Flashcards in Using magnetic fields to keep things working Deck (29):
What are the ends of a magnet called?
What is the magnetic field?
The ends of a magnet are called the magnetic poles, there is a north pole at one end and a south pole at the other.
The magnetic field is the region around a magnet in which a piece of iron or steel will be attracted to the magnet.
What will happen to iron filings placed near the magnet?
What are these lines called?
In which direction will a plotting compass placed in the magnetic field point?
Iron filings placed near a magnet will form a pattern of lines that loop from one pole to the other. These are lines of force or magnetic field lines.
A plotting compass placed in the magnetic field will always point along a field line.
If two magnets are brought together with like poles together, how will they behave?
If unlike poles are together how will the magnets behave?
They will repel each other.
They will attract each other.
What is produced around a wire when current flows through it?
When current flows through a wire, a magnetic field is produced around the wire.
How is an electromagnet made?
What happens when a current flows through the wire?
What happens when the current is switched off?
An electromagnet is made by wrapping insulated wire around a piece of iron, called the core.
When a current flows through the wire the iron becomes strongly magnetised, when the current is switched off, the iron loses its magnetism.
What makes electromagnets very useful?
Name four devices that use electromagnets.
Electromagnets are very useful because of the temporary magnetism when current flows through the wire.
1) Scrapyard cranes
2) Circuit breakers
3) Electric bells
What is the motor effect?
When is the force a maximum and when is it zero?
The motor effect is the force experienced by a wire carrying an electric current when it is placed in a magnetic field.
The force is a maximum if the wire is at an angle of 90º to the magnetic field and zero if the wire is parallel to the magnetic field.
For what is Fleming's left hand rule used?
How can it be represented on someones hand?
What does each of the fingers and the thumb represent?
Fleming's left hand rule is used to determine the direction of the force, in the motor effect.
The thumb and first two fingers on the left hand are all held at right angles to each other:
1) The first finger represents the magnetic field (pointing north to south)
2) The second finger represents the current (pointing positive to negative)
3) The thumb represents the direction of the force.
How can the size of the force in the motor effect be increased? (2)
1) Increasing the strength of the magnetic field.
2) Increasing the size of the current.
The direction of the force on the wire in the motor effect is reversed by what two things?
1) If the direction of the current is reversed.
2) If the direction of the magnetic field is reversed.
How can the speed of an electric motor be increased?
How can the direction of an electric motor be reversed?
The speed of the motor is increased by increasing the size of the current.
The direction of the motor can be reversed by reversing the direction of the current.
Why does the coil in a motor spin when a current passes through the coil? (2)
1) A force acts on each side of the coil due to the motor effect.
2) The force on one side of the coil is in the opposite direction to the force on the other side.
What does the split ring commutator do to the direction of the current?
Why is the coil always pushed in the same direction?
The split ring commutator reverses the direction of the current around the coil every half-turn.
Because both sides swap over each half-turn, the coil is always pushed in the same direction.
What happens if an electrical conductor 'cuts' through magnetic field lines?
If an electrical conductor 'cuts' through magnetic field lines, a potential difference is induced across the ends of the conductor.
What happens if a magnet is moved into a coil of wire?
What is this process called?
What happens if the wire or coil is part of a complete circuit?
If a magnet is moved into a coil of wire, a potential difference is induced across the ends of the coil. This process is called electromagnetic induction.
If the wire or coil is part of a complete circuit, a current passes through it.
In electromagnetic induction what will happen if the direction of movement of the wire or coil is reversed, or the polarity of the magnet is reversed?
When is a pd induced?
If the direction of movement of the wire or coil is reversed, or the polarity of the magnet is reversed, the direction of the induced pd is also reversed.
A pd is only induced while there is movement.
The size of the induced pd in electromagnetic induction is increased by increasing what three factors?
1) The speed of movement
2) The strength of the magnetic field
3) The number of turns of the coil
What does a transformer consist of?
What happens when an alternating current is passed through the primary coil?
What happens to this?
A trans former consists of two coils of insulated, called the primary coil and the secondary coil. These coils are wound on to the same iron core.
When an alternating current passes through the primary coil, it produces an alternating magnetic field in the core. This field continually expands and collapses.
What affect do the alternating magnetic field lines have on the secondary coil?
What if the the secondary coil is part of a circuit?
The alternating magnetic field lines pass through the secondary coil and induce an alternating potential difference across its ends.
If the secondary coil is part of a complete circuit and alternating current is produced.
Why are the coils of wire insulated?
Why is the core made of iron?
The coils of wire are insulated so that current does not short across either the iron core or adjacent turns of the wire, but flows around the whole coil.
The core is made of iron so it is easily magnetised.
What does a step-up transformer do?
What does a step-down transformer do?
A step-up transformer makes the pd across the secondary coil greater than the pd across the primary coil. Its secondary coil has more turns than the primary coil.
A step-down transformer makes the pd across the secondary coil less than the pd across the primary coil. It secondary coil has fewer turns than its secondary coil.
From what is the core made in a switch mode transformer?
What are the advantages of a switch mode transformer? (3)
A switch mode transformer has a ferrite core.
1) A switch mode operates at a much higher frequency.
2) It is lighter and smaller.
3) It uses very little power when there is no device connected its output terminals.
For what does the national grid use transformers and why is this necessary?
What are step-down transformers used for?
The national grid uses transformers to step up the pd from power stations, this is because the higher the pd at which electrical energy is transmitted across the grid, the less energy is wasted in the cables.
Step-down transformers are used to reduce the pd so that it is safe to be used by consumers.
What equation relates the pd across, and the number of turns on, the primary and secondary coils?
Vp/Vs = Np/Ns
Vp is the pd across the primary coil in volts, V
Vs is the pd across the secondary coil in volts, V
Np is the number of turns on the primary coil
Ns is the number of turns on the secondary coil
How efficient are transformers?
How is the efficiency of a transformer represented?
Vp x Ip = Vs x Is
Ip is the current in the primary in amperes, A
Is is the current in the secondary in amperes, A
What is an ECG used for?
An ECG or electrocardiogram is used to measure the potential differences generated by the heart.
What are electronic devices used for in hospitals?
What are digital thermometers used for?
Electronic devices are used to measure the blood pressure.
Digital thermometers are used to measure temperature.
What is an endoscope used for?
What are x-rays used for?
An endoscope contain bundles of fibre optics is used to look inside the body without making large incisions.
X-rays are used to take pictures of suspected broken bones.