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.
How do transformers work?
- when the ac flows through the primary coil, it produces a magnetic field that magnetises the iron core.
- because there is ac in the primary coil, the magnetic field in the iron core is alternating too
- the constantly changing magnetic field cuts through the secondary coil
- the changing field induces an alternating p.d. across the secondary coil by the generator effect
- if the secondary coil is part of a circuit, this p.d. causes a current to flow
What are step up vs step down transformers?
the one with a greater p.d. has more coils on its side
* step up has more on the secondary
* step down has more on the primary (less on the secondary)
