P3 - Using Magnetic Fields Flashcards
How is an electromagnet made?
- Wrapping an insulated wire around an iron core.
- Passing a current through the wire magnetises the iron core.
Why is steel unsuitable for use in an electromagnet?
Steel keeps its magnetism when the current is removed.
Give four examples of everyday uses of electromagnets.
- Scrapyard crane
- Circuit braker
- Electric bell
- Relay
How does a circuit breaker work?
- There is a switch in series with the electromagnet.
- The switch is held in place with an iron catch.
- When too much current passes through the electromagnet, the switch is pulled open and has to be reset manually.
How does an electric bell work?
- When connected to the battery, the iron armature is pulled onto the electromagnet.
- The circuit is opened so the electromagnet is switched off and the armature springs back onto the bell.
- The armature then closes the switch making the process repeat.
How does a relay work?
- When current passes through the electromagnet, the armature is pulled onto the electromagnet.
- The armature turns about the pivot and closes the switch.
- The small current through the electromagnet is used to turn on a larger current.
How can the size of the force of the motor effect be increased?
- Increasing the current.
- Using a stronger magnet.
How does the size of the force of the motor effect depend on the angle between the wire and magnetic field lines?
- Greatest when the wire is perpendicular to the magnetic field.
- Zero when the wire is parallel to the magnetic field lines.
What happens to the forces in the motor effect when the current or the magnetic field is reversed?
The direction of the force is reversed.
How can the speed of an electric motor be changed?
Changing the current.
How can the direction of an electric motor be changed?
Reversing the current.
What is a simple electric motor made of?
- Rectangular coil of insulated wire.
- Two graphite brushes.
- Split ring commutator.
Why does the coil in a motor rotate?
- A force acts on each side of the coil.
- The force on each side is opposite.
What is the purpose of the split ring commutator?
- It reverses the current round the coil every half turn.
- This means the coil is pushed in the same direction.
Why does the diaphragm of a speaker vibrate?
- When a current passes through the coil, the force makes the coil move.
- When the current changes direction, the force reverses its direction making the coil move forwards and backwards.
- The vibrations produce sound waves.
What is electromagnetic induction?
The process of creating a potential difference using a magnetic field.
How can a magnet induce a potential difference?
When a conductor cuts the lines of a magnetic field, a potential difference is induced across the ends of the conductor.
How can an electromagnet induce a potential difference?
When an electromagnet is switched on, the magnetic field is broken by the conductor.
Who discovered electromagnetic induction?
Michael Faraday.
Why do transformers only work with AC?
A changing magnetic field is necessary to induce AC in the secondary coil.
What is a transformer for?
A device used to change the size of the alternating pd.
What is the core of a transformer made from?
An iron core unless it is a switch mode transformer which has a ferrite core.
Give two ways in which a switch mode transformer is different from an ordinary transformer.
- A switch mode transformer is lighter and smaller.
- It operates at high frequency.
Why is an alternating pd induced in the secondary coil of a transformer when alternating current passes through the primary coil?
- Alternating current passing through the primary coil produces an alternating magnetic field.
- The lines of the alternating magnetic field pass through the secondary coil.
- The magnetic field is changing.
What is a step-up transformer?
A transformer that makes the pd across the secondary coil greater.
Which coil has more turns in a step-up transformer?
The secondary coil.
What is a step-down transformer?
A transformer that makes the pd across the secondary coil smaller.
Which coil has more turns in a step-down transformer?
The primary coil.
Why wouldn’t a DC transformer work?
There would be no moving magnetic field.
Why is the iron core of a transformer laminated?
To cut out induced currents in the iron which would reduce efficiency.
What range of frequencies do switch mode transformers work in?
50,000-200,000Hz
Give two main differences between a switch mode transformer and regular transformers.
- It is lighter and smaller.
- It uses very little power when no device is attached.
What are the three main circuits in a phone charger?
- Frequency converter.
- Switch mode transformer.
- Low voltage ac to dc converter.
What is the equation for power in a transformer that is 100% efficient?
Vp x Ip = Vs x Is
What is the transformer equation?
pd across primary / pd across secondary = turns on primary / turns secondary
How is an alternating magnetic field set up in a transformer?
There is an alternating current.
Give three ways of transporting 500MW.
- 500,000,000V at 1A
- 1V at 500,000,000A
- 500,000V at 1000A
What is the problem with transporting at a high pd?
There would have to be a lot of insulation or the current would jump through the air.
What is the problem with transporting at high current?
Thick wires would be needed to prevent melting.
What is the relationship between current and energy loss?
- When the current is doubled, the heat energy produced is quadrupled.
- Heat energy produced is directly proportional to current flowing squared.
Why does the pd have to be kept high in the national grid?
To keep current as low as possible to prevent energy loss.
What is Fleming’s left hand rule?
- First finger field.
- Second finger current
- Thumb movement
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