electricity and circiuts Flashcards
what is current
The flow of electrical charge.
Current will only flow if there is a potential difference accros that component and the circuit is complete
what is potential difference
the energy transferred per coulomb of charge that passes between to points in a circuit
energy transferred= charge moved x potential difference
p.d. = amount of energy transferred per unit of charge passed.
Therefore unit= joule per coulomb
what is resistance
anything that slows the flow of current down
unit: ohm
how resistance and potential difference affects current
higher potential difference the higher the current
greater resistance the smaller the current
charge equation
charge = current x time
why does resistance usually increase with temperature (full explanation)
1) when charge flows through a component it has to do work against resistance.
2) this causes a electrical transfer of energy
3) some energy transferred usefully but waste energy of thermal stores
4) so when current flows through resistor it heats up
5) Happens because electrons collide with the ions in lattice that make up the resistor . This gives them energy, which causes them to vibrate and heat up.
6) The more the ions vibrate the harder it is for electrons to get through resistor. This means that for a given p.d the current decreases as resistors heat up.
7) if too hot, no current will be able to flow. Only exception is thermistor
why does resistance usually increase with temperature (important bit)
Happens because electrons collide with the ions in lattice that make up the resistor . This gives them energy, which causes them to vibrate and heat up.
The more the ions vibrate the harder it is for electrons to get through resistor. This means that for a given p.d the current decreases as resistors heat up.
explain the standard test circuit to investigate the relationship between the current, p.d. , and resistance for a range of components
Check cgp guide pg 73 for drawing
Use Ammeter in series
Use a voltmeter in parallel to measure potential difference
To investigate a component:
1) change output potential difference of power supply. This allters the current flowing through the circuit and potential difference across the component.
2) take several readings.
3) Plot current against potential difference
4) use this data to work out the resistance of each measurement
5) make sure doesn’t get too hot if so let it cool. calculate mean of results
how do you investigate diodes using test circuit
Diodes- after finishing withtest circuit and taking measurements for a range of currents, remove the diode and swap its direction. Should find current can’t fkow through it anymore.
how do you investigate Thermistors using test circuit
Thermistor- Keep supply p.d. constant, gradually heat the thermistor (place by beaker of hot water). You should find that as temperature increases, current through the thermistor increases as the resistance decreases.
how do you investigate LDRs using test circuit
Conduct in dim room
keep p.d. supply constant
slowly adjust light levels near to the LDR
Should fin as light level gets brighter, the current through LDR increases as resistance decreases
current-potential difference graphs
pg 74 cgp guids
What is a linear component of a I-V graph
straight line
If line goes through 0,0 resistance of component is inverse of gradient of the line or 1/gradient.
The steeper the graph, the lower the resistance
what is an LDR
Light dependant resitors.
in bright light resistance falls
in darkness resistance is highest
applications- automatic night lights and burglar detection
what is a thermistor
temperature dependant resistor
hot conditions the resistance drops
cool conditions the resistance goes up
applications- temperature detectors
series circuits
all connected in a line between +ve and -ve.
If you disconnect one component the circuit stops working.
In practice very few things are connected in series
rules in setting up a series circuit
1) bigger supply p.d. when more cells are in series. e.g. when 2 1.5V batteries connected in parallel make 3V.
2) current same everywhere. Size of current depends on total p.d. and total resistance.
3) total p.d. is shared between components. The p.d. for each component depends on its resistance
4) total resistance increases as you add resistors
parrallel circuits
each component seperately connected to the +ve and -ve
If you remove a component it will hardly affect other one
Modern day circuits often contain a mixture of series and parallel parts
parrallel circuit rules
The potential differnece is same accross all components
current shared between branches. Total current = total current through seperate components
Total current into junctions = total current leaving a junction
total resistance decreases if you add a second resistor
how does adding resistors in series affect total resistance
Increases total resistance
total resistance of two components is just the sum of their resistance.
This is because by adding a resistor in series the two resistors have to share total p.d.
P.d. accross each resistor is lower so the current through each resistor is lower.
In a series circuit current is same everywhere so total current is reduced when resistor added.
therfore total resistance of circuit increases.
how does adding resistors to a parallel curcuit affect resistance
reduces the total resistance
In parralel, both resistors have the same potential difference acroos tham as the source.
by adding a loop the current has more than one direction to go in
core practical how to use a circuit to investigate properties of resistor and filament lamp
1) set up circuit with ammeter in series after a resistor and a votmeter in parralel
2) vary output p.d. record readings from ammeter and voltmeter
3) replace with filament lamp and repeat
4) Now connect a second lamp in parallel to the first, connect ammeters and voltmeter.
5) vary p.d. output and write current through each ammeter and the p.d. accross each component
core practical
test series and paralel circuits using resistors and filament lamps
method same as finding properties
For series: potential difference increases, current through resistors increases. V=IR.
but a non-linear relationship between p.d. and current for a filament lamp
for paralel: p.d. and current increases through bulb but non-linear relationship.
p.d. accross each bulb same as from source. sum current through the branches higher than number of branches before.
how does energy transfered depend on current, p.d. and time
1) when a charge goes through a change of p.d. then energy is transfered (as it works against resistance)
2) energy supplied at power source
3) charge gives up energy when it ‘falls’ through any potential drop.
4) the larger the current though or p.d. accross a component the more energy is transfered
equation to find the energy transfered to an electrical component
E= I x V x t
E= energy (joules)
I= current (amps)
V= p.d. (volts)
t = time (s)
energy is transfered from cells and other sources
e.g. kettle transferes energy electrically from a.c. supply to thermal stores
