magnetism and electromagnetism 6

Question 1

what do all magnets have?

Answer 1

north and south poles

Question 2

what are magnetic field lines?

Answer 2

used to show the size and direction of magnetic fields
always point from north to south

Question 3

what happens if you place two permanent bar magnets near each other (south and north)?

Answer 3

creates a uniform field between magnets

Question 4

how can a material become magnetised?

Answer 4

by placing an object in a magnetic field
domains align

Question 5

what are some properties of magnetically soft materials?

Answer 5

easy to magnetise and demagnetise
iron
electromagnets

Question 6

what are some properties of magnetically hard materials?

Answer 6

hard to magnetise and demagnetise
steel
permanent magnets

Question 7

whats the law of magnetism?

Answer 7

opposite poles attract
like poles repel

Question 8

do field lines go in or out of north and south poles?

Answer 8

south - in
north - out

Question 9

practical
finding magnetic field lines using a compass:

Answer 9

compass shows pattern and direction
- carry out on a blank piece of paper
- bar magnet is placed in the centre of the paper
- start at the edge of the north pole and put a dot where the arrow of compass Is pointing
- repeat this method until you get to the south pole

Question 10

practical
finding magnetic field lines using iron fillings:

Answer 10

iron fillings show the pattern
- place magnet under piece of paper
- sprinkle iron on top of paper
- iron become induced magnets and lines up with field
- tap paper until iron fillings create a clear pattern

Question 11

how do you create a uniform field with two bar magnets?

Answer 11

place two magnets next to each other with the north and south pole facing
must be close but not too close
field lines will be evenly spaced and parallel

Question 12

what do magnetic field lines never do?

Answer 12

intersect

Question 13

what is a magnetic field?

Answer 13

a region where magnetic materials experience a force

Question 14

like poles…

Answer 14

repel each other

Question 15

opposite poles…

Answer 15

attract each other

Question 16

how is magnetism induced?

Answer 16

• magnets affect magnetic materials and other magnets
• poles attract magnetic materials that aren’t magnets
• when magnetic materials are brought near to a magnet (into its magnetic field) that material acts as a magnet
• the original magnet has induced this magnetism
• the closer the magnet and the magnetic material get, the stronger the induced magnetism will be

Question 17

how does a current-carrying wire create a magnetic field?

Answer 17

• an electric current in a conductor produces a magnetic field around it
• the larger the electric current, the stronger the magnetic field
• the direction of the magnetic field depends on the direction of the current

Question 18

what is the magnetic field around a straight wire?

Answer 18

the field is made up of concentric circles with the wire in the centre

Question 19

what is the magnetic field around a flat circular coil?

Answer 19

similar to that of a bar magnet
there are concentric ellipses (stretched circles) of magnetic field lines around the coil

Question 20

what is the magnetic field around a solenoid?

Answer 20

the magnetic field inside a current-carrying solenoid (coil of wire) is strong and uniform
- outside the coil, the field is just like the one around a bar magnet
- this means that the ends of a solenoid act like the north and south poles of a bar magnet
this type of magnet is called an electromagnet

Question 21

how can you increase the strength of a magnetic field around a solenoid?

Answer 21

by adding a magnetically soft iron core through the middle of the core

Question 22

how does current in a magnetic field experience a force?

Answer 22

• when a current-carrying wire is put between magnetic poles, the two magnetic fields affect one another
• this results in a force acting on the wire
• this can cause the wire to move
• this is called the motor effect
• this is due to charged particles moving through a magnetic field experiencing a force as long as they are not moving parallel to the field lines

Question 23

explain the motor effect:

Answer 23

• the wire has to be at 90 degrees to the magnetic field (if the wire runs along the magnetic field it won’t experience any force at all)
• the force always acts in the same direction relative to the magnetic field of the magnets and direction of the current in the wire
• the strength of the force increases with the strength of the magnetic field
• the force also increases with the amount of current passing through the conductor
• reversing the current or the magnetic field also reverses the direction of the force

Question 24

what is fleming’s left-hand rule?

