connections are reversed.
- This means that the current to the external circuit always flows in the same direction
22
Q
How does a microphone work?
A
- 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 potential difference is induced in the coil
- the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
the changing size and direction of the induced current matches the vibrations of the coil (and sound wave) - the electrical signals generated match the pressure variations in the sound waves
23
Q
How does a loudspeaker work?
A
- a current in the coil creates an electromagnetic field
- the electromagnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
- the current is made to flow in the opposite direction
- the direction of the electromagnetic field reverses
- the force on the cone now pulls it back in
repeatedly alternating the current direction makes the cone vibrate in and out
the cone vibrations cause pressure variations in the air, which are sound waves
To make a loudspeaker cone vibrate correctly, the electric current must vary in the same way as the desired sound.
24
Q
What is a transformer?
A
a device that can change the potential difference or voltage of an alternating current
25
A step-up transformer...
increases the voltage
has more coils on the secondary coil
26
A step-down transformer...
reduces the voltage
has more coils on the primary coil
27
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
28
Why can transformers only work with AC?
because you need a changing magnetic field to induce p.d on the secondary coil.
29
What are transformers' roles in power?
- The National Grid carries electricity around Britain.
- The higher the current in a cable, the greater the energy transferred to the surroundings by heating.
- This means that high currents waste more energy than low currents
- To reduce energy transfers to the environment, the National Grid uses step-up transformers to increase the voltage from power stations to thousands of volts, which lowers the current in the transmission cables.
- Step-down transformers are then used to decrease the voltage from the transmission cables, so it is safer to distribute to homes and factories
30
What is a magnetic field
A region where magnets, magnetic materials and wires carrying currents experience a force acting on them
31
How to increase the strength of the magnetic field of a solenoid
By putting a block of iron in the centre of the coil
-> the iron core becomes an induced magnet whenever current is flowing + the magnetic field of the core and coil combine to make a stronger magnet overall
32
Electromagnet
Magnet whose magnetic field can be turned on and off by an electric current
33
Why are electromagnets useful
- they can be quickly turned on and off
- they can create a varying force
34
Use of electromagnets in relay switches
- relay switches join 2 circuits together so that turning on one causes the other to turn on too
- they are used to turn on very high current circuits using a low current circuit protecting the user
35
How relay switches work
- when the switch in the low current field is closed, current flows through the electromagnet, producing a magnetic field
- the electromagnetic attracts the iron contact which pivots and closes the contacts in the high current circuit
- current flows through the motor and the motor spins
- when the low current switch is opened the electromagnet stops pulling so the rocker returns and the high current circuit is broken again
36
Magnetic flux density
How many field lines there are in a region
