P4 Flashcards
How do magnetic fields explain the behaviour of magnets?
-like poles repel, unlike poles attract just like electric charges
-you can explain the movement of magnets with magnetic field lines
-Field lines are a way of modelling a field
-Field lines represent magnetic flux and the number of field lines passing through a particular area is called a magnetic flux density
-we also call magnetic flux density, magnetic field strength
-the magnets move in the direction that makes the field lines shorten
-the density of the field lines show you the strength of the magnetic field
What is an induced magnet?
-A steel paper clip is attracted to a magnet
-even though a paper clip is not a permanent magnet, it is an induced magnet
-In the domain model of magnetism, a permanent magnet is made up of many small magnetic regions (domains) that all line up.
-steel or iron has regions that are not lined up, but when they are in a magnetic field they do line up
-in some (hard) magnetic materials, the domains continue to be lined up when you remove the magnetic field.
-in other (soft) magnetic materials the domains return to their original direction.
Why do compasses point “north”?
If you hang a magnet up it will line up so that one end of it points towards the magnetic North Pole.
-This point in the surface of the earth is not the same as the North Pole, which is the point about which the earth spins.
A compass points towards the magnetic North Pole
-it is a north seeking pole
-The earth behaves as if it has a large bar magnet at its centre
-this is a model of the earth and it’s magnetic field
-Scientists are not sure what causes the field
-It could be produced by convection currents in the molten iron core of the earth
-many compasses are “weighted” so that they lie horizontally. They need to be weighted because the earths magnetic field is like that of a bar magnet
- the angle between the field lines and a line horizontal to the surface of the Earth is the “dip”
-it is 90 degrees at the north and south magnetic poles, and zero at the magnetic equator. In the UK the field has a dip of about 70 degrees
What dimension is a magnetic field?
3 dimensional
What is the magnetic field around a wire?
An MRI scanner needs to produce a very strong magnetic field
-the field is stronger than any produced by a permanent magnet
-in 1820, Hans Christian oersted discovered that there is a magnetic field around a wire that is carrying a current
What does it mean if the current in a wire is coming towards you?
The field lines are anti-clockwise
What does the strength of the field around a wire depend on?
The strength of the field, which is called magnetic field strength, or magnetic flux depends on:
-the magnitude of the current: a bigger current = a stronger field
-the distance from the wire: nearer the wire = a stronger field
What is the strength of a magnetic field measured in and what are they like?
The strength of a magnetic field is measured in teslas (T).
-The strength of the Earths magnetic field is about 0.01 mT the strength of the field 1 cm from a wire carrying a current of 5A is about 0.1mT.
-A very strong permanent magnet has a magnetic field near its poles of about 1T
How does a solenoid enhance the magnetic effect?
-You can make a loop of wire that carries a current. The magnetic field in the centre is a straight line
-Many parallel loops make a coil. Or solenoid
-Adding together many fields produces a much stronger field than that of a single wire
-You can make the field even stronger by putting a magnetic material inside the core
-the field produces an induced magnet
-You can make an electromagnet that is much stronger than any permanent magnet.
-the electromagnets used in MRI scanners have a strength of 10T, and the strongest continuous magnetic field produced is over 40T
-You can model the direction of the current in wires using darts
What happens when you combine fields?
-You can combine the field due to a wire with the field due to a permanent magnet. This produces a force on the wire
Why is there a force on the wire?
-If you work out what is happening in the combined fields you can predict what will happen to the wire.
-When you put two fields together that are in the same direction they add up, but if they’d re in opposite directions they will cancel out
The force on a wire creates a catapult field:
-The current is into the paper and the uniform field is left to right.
-You get a catapult field with stretched field lines above the wire and fewer stretched lines below it
-the wire moves down, and all the field lines straighten
-the movement of the wire has acted to shorten the field lines
-the current, magnetic field, and force are all at right angles to eachother
How can you remember which way the wire moves?
-drawing the fields and combining them
Or
-use Flemings left hand rule
What is Flemings left hand rule?
-Thumb= force(movement)
-first finger = field (north to south)
-second finger = current (positive to negative)
-start by lining up your first finger with the field, then put your second finger in line with the current.
-the direction of your thumb tells you the direction of the force
How do you calculate the size of force?
-you know that the strength of the field around a wire depends on the current through it.
-the five in the wire depends on the current, the field that it is in, and the length of wire in the field (assuming the wire and the field are at 90 degrees)
-you need to be able to apply this equation:
Force on a conductor ( at right angles to s magnetic field) carrying a current (N) = magnetic flux density (T) x current (A) x length (m)
-this is how scientists define one ampere (amp).
-One amp is the size of current that produces a force of 0.2 micronewtons on two wires 1 metre apart
-you saw how parallel wires carrying current experience forces
What do you need to make a robot move?
An electric motor
How can you get a coil or wire to spin?
-A motor needs to spin
-You know that a combination of magnetic fields can cause a force on a wire that has current flowing in it.
-You can make a very simple motor by making a piece of wire into a loop and placing the loop in a magnetic field
-when you connect the wire to a battery a current flows
-one side of the wire goes upwards and the other goes downwards
-however this motor would not spin very well
-the coil will start to rotate back the other way as soon as it passes vertical position
How does an electric motor work?
-you need a way to keep the current moving in the right and out on the left all the time, but still allowing the coil to spin.
-you can do this with a split ring commutator
What does the split ring commutator do?
The split ring commutator
-enables the current to flow the same way from the battery, but change to different halves of the coil as it spins.
-this makes sure that the force on the left hand side of the coil is always upwards, and the force of the right hand side of the coil is always downwards
How do you change the speed of the motor?
By changing the:
-magnitude of the current flowing in the coil
-the strength of the magnetic field
-the number of coils of wire
-the length of coil
What is electromagnetic field induction?
Electromagnetic induction is one of the most important discoveries in the history of science
-you produce an induced potential difference across the ends of a wire
If the wire is in a changing magnetic field so that it cuts field lines, or flux lines.
-this can happen when you move a conductor in magnetic field or make a magnetic field change around a conductor
-one way of producing an induced potential difference is if there is a relative motion between the wire, or coil, and the magnets
-if the wire does not cut field lines there is no induced potential difference
-the wire cuts field lines as you move it
-the potential difference that you induce depends on the length of wire in the field, and the rate at which you cut field lines
How can you induce a potential difference across a coil?
you can induce a potential difference across a coil by moving a magnet
