Topic 7- Magnetism and electromagnetism Flashcards
What is a magnetic field?
A magnetic field is a region where magnets, magnetic materials like iron and steel and also wires carrying a current, feel a force acting on them.
What rule is used to find the direction of a magnetic field around a wire?
Right hand thumb rule
What are the two types of magnet
permanent and enduced magnets
How can you increase the electrical field strength of a wire?
Wrapping it into a solenoid
What is an electromagnet?
A solenoid with an iron core
Describe how a simple dc electric motor works
- A loop of wire that’s free to rotate about an axis is placed in a magnet
- When a direct current flows through the loop, the two side arms, which are at 90° to the field, each experience a force due to the motor effect.
- They experience forces in opposite directions because the direction of the current in each arm is opposite.
- The loop will start to rotate around its axis because the forces act one up and one down.
- When the wire loop reaches a vertical position the forces will be acting one up and one down
- For the motor to keep rotating in the same direction, the forces acting on the arms of the loop need to swap direction.
- The split ring commutator does this and therefore allows the loop to continue rotating
How can the speed of a simple electric motor be increased?
- By increasing the current
- by increasing the strength of the magnetic field
How could you change the direction of a simple electric motor?
- swapping the polarity of the DC supply
- swapping the magnetic poles over
How does a loud speaker work
- alternating current sent through coil of wire attatched to the bass of the cone
- the coil surrounds pole of permanent magnet
- when the current reverses, the force acts in the opposite direction causing the cone to move in the opposite direction
- variations in the current cause the cone to vibrate which creates vibrations in air particles forming a sound wave
When does the generator effect happen?
When a conductor ‘cuts’ through the magnetic field lines. So it can be influenced in two ways:
- by moving the electrical conductor in a magnetic field
- by moving or changing a magnetic field
What are the two generators of electricity?
Dynamos and alternators
Explain how an alternator works
- as the coil spins, a current is induced in the coil
- every half turn, the current changes direction
- alternators use slip rings and brushes so that the contacts don’t swap every half-turn
- this generates alternating current
Explain how a dynamo works
- as the coil spins, a current is induced in the coil
- every half turn, the current changes direction
- dynamos use split ring commutators to make sure the current isn’t changing direction
explain how a microphone works
- soundwaves hit a flexible diaphragm that is attatched to a coil of wire wrapped around a magnet
- this causes the coil of wire to move back and forth which generates a current
What do transformers do?
Transformers can change the potential difference of an electrical supply using the generator effect. THEY WILL ONLY WORK WITH ALTERNATING CURRENT NOT DIRECT CURRENT.
