Electromagnetism Flashcards
(36 cards)
How can you plot the magnetic field around a bar magnet?
- Place the bar magnet on a piece of paper.
- Place the compasses around the bar magnet and mark the direction of the needle on the paper.
- Remove the compasses and join up the points to show the magnetic field lines, adding arrows to show the direction.
The geographic south pole of the Earth is the magnetic ____ pole.
north
What’s a solenoid?
A coil of wire that acts like a magnet when a current is passed through it.
What’s an electromagnet?
A solenoid with an
iron core.
A magnet can be p__ or i__.
permanent
induced
What three things can you do to increase the magnetic field around a current-carrying wire?
- Increase the current.
- Move closer to the wire.
- Coil the wire into a solenoid.
Give two advantages of an electromagnet over a solenoid.
- It can be turned on and off.
- The strength of the electromagnet can be varied by changing the current.
Explain how a loudspeaker works. [4 marks]
- One iron core pole is surrounded by a coil and the other magnetic pole.
- This creates a magnetic field, and when a current flows through the wire, the magnetic fields interact which causes the wire to experience a force due to the motor effect.
- This causes the paper cone to vibrate forwards & backwards and so air particles vibrate at the same frequency as the AC. This creates variations in pressure of the air which form the sound waves.
- By changing the frequency of the AC, you can change the frequency of the sound wave produced (the frequency of the AC = the frequency of the sound wave produced).
Explain how a microphone converts electrical signals into sound waves. [4 marks]
- The sound waves cause the diaphragm to vibrate.
- This causes the coil to move in the magnetic field, cutting magnetic field lines.
- This induces a potential difference due to the generator effect.
- The coil is connected to a circuit, so a current is induced.
- When the coil moves backwards, the current reverses, so an alternating current is induced.
- The frequency of the ac is the same as the frequency of the sound wave.
Explain how an electric bell works. [4 marks?]
- When the switch is closed, a current flows through the circuit.
- The electromagnet turns on and becomes magnetised, creating an magnetic field which attracts the iron armature.
- This causes the contacts? to push together, which causes the hammer to hit the bell.
- However this breaks the circuit, so the electromagnet turns off - the armature is no longer attracted to the electromagnet, causing the arm to fall back to its original position.
- This completes the circuit, so the cycle repeats until the switch is opened.
Define the motor effect.
A current-carrying wire/conductor in a magnetic field experiences a force (the magnet & conductor exert forces on each other as the magnetic fields interact).
How can you increase the motor effect?
- Increase the strength of the magnetic field by using stronger magnets.
- Increase the current.
How do electric motors work?
The two sides of the current-carrying coil will experience forces in opposite directions due to the motor effect & Fleming’s left hand rule.
Why is a split-ring commutator used in a motor?
It reverses the direction of the current every half-turn so the coil continues to rotate in the same direction.
What is the pupose of the carbon/graphite brushes in a motor?
The two halves of the coil swap from one carbon brush to the other.
The carbon brushes are electrical conductors and complete the circuit as the wire rotates - if they weren’t used, the wire would all get tangled up.
Define the generator effect.
When a conductor moves relative to a magnetic field or a magnetic field changes around a conductor, magnetic field lines are cut so a potential difference is induced. If the conductor is connected to a complete circuit, a current is induced.
Give three ways you can increase the size of the induced potential difference/current from the generator effect.
- Increase the speed of the conductor’s movement.
- Increase the strength of the magnetic field.
- Increase the number of turns on the coil.
Give 2 ways to reverse the direction of the generator effect.
- Reverse the magnet’s polarity.
- Reverse the conductor’s direction of movement.
What is the difference between an alternator and a dynamo?
An alternator produces ac (the dirction of the current changes every half revolution) whereas a dynamo produced dc.
Explain how an alternator works.
- Coil of wire rotates in a magnetic field.
- The ends of the coil are connected to slip rings and carbon brushes, ensuring there’s a continuous connection allowing the induced current to flow through the coil in a circuit (due to the generator effect).
- There is no split ring commutator, so the direction of the current changes every half revolution - this is ac.
Explain how a dynamo works.
- Coil of wire rotates in a magnetic field.
- The ends of the coil are connected to a split ring commutator, which makes the ends of the coil swap contacts (from one carbon brush to the other) every half revolution.
- This means the induced current (due to the generator effect) reverses every half revolution so it is always flowing in the same direction - this is dc.
Give three ways you can increase the maximum potential different induced by an alternator.
- Increase the rate of rotations.
- Increase the strength of the magnetic field.
- Increase the number of turns on the coil.
When the coil in an alternator is at 0°/180° to the magnetic field, what is the induced potential difference?
0V as the coil moves parallel to the direction of the magnetic field - no magnetic field lines are cut.
When the coil in an alternator is at 90° to the magnetic field, what is the induced potential difference?
Maximum potential difference as the coil moves perpendicular to the direction of the magnetic field - the most magnetic field lines are cut.
