Topic 12 - Magnetism and the Motor Effect Flashcards
two unlike magnetic poles will…
attract
Two like magnetic poles will…
repel
What are the main 3 magnetic materials
- Iron (steel)
- Cobalt
- Nickel
What is the main difference between permanent and induced magnets?
Permanent magnets produce their own magnetic fields ALL THE TIME
Induced magnets only produce a magnetic field while they’re in another magnetic field
How does induction explain why a force between a magnetic material and a magnet is always attractive?
The south pole of the magnet induces a north pole in the material, (and the north pole of the magnet induces a south pole of the material)
How fast do magnetically ‘soft’ materials like iron and nickel iron alloys lose their magnetism?
Very quickly
How fast do magnetically ‘hard’ materials like steel lose their magnetism?
More slowly
What are permanent magnets made out of?
Magnetically hard materials
How are magnetic materials used in fridge doors?
There is a permanent magnetic strip in your fridge door to keep it closed
How do cranes move magnetic materials?
These use induced electromagnets to attract and move magnetic materials e.g. moving scrap metal in scrap yards
How are magnetic materials used in doorbells?
These use electromagnets which turn on and off rapidly, to repeatedly attract and release an arm which strikes the metal bell to produce a ringing noise
What are magnetic materials used for in recycling plants?
These are used, in recycling plants, to sort metal items
How are magnetic materials used in Maglev trains?
These use magnetic repulsion to make trains float slightly above the track (to reduce losses from friction) and to propel them along
How are magnetic materials used in MRI machines?
These use magnetic fields to create images of the inside of your body without having to use ionising radiation
In a bar magnetic, where is the magnetic fields/forces strongest at?
The magnetic fields and forces are strongest at the poles of a magnet
Describe how you could use a compass to show the directions of magnetic fields?
- Put magnet on a piece of paper and draw around it
- Place compass on the paper near magnet. The compass will point in the direction of the field line at this position
- Mark the direction of the compass needle by drawing two dots - one at each end of the needle
- Move the compass so that the tail end of the needle (in the compass) is where the tip of the needle was in the previous position and put a dot by the tip of the needle. - repeat this and join up marks you’ve made so you’ll end up with a drawing of 1 field line around magnet
- Repeat at different point around the magnet to get several field lines and draw arrows from N to S
How do we know that the earth’s core must be magnetic?
When compasses are not near a magnet, they always point towards the earth’s North Pole. This is because the earth generates its own magnetic field (and the north pole is actually a magnetic south pole)
How could you show that current flows a long straight conductor and a magnetic field is created around it?
The right hand thumb rule
- Point your thumb in the direction of current (upwards) and curl your fingers. The direction of your fingers is the direction of the field
Describe the correlation between the current through the wire and the strength of the field?
The larger the current through the wire, or the closer to the wire you are, the stronger the field is
What is a solenoid?
A long coil of wire
How can you increase the strength of the magnetic field produced by a length of wire?
By wrapping the wire into a long coil with lots of loops, called a solenoid
What happens inside the solenoid?
You get lots of field lines pointing in the same direction. The magnetic field is strong and almost uniform
What happens outside the coil?
The overlapping field lines cancel each other out - so the field is weak apart from the ends of the solenoid
A solenoid is an example of an ……
Electromagnet
What is an electromagnet?
A magnet with a magnetic field that can be turned on and off using an electric current
How can you increase the field strength of the solenoid EVEN MORE?
By putting a block of iron in the centre of the coil. This iron core becomes an induced magnet whenever current is flowing
When a current-carrying conductor (wire) is put between magnetic poles, the two magnetic poles….., The result is a …….. on the wire. The wire also exerts an …. and …. force on the magnet (newtons third law)
- interact
- force
- Equal and opposite
What angle does the wire have to be (to the magnetic field) in order to experience the full force?
90 degrees
Does the force always act in the same direction relative to the magnetic field, and the direction of the current in the wire and what does this mean?
Yes - this means changing the direction of the magnetic field or the current will change the direction of the force
What is Fleming’s left hand rule?
A rule to find the direction of the force on a current carrying conductor
1- Using your left hand, point you First finger in the direction of the magnetic Field and your seCond finger, in the direction of the Current
2- The thuMb should point in the direction of the Motion
What is the formula to work out the size of a force using current?
Force = magnetic flux density (how many field lines in a region) * current (through conductor) * length (of conductor in magnetic field)
In an electric motor, where would the forces act?
On the two side arms of a coil of wire that’s carrying a current. The forces act in opposite directions on each side, so the coil rotates
What is the split-ring commutator used for?
The split ring commutator is used to swap the contacts every half turn to keep the motor rotating in the same direction
How can the direction of the motor be reversed?
It can be reversed either by swapping the polarity of the d.c. supply (reversing the current) or swapping the magnetic poles over (reversing the field)
How could you work out which way the coil in an electric motor will turn?
By using Fleming’s left hand rule
