Flashcards in paper 2 physics electricity Deck (60):
how should a plug be wired?
each wire is connected to the right pin &firmly screwed in
no bare wires are showing inside the plug
the cable grip is tightly fastened over the cable outer layer
what are the metal parts in plugs made from and why?
copper and brass-good conductors
what are the 3 wires in a plug and colours?
1)live wire-brown- alternates between high +ve and -ve voltage of about 230v
2) neutral wire- blue-always at 0v
3) earth wire- green and yellow with fuse work together for safety
what can earthed conductor never become?
what is double insulation?
the appliance has a plastic casing and no metal parts showing
why don't you need an earth wire with double insulation?
because plastic insulates the appliance and stops a current flowing through so can't get a shock- just need live and neutral
how does a fuse work?
a fault develops in which the live touched the metal case- which is earthed, so a big current flows in through the live, through the case and out through the earth wire.
this serge in current melt the fuse which cuts off the live supply
this isolates the whole appliance making it impossible to get an electric shock- prevents risk of fire caused by the heating effect of a large current
how do residual current circuit breakers work?
usually same current flows through live and neutral, if somebody touches the live wire, a small but deadly current flows through them to the earth. so the neutral wire carries less current than the live wire the RCCB detects this difference in current and cuts off the power by opening a switch
how do circuit breakers work?
when they detect a surge in current they break the circuit by opening a switch
what are the advantages of circuit breakers over fuses?
they can be easily reset by flicking a switch-fuse doesn't have to be replaced
they can detect even small current changes that might not be large enough to melt a fuse- more effective at protecting against electrocution
what happens when an electric current passes through a resistor?
it gets hot as there is an energy transfer
what does the heating effect do to the resistance and current?
it increases the resistor's resistance so less curren will flow
what is electrical power?
the rate at which an appliance transfers energy
an appliance with a high power rating does what?
transfers a lot of energy in a short time
a high power rating draws what from the supply?
a large current
electrical power (W)=
current x voltage
current x voltage x time
what is current?
the rate of flow of charge round the circuit-only flow through a component if there's a voltage across it
what is voltage?
the driving force that pushes the current round. it's the energy of the electrons. it's the potential difference
if you increase voltage?
more current will flow
less current will flow
current x resistance
the gradient for v-i graphs is?
describe metal filament lamp graph
as temp of metal filament increases the resistance increases so a curve
describe a wire graph
the current through a wire is proportional to voltage so linear
describe graph for resistors
the current though a resistor is proportional to voltage- diff resistors have diff resistances so different slopes
describe diode graph
current will only flow in one direction- goes along x and curves up
what are LEDs?
- emit light when a current flows through them in forward direction
- used on digital clocks, traffic lights, remote controls
-used in appliances to show they are switched on
what are LDRs?
light-dependent resistors- changes its resistance depending on the light that falls on it
- in bright light-resistance falls
-in darkness -resistance is highest
-good for burglar detectors
what are thermistors?
- in hot conditions, resistance drops
-in cool conditions- resistance goes up
- useful as car engine temperature sensors
what happens in series circuit?
current stays constant
voltage decreases as you go round circuit
what happens in parallel circuits?
voltage stays constant
resistors combine= 1/Rtotal= 1/R1+1/R2 etc
in solid metal conductors what is charge carried by?
negatively charged electrons
current x time
2 metals that conduct?
copper and silver- a current can flow through them
plastic and rubber- current can't flow through them
What happens when a charge drops through a voltage?
It transfers energy
Where is energy supplied to the charge?
At the power source to 'raise' it through a voltage
When does the charge give up this energy?
When it 'falls' through any voltage drop in components elsewhere in the circuit
When is more energy transferred?
The bigger the change in voltage, the more energy is transferred for a given amount of charge passing through the circuit
What is the voltage?
The energy transferred per unit charge passed
What is one volt defined as?
