Topic 12 - Magnetism And The Motor Effect Flashcards
When do poles attract and repel?
unlike poles attract
like poles repel
What is the difference between permanent and induced magnets?
Permanent magnets always make their own magnetic fields whilst induced magnets only create a magnetic field when in another magnetic field.
Describe the uses of permanent and temporary magnetic materials including cobalt, steel, iron and nickel
Fridge doors - permanent magnetic strip to keep it closed
Cranes - induced electromagnets to attract and move magnetic materials
Doorbells - electromagnets which turn on and off rapidly, to repeatedly attract and release an arm which strikes a metal bell to make a ringing noise.
Magnetics separators - used in recycling plants to sort metal items.
Maglev trains - magnetic repulsion to make trains float slightly above the track and propel them along.
MRI machine - use magnetic fields to create images of the inside of your body without having to use ionising radiation
Speakers and microphones
Describe the magnetic field of a bar magnet.
- The magnetic field lines never cross each other
- The closer the lines, the stronger the magnetic field
- The lines have arrowheads to show the direction of the force exerted on a magnetic north pole
- The arrowheads point from the north pole of the magnet to its south pole.
How do you use plotting compasses to show directions of field lines?
1) Put the magnet on a piece of paper and draw around it.
2) Place the compass on the paper near the magnet. The needle will point in the direction of the field line at this position.
3) Mark the direction of the compass by drawing two dots-one at each end of the needle.
4) Then move the compass so that the end of the needle is where the tip of the needle in the previous position and put a dot of the needle. Repeat this and then join the marks you’ve made-you’ll end up with a drawing of a field line around the magnet.
5) Repeat this method at different points around the magnet to get several field lines. Make sure you draw arrows from north to south on your field lines.
Explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic.
When they’re not near a magnet, compasses always point toward the Earth’s North Pole. This is because the Earth generates its own magnetic field (and the North Pole is actually magnetic south pole). This shows the inside (core) of the earth must be magnetic.
Describe how to show that a current can create a magnetic effect around a long straight conductor, describing the shape of the magnetic field produced and relating the direction of the magnetic field to the direction of the current
When a current flows through a long, straight conductor (e.g. a wire) a magnetic field is created around it.
The field is made up of concentric circles perpendicular to the wire, with the wire in the centre.
Changing the direction of the current changes the direction of the magnetic field - use the right-hand thumb rule is the way to go.
What does the strength of the field depend on?
The larger the current through the wire, or the closer to the wire you are , the stronger the field is.
Where is the magnetic field strongest with a solenoid?
Inside a the fields from individual coils
add together to form a very strong almost uniform field along the centre of the solenoid and cancel to give a weaker field outside the solenoid.
What happens when a current carrying conductor is put between magnetic poles?
A current carrying conductor placed between magnet poles experience a force and that an equal and opposite force acts on the magnet
Why are there forces when a current is place between two magnetic poles?
The two magnetic fields interact when a current is placed between magnetic poles causing a force.
Describe Fleming’s left-hand rule.
Fleming’s left-hand rule is used to find the direction of the force on a current-carrying conductor.
Using your left hand, point your FIRST finger in the direction of the magnetic field and your SECOND finger in the direction of the current. Your THUMB will then point in the direction of the Force (Motion).
How do you work out force?
Force = Magnetic flux density (tesla) x Current (Amps) x Length (m)
F = B (T) x I (A) x L (m)
What does force acting on a conductor in a magnetic field depend on?
MAGNETIC FLUX DENSITY - how many field (flux) lines there are in a region
SIZE OF CURRENT
LENGTH OF CONDUCTOR in the magnetic field.
Explain how the force on a conductor in a magnetic field is used to cause rotation in electric motors
These forces act in opposite directions on each side, so the coil rotates. The split-ring commutator is a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction.
