Electromagnets Flashcards
Which metals are magnetic?
- Iron
- Cobalt
- Nickel
- Steel
Which direction do magnetic field lined flow in?
From north to south.
What is a permanent magnet?
A material that produces its own magnetic field.
What is an induced magnet?
A material that becomes a magnet when placed in a magnetic field.
What causes the Earth’s magnetic field?
The molten iron outer core.
Why do compasses point to the north pole?
It is the magnetic south pole.
What is a magnetic field?
The area around a magnet where a force will be exerted on another magnet/magnetic material.
Which end is the south pole of a solenoid?
The end where the current flows clockwise.
Which end is the north pole of a solenoid?
The end where the current flows anti-clockwise.
How can the strength of a solenoid by increased?
- Increasing the number of turns
- Increasing the current
- Adding an iron core
What is Fleming’s left hand rule?
Thumb: Motion
First finger: Magnetic field
Second finger: Current
How can the force on a wire in a magnetic field be increased?
- Increasing the current
- Increasing the magnetic flux density
- Increasing the length of wire in the magnetic field
What happens if current flows through a wire in the same direction as the magnetic field?
The force on the wire is 0N
What is the motor effect?
Force acting on a wire placed in a magnetic field when current flows through it.
Why is a split ring commutator used in a DC motor?
It reverses the direction of the current every half turn so the coil experiences a force in the same direction, despite each side cutting through the magnetic field line in the opposite direction.
Why does the motor continue spinning even though the brushes disconnect every half turn?
Its momentum keeps it spinning until the brushes reconnect.
Why does the coil of a motor spin?
- The current in the coil creates a magnetic field
- This interacts with the permanent magnet
- The current flows in opposite directions on the two sides of the coil
- The forces act in opposite directions, making it spin.
How does a loudspeaker work?
- AC current in coil reverses poles of solinoid
- Interaction between magnetic fields causes vibrations in the cone
- This moves air molecules, creating the pressure variations needed for sound waves.
What is electromagnetic induction?
Potential difference induced in a conductor when it cuts through magnetic field lines, causing current to flow.
Why do you have to do work when moving a magnet through a coil of wire?
- The polarity of the solenoid will change to oppose the movement
- Energy used is transferred along the coil of wire
When moving a bar magnet in a coil of wire to induce a potential difference, how do you create a higher potential difference?
Moving the magnet faster.
What is an alternator?
An AC generator.
How can an induced current be increased?
- Increasing the speed of the coils rotation
- Increasing the strength of the magnetic field
- Increasing the number of turns in the coil
- Increasing the total area of the coil
What is a dynamo?
A DC generator.
Why are slip rings and brushes used in an AC generator?
- They ensure that the wires do not get tangled
- The brushes are an electrical contact
What happens if a DC generator is rotated twice as fast?
- The frequency is doubled
- The amplitude is doubled
Why are electromagnets used instead of permanent magnets for a scrapyard crane?
They can be turned off to drop objects.
How does a circuit breaker work?
- Push switch is held in place iron bolt
- Electromagnet is put into the circuit next to the bolt
- When the current gets too high, the magnet pulls out the bolt
How does an electric bell work?
- Electromagnet attracts metal arm
- Arm hits the gong and breaks the circuit connection
- This switches off the electromagnet
- Arm moves back
- The circuit is completed again
What is a relay switch?
- An electromagnetic switch
- Allows a small circuit to turn on/off a much larger circuit.
What are the 2 ways a relay switch can be used?
- A switch
- An amplifyer
How does a relay switch work?
- It has a coil of wire which becomes a temporary magnet
- This attracts a switch for a larger circuit, turning it on/off
How does a microphone work?
- Sound waves move diaphragm
- Diaphragm moves a coil, making it cut through magnetic field lines
- This induces a potential difference in the coil
- Electrical signal matches the sound waves
How does a transformer work?
- AC in primary coil induces a changing magnetic field in the core
- Secondary coil cuts through changing fie;d lines, so alternating potential difference is induced
- Number of turns on each side determines the voltage output
What does a step up transformer do?
Increases voltage and decreases current.
What does a step down transformer do?
Decreases voltage and increases current.
Which side is the primary coil on a step up transformer?
The coil with less turns on it.
Which side is the primary coil on a step down transformer?
The coil with more tuns on it.
What are the 2 uses of transformers?
- Increasing efficiency in power transmission
- Changing mains voltage for different devices
What are transformer cores made from?
Iron.
