P3 - Magnetic Fields Flashcards
How do you draw a magnetic field for a bar magnet?
You draw a metal bar with an N on one side and an S on the other
Draw magnetic field lines (with arrows) going from north to south over the top and under the bottom of the magnet and also spreading out from the north side and coming in from the south side
What is a magnetic field?
A region where magnetic materials and also wires carrying currents experience a force acting on them
How do you draw a magnetic field for a wire with a current?
You use the right hand rule (stick up thumb and curl fingers)
The thumb represents the direction of the current and the fingers represent the magnetic field going round the wire and that direction
In a diagram, the current and magnetic field lines are drawn as lines with arrows on them
What is an electromagnet?
A magnet with a magnetic field can be turned on and off with an electric current
Describe the electromagnet
Physically, an electromagnet is a coil of wire with a current flowing through it
The magnetic field inside the coil of wire is strong and uniform
Outside the coil, the magnetic field is just like the one round a bar magnet
How can you make an electromagnet have a stronger field?
Add a magnetically soft iron core through the middle of the coil
A magnetically soft material is one that magnetises and demagnetises very easily
What is the motor effect?
Passing an electric current through a wire produces a magnetic field around the wire
If you put that wire into a magnetic field, you have 2 magnetic fields combining which puts a force on the wire
Using the motor effect, what are the 2 ways that you can increase the force on the wire?
Increasing either the magnetic field or the current
How do we get the highest amount of force on the wire if we can’t change the magnetic field and current?
Using the Fleming’s left hand rule to work out the direction of the force according the direction of the magnetic field and current
If it is not quite at the right angle, you will still get some force on the wire but less
How do you know the direction of the magnetic field?
The direction of the magnetic field is from the north magnet to the south magnet
How do you use the Fleming’s left hand rule?
Put fingers on the left hand into practiced position
First finger points in direction of magnetic field
Second finger points in direction of current
Your thumb will point on the direction of the force
Describe how electric motors work
There are 2 side arms of a coil of wire between the north and south magnets
When the current flows through the coil and side arms the force on one side arm is up and one is down
The coil is on a spindle so this causes the coil to rotate
Every half turn, the split-ring commutator swaps round the contacts in order to keep the motor spinning in the same direction
What are the 2 ways that the direction of the motor can be reversed?
Swapping polarity of DC
Swapping magnet poles over