Answer 24

helps you work out motor effect

• LEFT hand
• first finger = direction of the field
• second finger = direction of current
• thumb = direction of the force

Question 25

what 4 factors can speed up the motor effect?

Answer 25

- more current - more turns on the coil - stronger magnetic field - a soft iron core in the coil

Question 26

what is a dc electric motor?

Answer 26

- when a force is acting on two sides of the coil - the forces are the usual ones that act on any current in a magnetic field - the coil is a spindle and the forces act one up and one down it rotates - the split-ring commutator is a clever way of swapping the contacts every half-turn to keep the motor rotating in the same direction - the direction of the motor can be reversed either by swapping the polarity of the DC supply or swapping the magnetic fields over

Question 27

how do speakers work due to the motor effect?

Answer 27

- AC electrical signals from an amplifier are fed to a coil of wire in the speaker, which is wrapped around the base of a cone - the coil is surrounded by a permanent magnet so that the AC signals cause a force on the coil and make it move back and forth -these movements make the cone vibrate and this creates sounds

Question 28

what is electromagnetic induction?

Answer 28

the creation of voltage (and maybe current) in a wire which is experiencing a change in the magnetic field

Question 29

what is the dynamo effect?

Answer 29

- using electromagnetic induction to generate electricity using energy from kinetic energy stores is called the dynamo effect

Question 30

dynamo effect: what are the two different situations where you get electromagnetic induction?

Answer 30

- electrical conductor (a coil of wire is often used) moves through a magnetic field - the magnetic field through an electrical conductor changes (gets bigger or smaller or reverses)

Question 31

dynamo effect: how can you test the two different situations where you get electromagnetic induction?

Answer 31

you can test by connecting an ammeter to a conductor and moving the conductor through a magnetic field if the direction of movement is reversed then the induced voltage/current will be reversed too

Question 32

electromagnetic induction: how can you get a bigger voltage?

Answer 32

- increase the strength of the magnet - increase the number of turns of the coil - increase the speed of movement

Question 33

how do AC generators work?

Answer 33

- generators rotate a coil in a magnetic field - their construction is pretty much like a motor - as the coil spins, a current is induced in the coil, this current changes direction every half-turn - instead of a split ring commutator AC generators have slip rings and brushes so the contracts don't swap every half-turn - power stations use AC generators to produce electricity, they just get the energy needed to turn the coil or magnetic field in different ways

Question 34

how do transformers work?

Answer 34

- transformers change the voltage of an alternating current - they all have two coils, the primary and secondary joined by an iron coil - when an alternating voltage is applied across the primary coil, the magnetically soft iron core magnetises and demagnetises quickly = this induces an alternating voltage in the secondary coil - the ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils

Question 35

what do step-up transformers do?

Answer 35

step-up transformers increase the voltage, they have more turns on the secondary coil than the primary coil;

Question 36

what do step-down transformers do?

Answer 36

step-down transformers decrease the voltage, they have more turns on the primary coil than the secondary

Question 37

how do you calculate the output voltage from a transformer from the input voltage and the number of turns on each coil?

Answer 37

input (primary) voltage / output (secondary) volatge = number of turns on primary / number of turns on secondary

Question 38

transformers are nearly 100% efficient so...

Answer 38

power in = power out primary voltage x primary current = secondary voltage x secondary current

Question 39

how have transformers made transmitting mains electricity more efficient?

Answer 39

step-up and step-down transformers are used when transmitting electricity across the country - voltage produced by power stations is too low to be transmitted efficiently so the lower the voltage the higher the current for a given amount of power therefore the current causes wires to heat up - a step-up transformer is used to boost the voltage before it is transmitted - step-down transformers are used at the end of the journey to reduce the voltage so its safer and more useful