Physics: Magnetism and Electromagnetism - Unit 6

two poles of a magnet

North and South

how do these different poles of the magnet interact

• like poles repel

• unlike poles attract

permanent magnet

• a magnet that can produce it’s own magnetic field and will repel and attract other magnets when it is in contact with them

• it is made from a magnetic material

• it cannot be demagnetized

hard magnetic material

this are materials that get magnetised when an external magnetic field is applied and stay magnetised permanently when the external field is removed

soft magnetic material

this are materials that get magnetised when an external magnetic field is applied but they get demagnetised (lose their magnetic field) when the external field is removed

examples of magnetic materials

• iron

• cobalt

• nickel

• steel (iron+carbon)

magnetic field

this is a region surrounding a magnet where other magnets will experience a force

direction of the field lines in a magnetic field point

north to south

effect of the concentration of field lines

• if the there is a high concentration of field lines, that means that the magnetic field is strong

• if there is a weak concentration of field lines, that means that the magnetic field is weak

production of uniform magnetic field

to produce a uniform magnetic field, you place opposite poles of two magnets across each other, you create a uniform magnetic field where magnetic field lines are separated with a fixed distance between two magnets

how to use a plotting compass

• place a compass (containing a needle magnet) on a piece of paper near the field

• move the compass to different places on the paper

• draw an arrow in each position in the same direction as the needle points

• once you have gather enough data, you can join the arrows to make a complete field pattern

what must happen for a magnetic field to be created around a wire

• when current flows through a wire, it induces a magnetic field

Fleming’s Left Hand rule

relationship between current, magnetic field and force

direction wise, they all act perpendicular to each other

how to increase the magnitude of the force acting on a current-carrying wire inside a magnetic field

• increase the size of a current

• increase the strength of magnetic field (use a stronger magnet)

motor effect

• a current carrying wire is placed in a magnetic field and experiences a force that pushes the wire

• the field from the current carrying wire interacts with the other magnetic field (the magnetic field it is placed in)

• so a force is applied on the wire

• the force pushed the wire, therefore the wire rotates

how do loudspeakers use current to work

• coil is placed inside a magnet

• an alternating current in this coil causes a magnetic field

• when the field interacts with the magnet, a force is produced (motor effect)

• this force pushes and pulls the wire/cone of the speaker, causing it to move in different directions

• due to alternating current, the force causes the cone to vibrate

• this causes pressure variations in the air (sound waves)

how do microphones convert sound into electrical signals

• when sound is produced, the pressure variations in the waves cause the diaphragm to vibrate

• coil of wire is connected to the diaphragm, so as the diaphragm moves, so does the coil of wire

• the coil of wire is in a magnet so when it moves a voltage is induced in the coil

• as there is a complete circuit, a current is also created

how are electric motors kept rotating

a commutator is used, this switches the current direction every half turn, which ensures that the coil keeps spinning

production of an electric current using a magnet and a conductor on a small-scale

• moving a coil of wire into a magnet

• moving a magnet into a coil of wire

• once a voltage is induced, assuming there is a complete circuit, a current will also be induced

how is an electrical current produced on a large-scale

an electromagnet is rotated around a coil

factors affecting the size of an induced voltage

• number of turns on the coil of wire

• how strong the magnetic field is

• how fast you move the magnet

how does a dynamo generate current

• coil of wire rotates inside a magnetic field

• a commutator is used to ensure it continues rotating in the same direction, therefore keeping the current flowing in the same direction

how is an electromagnetic induction used in alternators to generate alternating current

• a coil of wire rotates in a magnetic field

• the end of this coil is connected to slip rings which will cause the current to change direction while rotating

• this means, alternating current is produced

shape of the magnetic field created around a straight wire when a current is running through

the shape of the magnetic field i circular from north to south

the right hand grip rule

• this is used to determine the direction of a current or magnetic field around a wire

solenoid

a coil of wire which turns into an electromagnet when there is a current flowing through it

how can you make a magnetic field in a solenoid stronger

• increase the current

• use more turns of wire

• use an iron core

transformer

this is a device which consists of a primary and secondary coil, which both surround an iron core

• a transformer changes the size of an alternating voltage

in a transformer, how does an alternating current in one circuit cause an alternating current in the other

• two coils of wire are placed by an iron core

• one of the coils produces a changing magnetic field when an alternating current is passed though it

• this field can be carried to the second coil through the iron core

• the iron core can increase the strength of the field, causing the magnetic field to pass through the other coil

• the magnetic field then causes a current in the next coil

what happens in a step-down transformer

• in a step-down transformer, the secondary coil has fewer turns

• advantage of a step down transformer: it means that the voltage can be reduced to a value safe enough to be used in houses

what happens in a step-down transformer

• it can increase efficiency as it decreases the heat loss in transmission lines

• this is because, for the same power, a higher voltage will lead to a lower current (P=VxI)

• the lower the current, the less energy that is lost since P=I²R

how does a step-up transformer work

• the primary coil has fewer turns than the secondary

• an alternating current is connected to the primary coil

• this produces a magnetic field

• the iron core causes that magnetic field to pass through the secondary coil

• the magnetic field induces a current in the secondary coil, but as it has more turns in the coil, the voltage will be higher

equation to determine the voltage/number of turns on primary and secondary coils

where are step-up and step-down transformers used in the national grid

• step-up transformers are used at power stations

• step-down transformers are used locally

equation for transformers with a 100% efficiency (using voltage and current)

advantages of transmitting power in high voltage cables

high voltage cables lead to a lower current in the wires, reducing any energy lost to the environment via heating, and increasing the efficiency since P=1²R