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
what are the main points about parallel circuits?
- each component has its own loop, so if one component breaks, then the overall circuit is still intact
- all components get the full potential difference
- the current is shared between each loop. the way it splits depends on the resistance of each of the components in each loop. components with greater resistance take a lower share of the current
- the more components we add in parallel, the lower the total resistance
what is the formula linking energy, power and time?
energy = power x time
(E=Pt)
what is the formula linking energy, charge flow and potential difference?
energy transferred = charge flow x potential difference
(E=QV)
what is the formula linking power, current and voltage?
power = current x voltage
(P=IV)
what is the formula linking power, current and resistance?
power = current^2 x resistance
(P=I^2R)
what is the national grid?
the giant network of transformers and transmission cables that spread across the country to distribute electricity
- transmits huge amount of power, due to such a high demand
where is electricity in the UK generally generated from?
power stations
- generate lots of heat (e.g. through combustion of fossil fuels, nuclear fission)
- this thermal energy is used to turn water into steam, which can then rise and turn turbines
- the kinetic energy of the moving turbines can be converted to electrical energy, which is sent out across the national grid
what is the problem with a high current in transmission cables?
- P=IV, so to achieve lots of power, we need a high voltage and a high current
- a high current generates lots of heat, due to the resistance in the wire, so we’d lose lots of energy to the surroundings
- we must therefore keep the current very low, so the voltage must be very high
what is the role of a step-up transformer?
increase the voltage of the electricity just before it leaves the power station to reduce energy transfer in the transmission cables
- the voltage is increased to around 400,000 volts
describe the transmission cables and step-down transformers in the national grid:
- the electricity is transmitted by the wires all across the country, between huge pylons
- just before the electricity reaches its destination, e.g. a town, the voltage is reduced back down in step-down transformers, and the current is increased once again
why must the voltage of the electricity be lowered back down before reaching homes?
- lowered from 400kV to 230V
- high voltages are dangerous, and they’d also blow the appliances in our households (not made to withstand such high voltages)
what is alternating current (a.c.)?
- the direction of the current is constantly swapping back and forth. for instance, charge flows in one direction, then the other, then back again, etc.
- an alternating current is produced from an alternating potential difference, one that fluctuates from positive to negative
- all UK mains supply electricity is a.c.
- specifically, it’s 50Hz and 240V. a graph will fluctuate from positive 240 to negative 240 and back again 50 times in a second, so the current would also fluctuate at that same rate
what is direct current (d.c.) ?
- produced by a direct potential difference, which is either positive or negative the entire time
- the charge is always flowing in the same direction
- d.c. is always found in cells/batteries, e.g. like the one in a phone
what device is used to display the potential difference of an object over time?
- oscilloscope
- shows its voltage and frequency, displays how voltage changes over time
describe plugs:
- three-core cable (three wires)
- each wire is made of copper and coated in a layer of insulating plastic for safety
describe the different wires:
- live wire (brown). supplies the alternating potential difference of around 240V from the mains supply
- neutral wire (blue). acts to complete the circuit by carrying away current. has a potential difference of 0V
- earth wire (green and yellow striped). doesn’t normally carry a current, has a potential difference of 0v
what is the role of the earth wire?
- stops the appliance’s casing from becoming live, which could happen if the live wire became loose and touched the casing, meaning it now holds a voltage of 240V
- if we touched the casing, we’d get a big electric shock, as the current flows through us and to the ground
- the the Earth wire provides an alternative, very low resistance pathway for the current to flow away, so no dangerous electricity would flow through us
- a difference in voltage (between 240V and 0V) is what makes the current more likely to flow through the neutral and earth wires, and humans
what is the name for a sudden increase in current?
a surge
- can be caused when turning an appliance on/off
- a fault in the circuit/appliance
- can damage an appliance, cause fires or electric shocks
describe fuses:
- breaks the circuit whenever the current gets too high
- very thin piece of wire connected to the live wire
- if there’s a surge, a huge amount of current flows through the live wire and therefore the fuse
- as the wire is so thin in the fuse, as soon as it heats up from the current, it melts, breaking the circuit, so no more current can flow
describe the rating on a fuse:
- many different ratings, depending on what current you want to break the circuit
- if you have an appliance with normally a 3A current, you could use a 5A fuse, so it breaks when there’s a surge that makes the current rise above 5A
what are the pros and cons of fuses?
pros:
- simple
- cheap
cons:
- permanently broken after a single surge, must be replaced each time
describe circuit breakers:
- breaks the circuit whenever the current gets too high, protecting the electrical circuits from overloading or short circuiting
- if the current is too high, it detects the fault, the circuit breaker trips and turns off the circuit
- however, it can be reset, and it’s not permanently damaged, meaning it can be reused
what are the cons of circuit breakers?
more expensive than fuses
describe the double insulation of an appliance:
- the entire appliance is encoated in a plastic casing, so there’s no exposed metal parts to touch
- plastic doesn’t conduct electricity, so we can’t get an electric shock from touching it
- if an appliance has this, it won’t need an earth wire
what is static electricity?
the build-up of charge on insulating materials, and can cause sparks when discharged
what is the overall charge of most materials?
neutral, as their positive and negative charges cancel out
how would you create static electricity?
- rub two objects together. electrons will rub off one of them and onto the other due to the friction between the two objects
- this leaves a positive static charge on the material losing the electrons, and a negative static charge on the material gaining the electrons (which way the electrons are transferred depends on the specific materials)
- if an object gains more and more electrons, the size of the negative charge increases. a potential difference is induced between the charged material and earth/any earthed object
- if this potential difference is large enough, electrons can jump across the gap, and this is a spark
why is static electricity not created on conducting materials?
the electrons transferred simply flow back again, so no charge ever builds up
- conductors do sometimes build up charge, but it’s much less common
what do all charged objects have?
an electric field surrounding it, shown with field lines (lines with arrows on)
- the Earth has a gravitational field
- magnets have a magnetic field
where do the field lines point?
- always from positive to negative
- out of positive particles
- in to negative particles
- always perpendicular to the surface of the particle
describe electric fields:
strongest the closest to the particle, gets weaker the further away you get
- as charged particles get closer together, their electric fields will start to interact more and more
describe charged particles:
- oppositely charged particles are attracted to each other - there is an electrostatic force between them, therefore it’s electrostatic attraction
- this can be shown with field lines, which extend between the particles, from the positive to the negative (always goes from positive to negative)
- particles with the same charge repel
describe the interaction between electric fields and air:
a really strongly charged object will have a strong electric field around it. the nearby air particles have no charge, therefore is an electrical insulator
- this strong electric field might cause the air particles to lose electrons and become positive ions
- this is called ionisation, and once air has been ionised, it can conduct electricity
- this is how sparks travel between objects, through air
