Electricity - GCSE Flashcards
what is a current?
it is the rate of flow of charge round the circuit
what do electrons usually carry?
charge. They are negatively charged particles
where will current only flow through?
it will only flow through a component if there is a voltage across that component.
what is current measured in?
aperes, A
what is voltage?
it is what drives the current round the circuit. Kind of like “electrical pressure”
what could be another name for voltage?
potential difference
what is voltage measured in?
in volts, V
what is resistance?
it is anything in the circuit which slows the flow down
what happens if you add more components to the circuit?
there will be a higher resistance
how is there a balance between the voltage, current, and resistance?
the voltage is trying to push the current round the circuit, and the resistance is opposing it.
what decides how big a current will be?
The relative sizes of the voltage and resistance
what happens if you increase the voltage?
more current will flow
what happens if you increase the resistance?
the less current will flow (or more voltage will needed to keep the same current flowing)
IMPORTANT TO KNKOW THE CIRCUIT SYMBOLS
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what does the ammeter do?
measures the current flowing through the components
where is the ammeter placed?
it must be placed in the series anywhere in the main circuit, but never in a parallel
what does a voltmeter do?
it measures the voltage across the component
where is the voltmeter placed?
must be placed in parallel around the component under test, NOT around the variable resistor or the battery
what is the circuit used for?
testing components
what does it mean when you say that the component, the ammeter and the variable are all in series?
it means that they can be put in any order in the main circuit
what happens if you alter the resistance of the variable resistor?
the current flowing through the components changes
what does a.c. mean?
alternating current, meaning the current is constantly changing direction
what does d.c mean?
direct current, meaning the current keeps flowing in the same direction
would a main supply be d.c. or a.c.?
a.c. -> alternating current
would cells and batteries be d.c. or a.c.?
d.c. -> direct current
what is the formula linking Voltage and Current?
voltage = current x resistance
what do current-voltage graphs show?
how the current varies as you change the voltage
what happens when a metal filament lamp temperature increases?
the resistance increases
the current through a wire (at a constant temperature) is proportional to what?
proportional to voltage
the current through a resistor (at a constant temperature) is proportional to what?
to voltage
different resistors have different what?
different resistances
current will only flow through a diode in what?
in one direction
when do light-emitting diodes (LEDs) emit light?
it emits light when a current flows through them in the forward direction
what do LEDs have lots of? and what are LEDs used for?
- -> lots of practical applications
- -> used for the number on digital clocks, in traffic lights and in remote controls
what do LEDs do not have? (unlike light bulbs)
filament that can burn out
what do LEDs indicate?
the presence of current in a circuit
what is a Light-dependent resistors (LDR)?
a special type of resistor that changes its resistance depending on how much light falls on it
how does the resistance in LDRs change in bright light?
the resistance falls
how does the resistance in LDRs change in darkness?
the resistance is highest
what is a thermistor?
a temperature-dependent resistor
what happens to the resistance of a thermistor in hot conditions?
the resistance drops
what happens to the resistance of a thermistor in cold conditions?
the resistance goes up
LDRs are useful device for what?
for various electronic circuits
thermistors make useful what?
temperature detectors
in a series circuit, how are the different circuits connected?
in a line, end to end
what happens if you remove or disconnect one component of a circuit?
the circuit is broken and they all stop working
in a series circuit, the total resistance of the circuit depends on what?
the number of components and the type of components used
how do you calculate the total resistance?
it is the sum of the resistance of each component in the circuit.
you just need to add all the resistances up
in a series circuit, the total potential difference of the supply is shared between what?
between the components
in a series circuit, the size of the current depends on what?
on the total potential difference and the total resistance of the circuit
what are the 2 different types of circuits?
- parallel circuits (everything is independent)
- series circuits (all or nothing)
in a parallel circuit, what happens if you disconnect or remove one component?
it will hardly affect the others at all
how is the potential difference in a parallel circuit?
it is the same across all branches
what causes the total resistance of the parallel circuit to decrease?
if you add a second resistor in the circuit
what is the scientific name for saying ‘charge flows’?
current
what is current?
it is the rate of flow of electrical charge around a circuit
in solid metal conductors, charge is carried by what?
negatively charged electrons
charge through a circuit depends on what?
current and time
what is the formula for charge?
charge = current x time
how much charge passes around a circuit when a bigger current flows?
more charge
what happens when an electrical charge goes through a change in voltage?
energy is transferred
why is energy supplied to the charge at the power source?
to raise it through a voltage
when does charge give up the energy that was supplied at the power source?
