section 6 - electric and magnetic fields

Question 1

when does static build up

Answer 1

when there’s friction between insulators, as the electrons aren’t delocalised and so are moved from one object to the other

Question 2

when do sparks form

Answer 2

• as charge builds on an object, the potential difference between the object and the earth (which is 0V) increases
• a high potential difference causes a strong electric field between the objects
• the electric field causes electrons in the air particles to be removed (ionisation)
• air is usually an insulator, but when ionised it is much more conductive and current can flow through it. this is the spark!

Question 3

static electricity in spraying

Answer 3

• the spray gun is charged, which charges up the small drops of paint so that they repel each other, giving a fine and even spray
• the object being painted is given the opposite charge of the gun, so it attracts the paint and prevents waste
• parts of the object facing away from the spray gun still receive paint
• insecticide sprayers - similar except the crops aren’t given an opposite charge, they just charge by induction as the droplets come closer

Question 4

static electricity when refuelling cars

Answer 4

as fuel flows from the filler pipe into the fuel tank, static can build up

this can easily lead to a spark which may cause an explosion

Question 5

static on airplanes

Answer 5

as planes fly, the friction between the air and the plane causes the plane to become charged

the build up of static charge can interfere with communication equipment

Question 6

static and lightning

Answer 6

raindrops and ice bump together in storm clouds, which leaves the top of the cloud positively charged and the bottom of it negative

this creates a huge voltage and big spark, that can damage homes/start fires when it reaches the ground

Question 7

earthing

Answer 7

using a conductor to connect a charged object to the ground

• it provides an easy route for the static charge to travel into the ground so no charge can build up and give you a shock/form a spark
• electrons flow down the conductor to the earth if the charge is negative and vice versa
• important in fuel tankers because the fuel could explode

Question 8

direction of electric field lines

Answer 8

they go from positive to negative

Question 9

where do compasses point when they aren’t near a magnet

Answer 9

towards the earth’s North Pole (which is actually a magnetic South Pole), because the earth generates it’s own magnetic field

Question 10

magnetic materials

Answer 10

the main 3 magnetic elements: iron, nickel and cobalt

some alloys/compounds of these are also magnetic, like steel because it contains iron

Question 11

magnetically soft vs hard

Answer 11

magnetically soft materials- lose their magnetism quickly, eg. pure iron/nickel-iron alloys

magnetically hard materials - lose their magnetism more slowly eg. steel
- permanent magnets are made form magnetically hard materials

Question 12

uses of magnetic materials

Answer 12

• fridge doors - a permanent magnetic strip keeps it closed
• cranes - induced electromagnets attract and move magnetic materials (like scrap metals)
• doorbells - use electromagnets that turn on and off rapidly, repeatedly attracting and releasing an arm that strikes the metal bell to make it ring
• magnetic separators - used in recycling centres to remove magnetic materials
• maglev trains - use repulsion to make trains float and reduce friction
• MRIs - use magnetic fields to take images of your body without using ionising radiation

Question 13

wires and magnetic fields

Answer 13

a moving charge creates a magnetic field

• a current flowing through a long, straight conductor forms a magnetic field around it
• the field is made up of concentric circles, perpendicular to the wire, with the wire in the centre

Question 14

what is the motor effect

Answer 14

when a current carrying conductor (eg. a wire), is put between magnetic poles, the two magnetic fields interact and this results in a force on the wire

the force is strongest when the wire is at 90˚ to the magnetic field

Question 15

how does a split-ring commutator work

Answer 15

it swaps the contacts every half turn, to keep the motor rotating in the same direction

Question 16

solenoids

Answer 16

• inside, the field lines align and point in the same direction, almost forming a uniform magnetic field
• outside, the overlapping field lines cancel each other out, so the field is weak apart from at the ends

it is an electromagnet

Question 17

how do transformers work

Answer 17

• they use induction to change the size of the potential difference of an alternating current
• they all have 2 coils (primary and secondary), joined with an iron core
• when an alternating p.d. is applied across the primary coil, an alternating magnetic field is produced
• the iron in the core is a magnetic material that easily de/magnetises, the coil producing an alternating magnetic field causes the magnetisation in the core to also alternate
• this changing magnetic field induces a p.d. in the secondary coil

they are almost 100% efficient

Question 18

step up vs step down transformers

Answer 18

step up - increase the p.d., they have more turns on the secondary coil
step down - decrease the p.d., they have more turns on the primary coil

Question 19

dynamos

Answer 19

dynamos are d.c. generators

• they apply a force to rotate a coil in a magnetic field - their construction is v similar to a motor
• as the coil (or magnet) spins, a current is induces in the coil, this current changes direction every half turn
• they have a split-ring commutator that swaps the connection every half turn to keep the current flowing in the same direction

Question 20

alternators

Answer 20

they work similar to dynamos but:

• instead of a split-ring commutator, alternators have slip rings and brushes, so the contacts don’t swap every half turn
• this means that an alternator produces an alternating p.d. and therefore an alternating current, if the coil is part of a complete circuit

Question 21

microphones

Answer 21

• they use electromagnetic induction to generate an electrical signal
• sounds waves hit a flexible diaphragm, that is attached to a coil of wire which surrounds one pole of a permanent magnet and is surrounded by the other pole
• this means that as the diaphragm (and so the coil) moves, a current is generated in the coil
• this is how pressure variations in sound waves are converted into variations in current in a circuit

Question 22

loudspeakers

Answer 22

• the coil is wrapped around one pole of a permanent magnet, so the a.c. signal causes a force on the coil which moves the paper cone attached to the coil
• when the current is reversed, the force acts in the opposite direction
• this makes the cone vibrate, making the air around the cone vibrate and causing the variations in pressure that cause a sound wave

Question 23

generating electricity in power stations

Answer 23

• fuels are burnt in the boilers of big power stations, which is used to heat water and the steam turns a turbine
• the turbine is connected to a powerful magnet (usually an electromagnet), inside a generator - a huge cylinder wound with coils of copper of wire
• the coils are joined in parallel, to produce a single output from the generator

works similarly for hydroelectric, tidal and wind except the turbine just gets turned directly