P2 electricity Flashcards
what is current
current is the rate of flow of charged particles in a circuit
what is the unit of current
amps (A)
what is potential difference
the driving force that pushes the charge around of circuit
what is the unit of potential difference
volts (V)
what is the equation for charge
charge = current x time
what is the symbol for charge
Q
what is the symbol for current
I
what is the symbol for time
T
what is the equation triangle for charge
Q
I T
what is resistance
anything in the circuit that slows the flow down
what is the unit for resistance
ohms
what is the equation for potential difference
potential difference = current x resistance
what is the symbol for potential difference
V
what is the symbol for resistance
R
what is the equation triangle for potential difference
V
I R
what are some characteristics of an LDR
- dependant on the intensity of light
- in bright light resistance falls
- in darkness resistance is highest
what are some characteristics of a thermistor
- temperature dependant resistor
- in hot conditions resistance drops
- in cold conditions resistance increases
- useful temperature detectors
what is a sensing circuit used for
to turn on or increase the power to components depending on the conditions they are in
how is potential difference distributed in a sensing circuit
the potential difference of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and the fan according to their resistances - the bigger a components resistance the more of the potential difference it takes.
what happens to the resistance in a sensing circuit when the room gets hotter
the resistance of the thermistor decreases and it takes a smaller share of the potential difference from the power supply do the potential difference across the fixed resistor and the fan rises making the fan go faster
describe the features of a series circuit
-in series circuits, the different components are connected in a line end to end.
- if you disconnect or remove a component the circuit is broken and they all stop.
how is potential difference distributed in series circuits
the total potential difference of the supply is shared across the varies components so the potential differences round a series circuit always add up to the source potential difference
how is current determined in a series circuit
the size of the current is determined by the total potential difference of the cells and the total resistance of the circuit.
how is current distributed in a series circuit
current is the same everywhere
how is resistance distributed in a series circuit
resistance adds up - in series circuits the total resistance of two components is the sum of their resistances..
why does resistance get added in a series circuit
because by adding a resistor in series the two resistors have to share the total potential difference. the potential difference across each resistor is lower so the current through each resistor is lower as well. in a series circuit the current is the same everywhere so the total current in the circuit is reduced when a resistor is added.this means the total resistance of the circuit increases.
the bugger a components resistance the bugger its share of the total potential difference
describe the features of a parallel circuit
- in parallel circuits each component is separately connected to the +ve and -ve of the supply
- if you remove or disconnect one component it will hardly affect the others at all
how is potential difference distributed in parallel circuits
potential difference is the same across all components. this means that identical bulbs will all be the same brightness
how is current distributed in parallel circuits
current is shared between branches. in parallel circuits the total current flowing around the circuit is equal to the total of all the currents through the separate components. in a parallel circuit there are junctions when the current either splits or rejoins, the total current going into a junction is equal to the total current leaving it. if two identical components are connected in parallel then the same current will flow through each component
what happens if you add a resistor to a parallel circuit
the total resistance will be reduced. when you have two resistors in parallel their total resistance is less than the resistance of the smallest of the two resistors.
in more detail
- in parallel both resistors have the same potential difference across then as the source
- this means the pushing force making the current flow is the same as the source potential difference for each resistor that you add
- but by adding another loop the current has more than one direction to go in
- this increases the total current that can flow around the circuit. an increase in current means a decrease in total resistance of the circuit
is the mains supply ac or dc
ac
what are the three different wired in a plug
neutral wire -blue - the neutral wire completes the circuit
live wire - brown - the live wire provided the alternating potential difference from the mains supply
earth wire - green and yellow - the earth wire is for protecting the wiring and for safety, it stops the appliance casing from becoming live and it does not usually carry a current.
what is the national grid
a giant system of cables and transformers that cover the uk and connect power stations to consumers
how does the national grid work
-to transmit the huge amount of power needed you need either a high potential difference or a high current
- the problem with high current is that you loose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings
- it is much cheaper to boot the potential difference up really high and keep the current relatively low
- for a given power, increasing the potential difference decreases the current which decreases the energy lost by heating the wires and the surroundings. this makes the national grid and efficient way of transferring energy
how is potential difference changed by a transformer
- to get the potential difference to 400000V to transmit power requires transformers as well as big pylons with huge insulators but its still cheaper
- the transformers have to step the potential difference up at one end for efficient transmission and then bring it down to safe unstable levels at the other end
- the potential difference is increased by a step up transformer
- it is then reduced again for domestic use by a step down transformer
- transformers are almost 100% efficient so you can assume that the input power is equal to the output power.
what is build up of static caused by
friction
how are electrons transferred through static electricity
-when certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other
- this will leave the materials electrically charged with a positive static charge on one and an equal negative static charge on the other
- the direction in which the electrons are transferred depends on the two materials involved
what is a positvie static charge caused by
electrons moving away elsewhere. the material that loses electrons loses some negative charge and is left with an equal positive charge.
how are sparks caused
- an electric charge builds on an object. the potential difference between the object and the earth increases
- if the potential difference gets large enough electrons can jump across the gap between the charged object and the earth - this is the spark
- they can also jump to any earthed conductor that is nearby which is why you can get static shocks when getting out of a car. a charge builds up on the cars metal frame and then you tough the car the charge travels through you to earth
what is an electric field
an electric field is created around any electrically charged object.
the closer to the object you get the stronger the field is and the further you are from it, the weaker the field is.
- you can show an electric field around an object using field lines.