One joule per coulomb
What is build-up of static caused by?
What happens when two insulating materials are rubbed together?
Electrons are scraped off one and put on the other, this leaves a positive electrostatic charge on one and a negative electrostatic charge on the other
What do electrically charged objects attract?
Small objects placed near them
What happens when a polythene rod is rubbed with a cloth duster?
Electrons move from the duster to the rod. The rod becomes negatively charged and duster is left with an equal positive charge
What happens when an acetate rod is rubbed with a cloth duster?
Electrons move from the rod to the duster. The duster becomes negatively barged and the rod is left with an equal positive charge
What is a positive static charge caused by?
How can a charged conductor be discharged safely?
By connecting it to earth with a metal strap. The electrons flow down the strap to the ground if the charge is negative and flow up the strap from the ground if the charge is positive
When a charge builds up, what else does?
The voltage causing sparks
The greater the charge on an isolated object, the greater the what between it and the what?
The greater the voltage between it and the earth. If the voltage gets big enough there's a spark which jumps across the gap
What's the gold-lead electroscope experiment demonstrating electrostatic charge?
1- a gold- leaf electroscope has a metal disc connected to a metal rod, at the bottom of which are attached two thin pieces of gold leaf
2- put the electroscope in a glass beaker with a plug made of an insulator
3- when a rod with a known charge is brought near to the disc of the electroscope, electrons will either be attracted to or be repelled from the metal disc- depending on the charge of the rod
4- this induces a charge in the metal disc, which in then induces a charge in the gold leaves
5- both gold leaves will have the same charge, so will repel each other, causing them to rise
6- when the rod is taken away, the gold leaves will discharge and fall again
What is the experiment demonstrating electrostatic charge suspending a charged rod?
1- suspend a rod with a known charge on a thread and see if there is repulsion or attraction when the rod you're testing is brought close to it
2- if there is an attraction, then the test row has the opposite charge to the suspended rod
3- if there is a repulsion, then the test rod has the same charge as the suspended rod
How are van de Graff generator used to demonstrate electrostatic charge?
1- it's made up of a rubber belt moving round plastic rollers underneath a metal dome
2- an electrostatic charge is built up on the metal done as the belt goes round
3- if you stand on an insulated chair and place your hands on the dome, electrons will move between your body and the dome, giving your body a charge
4- the human body conducts charge and like charges repel so the charges will spread out as much as possible throughout the body. The charge is strong enough to make your hairs repel each other and stand on end
How does an inkjet printer work?
1- tiny droplets of ink are forced out of a fine nozzle making them electrically charged
2- the droplets are deflected as they pass between two metal plates. A voltage is applied to the plates- one negative and the other positive
3- the droplets are attracted to the plate of the opposite charge and repelled from the plate with the same charge
4- the size and direction on the voltage across each plate changes so each droplet is deflected to hit a different place on the paper
5- loads of tiny dots make the printout
How does a photocopier work?
1- the image plate is positively charged. An image of what you're copying is projected onto it
2- whiter bits of what's being copied make light fall on the plate and the charge leaks away in those places
3- the charged bits attract negatively charged black powder which is transferred into positively charged paper
4- the paper is heated so the powder sticks
How does clothing crackle?
When synthetic clothes are dragged over each other (tumble dryer or over your head) electrons get scraped off, leaving static charges on both parts leading to an attraction and sparks as the charges rearrange themselves
How does lightening happen?
Rain drops and I've bump together inside storm clouds, knocking off electrons and leaving the top of the cloud positively charged and the bottom of the cloud negative. This creates a huge voltage and a big spark causing thunder and a big sound
Why is fuel- filling dangerous?
1- as fuel flows out of a filter pipe, static can build up
2- this can lead to a spark and in dusty or fumy places
3- the solution is to make the nozzles out of metal so the charge is conducted away, instead of building up
4- it's also good to have earthing straps between the fuel tank and the fuel pipe