Topic 10 - Electromagnetic Induction and Magnets Flashcards
permanent magnet
a magnet that is always magnetic, such as a bar magnet
magnetic field
Space around a magnet where it can attract magnetic materials
induced magnet
When a piece of magnetic materials is in a magnetic field it becomes a magnet self
how can the shape of a magnetic field be found?
using a plotting compass
which ways do the lines in a magnetic field flow from
North to south
when is a magnetic field strongest
when the lines are closest together (closest to the poles)
How is the earths magnetic field caused
by electric currents in the molten outer core
How does the strength of the magnetic field depend on the size of the current?
The higher the current the stronger the field
electromagnet
A coil of wire with a current flowing through it
When is the magnetic field strongest around an electromagnet
It’s stronger closer to the wire and gets weaker with increasing distance
what is the magnetic field like around a wire
it’s in multiple circles around the wire
-> strength of magnetic field depends on size of current
-> if current changes direction so does the direction of magnetic field
solenoid
a wire made into a coil
what is the magnetic field of a solenoid like outside and inside the solenoid
The magnetic field from all the different parts of the wire form an overall magnetic field like the one around the bar magnet
-> the fields from individual coils add together to form a very strong field inside the solenoid
-> outside the solenoid the fields from one side of the coil tend to cancel out the fields from the other side to give a weaker field outside
how can the magnetic field of an electromagnet be made stronger?
- putting a piece of iron inside the coil -> this iron becomes a temporary magnet (it is only magnetic when the field from the electromagnet is affecting it)
- more coils of the wire
motor effect
when a wire with current flowing through it is placed in a magnetic field experiences a force - this effect is a result of 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, e.g. between two magnets
What magnetic fields are produced when a wire is placed between two magnets? (Force)
- The wire carrying a current experiences a force when it’s place between the two magnets as the current in the wire create a magnetic field around the wire and interact with the magnetic field between the magnets
- the force is greatest when the wire is at right angles to the magnetic field produced by the magnet and when the wire is in the same direction as this magnetic field
What is Flemming’s left-hand rule?
It shows how the directions of the force, magnetic field and current are related when a wire is placed in between two magnets
What’s the equation for the size of force on conductor carrying current at right angle to magnetic field?
Magnetic field strength (magnetic flux density) x current x length of wire
What causes an electric motor to rotate
The force on a conductor in a magnetic field is used to cause rotation in electric motors
What are ways to increase the total force turning the coil? (ELECTRIC MOTOR)
Using a coil with many turns of wire
function Split ring commutator in an electric motor
changes the direction of the current every half turn by swapping the positive and negative connections of the wire which ensures that the force acting on the coil always turn it in the same direction
When can a voltage or potential difference be induced (electromagnetic induction).
- when the wire experiences a CHANGE in the magnetic field
- If a wire is moved into a magnetic field or magnetic field is moved near the wire
-> this induced p.d. causes a current to flow -> only happens of ends of wire connect (close circuit)
size of induced p.d. depends on…
- number of turns in a coil of wire
- strength of the magnetic field
- how fast the magnetic field moves past the coil
What happens to the p.d. if you reverse the direction of the magnetic field change?
It reverses the direction of the induced potential difference
generator
A device that transforms mechanical energy into electrical energy typically by electromagnetic induction and it consists of a coil of water that is rotated inside a magnetic
Function of carbon brushes
to provide a good electrical connection between the coil and the external circuit
Function of slip ring in an alternator
to allow the alternating current to flow between the coil and the external circuit
alternator
Name given to generators that produce an alternating current
-> converts energy from motion into an electrical output
what is the need for the generator effect
- to create an a.c. in an alternator
- to create a d.c. in a dynamo
alternator structure
consists of: a rotating coil of wire between the poles of a permanent magnet and slip rings and brushes connected to an external circuit
Function of the rotating coil in an alternator
to cut the magnetic field as it rotates and allow an induced current to flow
operation of an alternator
- A rectangular coil rotates in a uniform magnetic field
- The coil is connected to an external circuit via slip rings and brushes
- A p.d. is induced in the coil as it cuts the magnetic field
- As a result of the alternating p.d., an alternating current is also produced as the coil rotates the coil rotates to 180° then rotated back to 0° as there’s no commutator switched the current every half turn
When is the maximum potential difference induced in an alternator?
