electricity (p2) Flashcards
what is a circuit?
a closed circuit containing a power source (e.g. cell) and something for electrons to flow through (e.g. wire)
- represented by circuit diagrams
what is current (I)?
a measure of the flow of electrons around the circuit, measured in amperes (A)
what is potential difference/voltage (V)?
the force driving the flow of electrons (current), provided by the cell/battery, measured in volts
what is resistance (R)?
everything that resists/opposes the flow of electrons, measured in ohms
which way round a circuit does current flow?
- the longer line on a cell/battery is the positive terminal
- as electrons are negatively charged, they flow around the circuit from the negative to positive terminal
- but when people first discovered electricity, they didn’t know this, and decided it flows from positive to negative, so we keep this as the ‘conventional current’
what is an equation linking potential difference, current and resistance?
potential difference (V) = current (I) x resistance (R)
what is the relationship between potential difference, current and resistance?
as long as the resistance stays constant, then as we increase the voltage, the current should increase proportionally (directly proportional)
how would you increase the potential difference?
use a bigger battery
describe graphs where current and potential difference are directly proportional:
- straight line through the origin
- we only get these graphs when the circuit is made up of only wires, or from resistors, as the resistance is constant
- in resistors, having a greater resistance would mean a shallower gradient, and a smaller resistance would mean a steeper gradient
- this is because a smaller resistor (less overall resistance), only a small potential difference would be needed to drive a large current
what is the relationship between temperature and resistance?
- current and potential difference are only directly proportional if the temperature remains constant
- as a higher temperature means a higher resistance, therefore the resistance would no longer remain constant
- higher currents do generally cause wires to heat up if they’re left on for a while
what is the potential difference:current graph for a filament lamp?
- lightbulbs containing a thin metal filament. when current flows through it, the wire heats up until it’s so hot that it emits light
- this heat massively increases the resistance, so the curve becomes less steep as we increase the current, telling us that less current can flow at higher potential differences, as the resistance is so great
- slightly s-shaped curve
what is the potential difference:current graph for a diode?
- devices that only allow current to flow in one direction
- have a high resistance in the reverse direction, so no current can flow in that direction
- straight line along x-axis until potential difference becomes positive, when it curves up
what is charge (Q)?
a measure of the total current that flowed within a certain period of time, measured in coulombs (C)
which equation links charge, current and time?
charge (Q) = current (I) x time (t)
what is the role of a cell/battery?
provides electric power to the circuit
what is the role of a switch?
control the flow of electricity. can be closed (allowing electricity to flow), or open (basically turning off the circuit as it’s no longer complete)
what is the role of a fuse?
breaks if too much current flows through the circuit
what is an LED?
- light-emitting diode
- emits light when current flows through it in the forward direction
describe ammeters and voltmeters:
ammeters: measure current, connected in series
voltmeters: measure potential difference, connected in parallel
what are the two basic types of resistors?
- fixed resistors: provide a certain number of ohms’ worth of resistance
- variable resistors: can modify how much resistance it provides
what is an LDR?
- light dependent resistor
- in light environments, there’s low resistance, so lots of current can flow
- in darkness, the resistance is so high that hardly any current can flow
what is a thermistor?
- resistance is dependent on temperature
- higher temperatures cause the resistance to fall, lower temperatures increase the resistance
- useful temperature receptors
what is the difference between parallel and series circuits?
- series circuits contain a single loop, and the components are all connected one after the other
- parallel circuits contain more than one loop
what are the main points about series circuits?
- disconnecting one component means they all stop working, as they’re all in series
- the potential difference of the cell/battery is shared across all the components
- current is the same everywhere in the circuit
- the total resistance is the sum of the individual resistances of each component
- components with a greater resistance will always have a higher share of the resistance, due to Ohm’s law. more force is required to push the charge through the areas of highest resistance