Magnetism and electromagnetism (Permanent and induced magnets (All magnets…
Magnetism and electromagnetism
Permanent and induced magnets
All magnets have 2 poles- north and south (north/ seeking). They produce a magnet field- region where other magnets/ magnetic materials experience a non-contact force.
Magnetic field is shown by drawing magnetic field lines. Always go from north to south and show which way a force would act on a north pole if it was put at that point of a field.
The closer the lines are, the stronger the magnetic field. The further away from a magnet you are, the weaker the field.
Magnetic field is strongest at the poles of a magnet- magnetic forces also strongest at poles. The force between a magnet and magnetic material is always attractive, no matter the pole.
If 2 poles of a magnet are put near each other, they will exert a force; attractive (unlike) or repulsive (same).
Inside a compass is a tiny bar magnet. North pole of magnet is attracted to south pole of any other close magnet. So compass points in the direction of the magnetic field it's in. You can move a compass around a magnet and trace its position on some paper to build a picture of what the magnetic field looks like; point north when not near magnet. This is because the Earth generates its own magnetic field which shows the core of the Earth must be magnetic.
2 types of magnet:
Permanent- Produce own magnetic field.
Induced- magnetic materials that turn into a magnet when they're put into a magnetic field.
The force between permanent and induced magnets is always attractive when you take away magnetic field, induced magnets quickly lose magnetism and stop producing a magnetic field.
When a current flows through a wire, magnetic field created around the wire. The field is made up of concentric circles perpendicular to the wire with the wire in the centre. You can see this by placing a compass near a wire that is carrying a current. As you move compass, it will trace the direction of the magnetic field. Changing direction of current changes direction of magnetic field.
Right hand thumb rule (work out which way field goes):
Using right-hand, point thumb in direction of current and curl your fingers. The direction of fingers is direction of the field.
The strength of the magnetic field produced changes with the current and distance from the wire. The larger the current through the wire/ the closer to the wire you are, the stronger the field.
You can increase strength of magnetic field that a wire produces by wrapping the wire into a coil called solenoid. This is because the field lines around each loop of wire line up with each other which results in lots of field lines pointing in the same direction that are close to each other- strengthens field.
The magnetic field inside solenoid is strong and uniform ( same strength and direction at every point in that region). Outside the coil, the magnetic field is like the one in a bar magnet- ends of solenoid are north and south pole- work out which is which by the right- hand rule.
You can 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.
If you stop the current, the magnetic field disappears. A solenoid with an iron core (a magnet whose magnetic field can be turned on and off with an electric current) is called electromagnet.
The motor effect
When a current carrying wire (or other conductor) is put between magnetic poles, magnetic field around wire interacts with magnetic field it has been placed in- causes magnet and conductor to exert force on each other---motor effect which may cause wire to move.
To experience full force, wire has to be at 90 degrees to magnetic field. If the wire runs parallel to magnetic field, it won't experience any force. At angles in between, feel some force. Force always acts at right-angles to magnetic field of magnets and the direction of current in wire.
Show direction of current by applying current to a set of rails inside a horseshoe magnet. A bar is placed on rails which completes circuit. This generates a force that rolls the bar along rails. The magnitude (strength) of the force increases with strength of magnetic field. Force also increases with the amount of current passing through conductor.
Force acting on a conductor in a magnetic field depends on:
Magnetic flux density- how many field lines there are in a region.
The size of the current through the conductor.
The length of the conductor that's in the magnetic field.
When a current is 90 degrees to magnetic field it's in, force acting on it can be found using:
Find direction of force by Flemings left-hand rule:
Using left-hand, point first finger in direction of magnetic field. Pont second finger in direction of current. The thumb will then point in the direction of force (motion).
This shows that if current or magnetic field are reversed, can be used for motors.
On a basic dc motor, forces act on 2 side arms of a coil of wire that's carrying current. These are usual forces which act on any current in a magnetic field. Because coil is on a spindle and the forces act: one up, one down; it rotates.
The split-ring commutator swaps contacts every half turn to keep motor rotating in same direction. Direction of motor can be reversed by swapping polarity of dc supply (reversing current) or swapping magnetic poles over (reversing field).
Speed of motor can be increased by increasing current, adding more turns to the coil or increasing the magnetic flux density. Fleming's left-hand rule used to work out which way coil will turn.