When the position and motion of the coil is perpendicular to the magnetic field -> this is because the greatest number of lines are cut when the coil is moving perpendicular to the field
When is no p.d induced in an alternator
When the position and motion of the coil is parallel to the magnetic field -> this is because no lines are cut when the coil is moving parallel to the field
differences between a dynamo and an alternator?
- an alternator has slip rings, however a dynamo has a slip ring commutator
- An alternator produces an a.c whereas a dynamo produces a d.c
Why does a dynamo produce a d.c and not an a.c
It has a commutator which switches over the connections every half turn of the coil to create a d.c
Operation of a dynamo
- As the coil rotates, it cuts through the field lines -> This induces a potential difference between the end of the coil
- The split ring commutator changes the positive and negative connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction -> This happens each time the coil is perpendicular to the magnetic field lines
- Therefore, the induced potential difference does not reverse its direction as it does in the alternator
- Instead, it varies from zero to a maximum value twice each cycle of rotation -> This means the current is always positive or always negative
How do the forces in a d.c. motor cause it to rotate
- Forces act in opposite directions on each side of the coil, causing it to rotate:
-> On one side of the coil (e.g. left) the current always travels towards the cell so the force acts upwards (using Fleming’s left-hand rule) causing it to turn up
-> On the other side (e.g. right), the current always flows away from the cell so the force acts downwards causing it to turn down
• Once the coil has rotated 90° (vertical), the split ring is no longer in contact with the brushes -> No current flows through the coil so no forces act but even though no force acts, the momentum of the coil causes the coil to continue to rotate slightly until the split ring reconnects with the carbon brushes and current flows through the coil again - Now the sides of the coil have flipped but the direction of current is still the same because the commutator swaps the connection so that force is the same all throughout the rotation to cause a full rotation
How can the speed at which the coil rotates on a generator be increased?
- Increasing the current
- Increasing the strength of the magnetic field
How can the direction of rotation of coil in the generator be changed?
- Reversing the direction of the current
- Reversing the direction of the magnetic field by reversing the poles of the magnet
How can the force supplied by the generator be increased?
- Increasing the current in the coil
- Increasing the strength of the magnetic field
- Adding more turns to the coil
microphones
Convert the pressure variations in sound waves into variations in current in electrical currents
loudspeakers
- Loudspeakers and headphones convert electrical signals into sound -> They work due to the motor effect
- They work in the opposite way to microphones
- A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet
Microphone structure
Operation of a loudspeaker
- An alternating current passes through the coil of the loudspeaker -> This creates a changing magnetic field around the coil
- As the current is constantly changing direction, the direction of the magnetic field will be constantly changing
- The magnetic field produced around the coil interacts with the field from the permanent magnet
- The interacting magnetic fields will exert a force on the coil -> The direction of the force at any instant can be determined using Fleming’s left-hand rule
- As the magnetic field is constantly changing direction, the force exerted on the coil will constantly
change direction -> this makes the coil oscillate and the oscillating coil causes the speaker cone to oscillate -> this makes the air oscillate, creating sound waves
Operation of a microphone
- 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 in 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
national grid
Electricity that is sent from power stations to homeschools and factories by a system of wires and cables
step up transformer
Increases the voltage and decreases the current
step-down current
decreases the voltage, increases the current
Structure of a transformer
Made using two coils of insulated wire wound onto an iron core
Pylon function in the national grid
Support the cables
Generator function in the national grid
Generates electricity that is then transported around the country
Cables function in the national grid
Transport electricity around the county
Transformer function in the national grid
Increase and decrease voltage at particular stages in the National Grid
transmission lines
cables between pylons
why do transmission lines have a low current but a high voltage
If they had a high current and low voltage would lead to a really high resistance! This would mean that the cables would get very hot and a lot of energy would be lost in transporting the electricity. To stop this, the voltage is kept high and the current is kept low in the cable.
what type of current do transformers only work with
alternating currents
current
number of coulombs per second
electrical power equation
electrical power = current x p.d
electrical power = current squared x resistance
power equation
energy transferred
time taken
how does a transformer work
