P15 - Electromagnetism Flashcards
Examples of magnetic materials
Iron, steel, cobalt and nickel
What is a permanent magnet
A magnet that produces its own magnetic field. Is usually made of steel as it doesn’t lose its magnetism easily
How can magnetic field lines be found?
- Iron filings
- Plotting compass which always points towards the South Pole
What is an induced magnet
A material that becomes a magnet when in a magnetic field. It always produces a force of attraction
What is a magnetic field
The region around a magnet where another magnet or a magnetic material experiences a force
What are magnetic field lines
- Show the direction of the force that would act on a north pole at that point
- They always point from a magnet’s north pole to its south pole
What does the strength of a magnetic field caused by a current in a wire depend on?
- The current through the wire
- The distance from the wire
How to remember the corkscrew rule
Right hand thumb rule
What is a solenoid?
A coil of wire that has a similar magnetic field to a bar magnet
What is the field like inside the solenoid?
Strong and uniform
Methods that increase the strength of a solenoid (3)
- Add an irone core (makes it an electromagnet)
- Increase current
- Tighter turns / more truns on solenoid
Devices where electromagnets are used
- Relays
- Electric bells
- Circuit breakers
How does a relay make use of electromagnets
- A relay contains 2 separate circuits
- There is a low voltage circuit containing an electromagnet which is safe to be switched on or off
- On the high voltage circuit there are 2 metal contacts, an iron block and a spring
- When the low voltage circuit is switched on, a current flows making a magnetic field around the electromagnet
- The electromagnet then attracts the iron block in the high voltage circuit
- This makes the contacts touch and a currnet can flow
How does a doorbell make use of an electromagnet
- The switch closes when the buzzer is pressed and a current can flow through the circuit
- The electromagnet then produces a magnetic field and the iron armature is attracted to the electromagnet
- When this happens, the hammer hits the bell and this breaks the circuit so no current flowing and no electromagnet
- This means the iron armature moves back to its original position and the circuit is complete again so the process repeats
Defenition of the motor effect
When a conductor carrying a current is in a magnetic field, the magnet and the conducter exert a force on each other
What factors increase the size of the force in the motor effect (3)
- Increasing the current
- Using a stronger magnet
- Length of the wire
What does each finger in Fleming’s left hand rule show
- Thumb - Force
- First finger - magnetic field
- Second finger - current
Equation that applies to Flemin’g left hand rule
Force = magnetic flux denisty x current x length
F = Bil
What is magnetic flux denisty and give its unit
The strength of the magnetic field. It is measured in tesla (T)
How does an electric motor work
- A loop of wire in a magnetic field rotates due to a force experienced on both sides of the wire - one side moves up and one side moves down creating a moment (turning force)
- When the coil gets to a vertical position, momentum keeps the wire turning and the split ring commutators change the direction of the current
- This keeps the motor rotating in the same direction
How does a moving coil loudspeaker work (5 steps)
- You have a cone with a coil or wire wrapped around one end and the coil is connected to an A.C. supply. A permanent magnet goes inside the coil of wire
- A.C through coil produces a magnetic field and this magnetic field interacts with the magnetic field of the permanent magnet
- Causes a resultant force due to motor effect and pushes cone out
- Current reverses, so force reverses, so cone moves back in
- This causes cone vibrations which are sound waves
What can you change to change the sound produced by a moving coil loudspeaker
- By changing the frequency of the A.C. supply, you can change the frequency of the cone vibrations and therefore the frequency of the sound waves
- If the size of the A.C. current changes, the amplitude and therefore volume of the sound changes
Defenition of electromagnetic induction
- Where a potential difference is induced across the ends of a conductor due to it moving through a magnetic field or the magnetic field is varying.
- When the condctor stops moving, the potential difference is lost
- If the conductor moves back the other away, a potential difference is induced again but it is in the opposite direction
What is the generator effect
- Same as defenition of induced electromagnetism
- If the conductor is connected to a circuit, an induced current is produced which creates its own magnetic field
- The induced current’s magentic field opposes themovement of the magnet (the original change)
- We only get the generator effect when the wire passes through the magentic field and not along it
These together make up the generator effect
What factors affect the direction of the induced current/ p.d. (2)
Generator Effect
- If the magnet is moved out of the coil
- If the other pole of the magnet enters the coil
What factors affect the size of the induced current/ p.d. (3)
Generator Effect
- Strength of magnetic field
- Number of turns on the coil
- Speed of magnet through coil
How does an alternator produce an alternating current
- An alternator is a coil or wire rotating in a magnetic field which is connected to 2 commutator rings
- One side of th wire always attaches to one commutator ring and the other side attaches to the other ring
- The commutators allow the current to pass out of the coil
- A potential difference is induced when the wire passes through the magnetic field
- This is alternating and as it is connected to a circuit, an A.C. is produced
Describe the potential difference graph produced by an alternator
- The maximum potential difference is when the coil is horizontal as the wire is moving through the magentic field at the fastest rate
- When the coil is vertical, the potential difference is 0 and the coil is moving parallel to the field
- As the coil moves round, we get a potential difference again but it has reversed direction because the two sides of the coil are moving in different directions to before
How does a dynamo produce a direct current
- It has a split-ring commutator so one side of the coil is connected to one side of the ring and vice versa
- A potential difference and current are induced
- When the coil is vertical, there is no potential difference as the coil is moving parallel to the magentic field
- When the coil moves round, the side of the coil moving down is connected to the same side of the split ring commutator and vice versa
- Therefore, the direction of the p.d. doesn’t change so the direction of the current doesn’t either
- 2 peaks equate to one rotation of the coil as each side of the coil passes through the field twice
How does a moving coil microphone work?
- The coil is attached to a diaphragm which is placed inside a permanent magnet
- Sound waves cause the diamphragm to vibrate
- This causes the coil to vibrate
- The coil moves in the magnetic field so a potential difference is induced across the ends of the wire
- The potential difference switches direction as the coil moves backwards and forwards
- The frequency of the changing potential difference is the same as the frequency of the sound waves
- The coil is part of a circuit so an induced current is produced
What is a transformer and how do they work generally
- 2 coils of wire - a primary coil and a secondary coil that are wrapped around an iron core (easily magnetised) which increases the strength of the magnetic field
- The primary coil is connected to an alternating current and as this flows through the coil, it generates an changing magnetic field
- This A.C. passes through the iron core and into the secondary coil and induces a potential difference in it
- Transformers can only work with A.C. as we need a changing magnetic field to induce a potential difference (generator effect)
Structure of a step up transformer
- The secondary coil has more turns that the primary coil
- This means that the induced p.d. in the secondary coil will be greater than the p.d. in the primary coil
Structure of a step down transformer
- The primary coil has more turns than the secondary coil
- Therefore, the induced p.d. in the secondary coil is less than the p.d. in the pirmary coil
Equation linking potential difference and turns in the coils of transformers
Vp/Vs = Np/Ns
Equation for the power in the coils in a transformer
- As P = VxI:
- Vp x Ip = Vs x Is
- This is bceause power is conserved between transformers (asssuming it is 100% efficient)
How are transformers used in the national grid
- First, a step up transformer increases the p.d. to 400000V
- This is because P = VI so to transfer the same amount of electrical power, a lower current is needed
- This makes the Natioanl grid more efficient because less current measn the cables are heated less so less energy is dissipated to the surroundings
- Then, a step down transformer decreases the voltage to 230V for UK homes
- This si to make it safer for everyday use