when it falls through any voltage drop in components elsewhere in the circuit
if the change in a voltage is bigger what will happen?
more energy is transferred for a given amount of charge passing through the circuit
if more energy is transferred for a given amount of change passing through a circuit, what will happen to a battery with a bigger voltage?
it will supply more energy to the circuit for every coulomb of charge which flows round it
why will a battery with a bigger voltage supply more energy to the circuit for every coulomb of charge which flows round it?
it will supply more energy to the circuit because the charge is raised up higher at the start, meaning more energy will be dissipated in the circuit too
what is voltage?
it is the energy transferred per unit of charge passed
what is one volt equal to?
one joule per coulomb
what is the formula for energy transferred?
energy transferred = charge x voltage
what is the formula for calculating the energy transferred by an amount of charge as it passes through a resistance?
energy transferred = charge x current x resistance
how many wires are there in a plug and what are they?
there are 3: live, neutral and earth
which of the 3 wires are usually needed?
the live and neutral, however if something goes wrong the earth wire stops you from getting hurt
what does the live wire do?
it alternates between a high positive and negative voltage of about 230 V
what V is the neutral wire always at?
at 0 V
in which wires does electricity normally flow through?
live and neutral wire
what are 3 things that are used for safety and work together?
- earth wire
- fuse
- (sometimes) circuit breaker
what do all appliances with metal cases must be done to reduce the danger of electric shock?
they must be earthed
what does earthing mean?
it means that the case (metal case) must be attached to an earth wire
what can an earthed conductor never become?
it can never become live
what is an appliance that has a plastic casing and no metal parts showing said to be?
a double insulated
what is an insulator?
it is a plastic case that stops a current flowing (this means you can’t get a shock)
anything with double insulation doesn’t need a what?
an earth wire, it just needs a live and neutral
what happens if the live wire somehow touches the metal case?
(3 steps)
1) Nothing will happen because the metal case is earthed, causing a big current to flow through the live wire, through the case and the earth wire.
2) Then the surge in current melts the fuse which cuts off the live supply.
3) this isolates the whole appliance, making it impossible to get an electric shock from the case. This also prevents the risk of fire caused by the heating effect of a large current
what is a circuit breaker?
an electrical safety devise used in some circuits. They protect the circuit from damage if too much current flows
what happens when a circuit breaker detects a surge in current in a circuit?
they break the circuit by opening a switch, making circuit breakers more convenient than fuses (which have to be replaced once they’ve melted)
what is one type of circuit breaker?
a Residual Current Circuit Breaker (RCCB)
what happens if somebody touches the live wire?
a current will flow through them to the earth. This means the neutral wire carries less current than the live wire
what does the RCCB detect?
the difference in a current and cuts of the power by opening a switch
why can the RCCB operate much faster than fuses?
because they break the circuit as soon as there is a current surge (no time is wasted waiting for the current to melt a fuse. This also makes them safer)
why do RCCB’s even work for small current changes that might not be large enough to melt a fuse?
because even small currents could be fatal, this means RCCB’s are more effective at protecting against electrocution
what happens to resistors when an electric current passes through them?
they get hot
why do resistors get hot when there is an electric current?
there is an energy transfer which heats the resistors
explain how a energy transfer is caused in resistors?
electrons collide with the ions in the lattice that make up the resistors as they flow through it. This gives the ions energy, which causes them to vibrate and heat up
what can the heating effect cause in resistors?
- it can increase the resistor’s resistance - so less current will flow, or a greater voltage will be needed to produce the same current
- it can cause components in the circuit to melt - this means the circuit will stop working, or not work properly.
why do fuses use the effect of heating in resistors?
they use it to protect the circuits - they melt and break the circuit if the current gets too high.
what is one advantage for the heating effect caused in resistors?
it is useful if you want to heat something
give an example of the heating effect in resistors:
toasters contain a coil of wire with a really high resistance. When a current passes through the coil, its temperature increases so much that it glows and gives off infrared (heat) radiation which cooks the bread
what do most electrical appliances come with?
with a power rating and a voltage rating
what is electrical power?
it is the rate at which an appliance transfers energy
how much energy does an appliance with a high power rating transfer?
a lot of energy in a short time
where does the energy an appliance transfer come from?
it comes from the current flowing through it. This means that an appliance with a high power rating will draw a large current from the supply.
what is power measured in?w
watts (W)
most electrical goods show what?
their power rating and voltage rating
what do fuses have?
they have current ratings and should be rated as near as possible but just higher than the normal operating current
what does energy transfer by appliance depend on?
it depends on the power of the appliance and how long it is on for
