P7 - magnetism and electromagnetism

227 - permanent and induced magnets

• permanent magnet is one that has its own magnetic field - a bar magnet

• induced magnet is a magnetic material like iron that become a magnet when they’re put in a magnetic field - meaning they’re placed by an induced magnet

• after the permanent magnet is taken away, induced magnets lose their magnetism and stop producing a magnetic field

• permanent magnets have a north and south pole

• the north is attracted to the south and the south is attracted to the north

• if the same poles are put together (south and south) they repel

• if something is a magnet, it will attract magnetic materials and not repel them, but they can both attract and repel magnetic materials based on their poles

• a magnet’s magnetic field can be shown on a diagram

• magnetic field lines go from north to south and show which way the force would go

• magnetic field lines also show the strength of a magnet

• direction is shown by the way the arrow is pointing and strength is shown by how close the arrows are to each other

• this means that magnetic fields and force are both vector quantities!

• magnets are always strongest at the poles

• compasses contain a bar magnet

• the north pole of the magnet inside the compass is always attracted to the south pole of any magnet it is close to, meaning it points in the direction of the magnetic field it is in

• when they’re not near a magnet, compasses always point north because the earth’s core generates its own magnetic field, which shows earth’s core is magnetic

228 - electromagnetism

• you can use the right-hand thumb rule to find out which direction the magnetic field is going in when a current is present

• if the current is going up, make a thumbs up and look at the direction your fingers curl in, this is the direction of the magnetic field

• a bigger current creates a bigger magnetic field - they are directly proportional

• this is because a current contains moving electrons, the bigger the current the more moving electrons

• electrons have a spin, which produces a tiny magnetic field

• when there’s lots of electrons in a current, there’s more alligned ‘spins’ so therefore there’s more tiny magnetic fields which build up to make a strong magnetic field around the wire carrying the current

• a solenoid is a wire that is wrapped in a coil

• the solenoid is only magnetic so long as a current is flowing through it providing it with a magnetic field - making it behave like an induced magnet

• this produces a strong magnet, as there are lots of field lines close together acting in the same direction

• the magnetic field of a solenoid looks like that of a bar magnet (you can figure out which are the north and south poles using the right hand rule) - it has close field lines coming out of each end of the solenoid, all acting in the same direction, with other close field lines moving left to right

• the magnetic field inside the solenoid is strong and uniform

• outside the coil it is the same as a bar magnet

• you can increase the strength of a solenoid by adding a soft iron core into the center of the coil, this becomes an induced magnet when current is flowing

• a solenoid with a soft iron core is only magnetic until the current is turned off , as this makes the magnetic field disapear making it an ELECTROMAGNET

229 - the motor effect

• when a current carrying wire is placed between magnetic poles, their fields interact with one-another and exert force on each-other, causing the wire to move

• the wire has to be at 90 degrees (perpendicular) to the magnetic fields to feel the full force, this is because the force is acting at it’s maximum angle to the current

• if the wire runs parallel to the magnetic field it wont feel any force at all

• at angles in between, it will feel some force

• the magnitude/strength of the magnetic field increases with the magnitude/strength of the force

• the force also increases with the amount of current flowing through the wire/conductor

• you can see the direction of the force using a horseshoe magnet, which is a permanent magnet with rails placed between the poles, a bar (the conductor) is placed on the rails and completes the circuit, the magnetic field produced by the bar interacts with the magnetic field produced by the horseshoe magnet, causing the bar to roll along the rails

• the bar becomes an induced magnet and completes the circuit, as the rails are made of a conductive material and connected to a power supply, providing it with a current which travels into the bar, completing the circuit when it’s placed on the rails and producing its own magnetic field due to the current flowing through it

• you can calculate the size of the force acting on a conductor in a magnetic field with the equation - force (n) = magnetic flux density (T tesla) x current (A) x length (m)

230 - electric motors

• you can use Fleming’s left hand rule to find the direction of a force exerted on any current-carrying conductor in a magnetic field

• first finger = direction of magnetic field

• second finger = direction of the current

• your thumb will then point in the direction of the force/motion

• this shows that if the direction of the magnetic field or current are reversed, so will the direction of the force/motion

• this can be used with motors

• inside the motor there is a coil of wire with a current flowing through it

• this means that the coil produces a magnetic field

• the coil’s magnetic field interacts with the magnetic field of the permanent magnets or electromagnets (depending on the motor) - this means force is exerted as the fields interact which cause the coil to rotate

• a split-ring commutator is used to reverse the direction of the current at the right times, keeping the coil spinning continuously in the same direction

• the constant switching of the direction of the current means that the force always pushes the coilin the same direction, making sure it keeps rotating , which allows the motor to work