Using Magnetic Fields to Keep Things Moving Flashcards
What is a magnetic field?
The region around a magnet in which a magnetic material (e.g. iron) will be attracted to the magnet
Define ‘line of force’
Line in a magnetic field along which a magnetic compass points.
Also called magnetic field lines.
Describe the force between two magnets
Like poles repel, same poles attract
Give 5 examples of where electromagnets are used.
- Scrapyard cranes
- Circuit breakers
- Electric bells
- Relays
- Fire alarms
What happens when a current flows through a wire?
A magnetic field is created
How are electromagnets made?
- Insulated wire is wrapped around a piece of soft iron, this will become 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
This temporary magnetism makes electromagnets very useful
What is the motor effect?
When a current is passed along a wire that has been placed inside a maganetic field, a force is exerted on the wire by the magnetic field.
This only works if the wire is not parallel to the lines of the magnetic field
When is the force felt by the wire in a magnetic field greatest?
When the wire is at 90° to the magnetic field (perpendicular)
When is the force felt by the wire in a magnetic field smallest?
When the wire is parallel to the magnetic field
What is Fleming’s Left Hand Rule used to determine?
The direction of the force felt by a wire expiriencing the motor effect (in a magnetic field)
In what position are the fingers in Fleming’s Left Hand Rule at in accordance to one another?
They are at right angles to each other (perpendicular)
What do the fingers in Fleming’s Left Hand Rule represent?
- Thumb - Thrust (direction of force)
- First Finger - Field (north to south)
- Second Finger - Current (positive to negative)
How can the force felt through the motor effect be increased?
Increasing the strength of the magnetic field
Increasing the size of the current
How can the direction of the force on the wire undergoing the motor effect reversed?
The direction of the current is reversed
The direction of the magnetic field is reversed
What happens to the direction of the force on a wire carrying a current in a magnetic field if:
Both the current and magnetic field are reversed?
It stays the same
What happens to the direction of the force on a wire carrying a current in a magnetic field if:
The direction of the current changed?
It is revesed
What happens to the direction of the force on a wire carrying a current in a magnetic field if:
The direction of the magnetic field is changed?
It reverses
How can the direction of a motor be changed?
By reversing the direction of the current (or magnetic field, but current is more easy/more common)
How can the speed of a motor be increased?
By increasing the size of the current (or magnetic field, but current is easier)
When a current passes through the coil of a motor, why does the motor spin?
Because a force acts on each side of the coil due to the motor effect.
The force on one side of the coil is in the opposite direction to the force on the other side.
This causes the motor to spin.
What is a split-ring commutator?
Metal contacts on the coil of a DC motor that connects the rotating coil continuously to its electrical power supply.
What does a split-ring commutator do?
Reverses the direction of the current around the coil every half-turn.
Because the sides swap over every half-turn, the coil is always pushed in the same direction.
What happens when an electrical conductor ‘cuts’ through magnetic field lines?
A potential difference (voltage) is induced across the ends of the conductor.
What is electromagnetic induction?
The process of inducing a potental difference (voltage) in a wire by moving the wire so it cuts across the lines of force of a magnetic field
What happens when a maget is moved into a coil of wire?
A potential difference (voltage) is induced across the ends of the coil.
This is electromagnetic induction.
If the wire or coil is part of a circuit, the current passes through it.
In an electromagnet, what happens if the movement of the wire/coil is reversed?
The direction of the induced potential difference (voltage) is also reversed.
In an electromagnet, what happens if the polarity of the magnet is reversed?
The direction of the induced potential difference (voltage) is also reversed.
When can a potential difference only be induced?
When there is movement.
How can the size of the induced potential difference (in an electromagnet) be increased?
By increasing:
- The speed of movement
- The strength of the magnetic field
- The number of turns on the coil
Why is there no potential difference induced when a bar magnet is held stationary inside a coil of wire?
Because no magnetic field lines are being cut
What is the effect on the induced potential difference of reversing the direction of the current in a conductor cutting magnetic field lines?
The direction of the induced potential difference is reversed
When an electromagnet is being used to induce a potetial difference, what state does it need to be in?
On.
What does a transformer consist of?
- Two coils of insulated wire called the primary and secondary coils.
- These are wrapped around a soft iron core
How do transformers work?
- When an AC passes through the primary coil, it produces an alternating magnetic field in the core. This field continually expands and collapses.
- 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, an AC is produced.
Why does a transformer only work on AC?
Because a changing/alternating magnetic field is needed to induce AC in the secondary coil.
Do all transformers have the same type of core?
No.
A switch-mode transformer has a ferrite core
What type of core does a switch-mode transformer have?
A ferrite core
Why are the wires in a transformer insulated?
So that the current does not short across either the iron core or the adjacent turns of wire, but instead flows around the whole coil.
Why is the core of a transformer made from soft iron?
Because it is easily magnetised
Where are transformers used?
The National Grid
What is a step-up transformer/ what does it do?
Makes the potential difference across the secondary coil greater than the potential difference across the primary coil.
Its secondary coil has more turns than its primary coil.
What is a step-down transformer/ what does it do?
Makes the potential difference across the secondary coil less than the potential difference across the primary coil.
Its secondary coil has fewer turns than its primary coil.
What are the features of a switch-mode transformer compared to a traditional transformer?
- Operates at a much higher frequency
- Lighter and smaller
- Uses very little power when there is no device connected across its output terminals
What does the National Grid use step-up transformers for?
Stepping-up the potential difference from power stations.
Why does the National Grid use step-up transformers?
To increase the potential difference at which electrical energy is transmitted. This is done because the higher the pd at which electrical energy is transmitted, the smaller the energy wasted in the cables.
Why are step-down transformers used in the National Grid?
To reduce the potential difference so that it is safe to be used by consumers.
Does a step-down transformer have more or less turns on the secondary coil than the primary?
A step-down transformer has fewer turns on the secondary coil than the primary
Does a step-up transformer have more or less turns on the secondary coil than the primary?
A step-up transformer has more turns on the secondary coil than the primary.
How efficient are transformers (when used in calculations - specifically the formula Vp X Ip = Vs X Is)
100% efficient
In the equation: Vp X Ip = Vs X Is, what are V, I, and <em>s</em> and<em> p</em>?
- V* = Voltage/pd across either primary or secondary coil in volts
- I* = Current across either primary or secondary coils in amperes
<em>p</em> and <em>s</em> = Primary or secondary coils repsectively
