P2 - Electicity Flashcards
To revise/learn topic p2
What is a diode?
An electrical component which can be used to regulate the potential difference is circuits and to make logic gates.
What does LED stand for?
Light Emitting Diode
When do LEDs produce light?
When the current flows through them in a forward direction.
What are LEDs used for?
They are often used for indicator lights In electrical equipment, such as computers and television sets.
Why is the use of LEDs increasing?
They use a much smaller current than other types of lighting
Describe the resistance and direction of flow in a diode.
It has very high resistance in one direction, which means that current can only flow in the other direction
What are thermistors?
temperature-dependent resistors
What does LDR stand for?
Light dependant resistor
In the most common type of thermistor, what happens to the resistance when the temperature increases?
It decreases - the current can now flow through it.
What happens to the resistance of LDRs as the light intensity increases?
It decreases, allowing more current to flow through it.
Give some applications of thermistors
Fire alarms, car engine temperature sensors and electronic thermostats
Give two equations to find the charge flow (Q).
Charge flow = current x time (Q = I t)
Charge flow = energy transferred / voltage (Q = E/V)
Give the equation that connects current (I) , charge flow (Q) and time (t).
Q = I t
What is the equation that connects resistance, potential difference and current in an ohmic resistor.
V = I * R
Give the equation that connects power, energy transferred and time
E = P * t
Give the equation that connects potential difference, charge flow and energy transferred.
E = Q * V
Give three formulas to find the power of a component in watts
P = E / t P = VI P = I^2 * R
What is the difference between a fuse and a circuit breaker?
A fuse has a wire that blows every time the current is too high. A circuit breaker is mechanical and when the current is too high, a magnet flips a switch that breaks the circuit.
What are the advantages and disadvantages of a fuse?
Advantages: It is cheap and stops the current quickly.
Disadvantages: once they blow you have to replace them.
What are the advantages and disadvantages of a circuit breaker.
Advantages: they can be reset and don’t have to be replaced.
Disadvantages: they are slower to stop the current and are mechanical so can be broken easier. They are also more expensive
What are two ways to increase the magnetism of a solenoid?
Increase the current and the number of coils.
Is magnetic charge stronger on flat areas or pointed ones?
Pointed ones.
what is current?
the flow of electrical charge
what is the unit of current?
ampere, A
what is potential difference?
the driving force that pushes the charge round
what is the unit of potential difference?
the volt, V
what is resistance?
anything that slows the flow down
what is the unit of resistance?
Ohm, Ω
what does total charge through a circuit depend on?
current and time
does the circuit symbol for a cell have one or two?
1
does the circuit symbol for a battery have one or two?
2
what do ohmic resistors have?
a constant resistance
what is an I-V characteristic?
a graph which shows how the current (I) flowing through a component changes as the potential difference (V) across it increases
what type of components have an I-V characteristic that’s a straight line?
linear components (e.g. a fixed resistor)
what type of components have curved I-V characteristics?
non-linear components (e.g. a filament lamp or a diode)
describe the I-V characteristic of a Ohmic resistor
a straight, diagonal line passing through the origin
describe the I-V characteristic of a filament lamp
a curved line that forms a sort of S shape and passes through the origin
describe the I-V characteristic of a diode
a line along the x-axis (V) until just after it passes through the y-axis, then it curves sharply up before straightening into a straight line
explain the I-V characteristic of a filament lamp
as the current increases the temperature of the filament increases, so the resistance increases. This means less current can flow per unit pd, so the graph gets shallower, hence the curve
explain the I-V characteristic of an ohmic resistor
at a constant temperature the current through an ohmic conductor is directly proportional to the potential difference so you get a straight line
explain the I-V characteristic of a diode
Current will only flow through a diode in one direction. The diode has a very high resistance in the reverse direction
how can you calculate the resistance at any point on the I-V characteristic?
using the formula R = V / I (V=IR)
what are some applications of LDRs
automatic night lights, outdoor lighting and burglar detectors
what can sensing circuits be used for?
to turn on or increase the power to components depending on the conditions that they are in
the bigger a components resistance, the bigger/smaller its share of the potential difference.
bigger
how is energy transferred in a kettle?
kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
how is energy transferred in a handheld fan?
Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor
with a higher current is more or less energy transferred to the thermal energy stores of the components (and then the surroundings)?
more
what does the total energy transferred by an appliance depend on?
how long the appliance is on for and its power
what is the power of an appliance?
the energy that it transfers per second
what is a power rating? what does it tell you?
appliances are often given a power rating - they’re labelled with the maximum safe power that an appliance can operate at. You can usually take this to be their maximum operating power. The power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use
how does the power rating help customers choose between models?
the lower the power rating the less electricity an appliance uses in a given time and so the cheaper it is to run.
However, a higher power doesn’t necessarily mean that it transfers more energy usefully. An appliance may be more powerful than another, but less efficient, meaning that it may only transfer the same amount of energy (or even less) to useful stores.
what is energy transferred per charge passed?
potential difference
what happens when an electrical charge goes through a change in potential difference?
energy is transferred
what is the national grid?
a giant system of cables and transformers that covers the UK and connects power stations to consumers
what are some events that could cause the demand for electricity to increase?
demand increases when people get up in the morning, come home from school or work, and when it starts to get dark or cold outside. popular events like a sporting final being shown on TV could also cause a peak in demand.
what are some measures power stations have in place to cope with an unexpectedly high demand?
power stations often run well below their maximum power output, so there’s spare capacity to cope with a high demand, even if there is an unexpected shut down of another station. Lots of smaller power stations that can start up quickly are also kept on standby, just in case
does the national grid use a high or low potential difference? what about current?
it uses a high pd and a low current
why does the national grid use a high potential difference and a low current?
to transmit the huge amount of power needed, you need either a high potential difference or a high current (P=VI). A higher current would mean losing a lot of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings, so it’s much cheaper to boost the potential difference up really high and keep the current as low as possible. This makes the national grid an efficient way of transferring energy.
what is the potential difference in the wires of the national grid?
400,000 volts
what is used in the national grid to get the voltage up to 400,000 volts?
step-up transformers (and big pylons with huge insulation)
what do all transformers have?
two coils, a primary coil and a secondary coil, joined with an iron core
how do step-up transformers work?
step-up transformers have more turns on the secondary coil than the primary coil. As the pd is increased by the transformer, the current is decreased
do step down transformers have more turns on the primary or secondary coil?
primary
what is the equation linking the pd and current of both the coils in a transformer? Why is this?
pd across primary coil (V) x current in primary coil (A) = pd across secondary coil (V) x current in secondary coil (A)
VpIp = VsIs
this is because transformers are nearly 100% efficient, so the power in primary coil = power in secondary coil
how efficient are transformers?
nearly 100%
is the power in the primary coil greater than, equal to or less than the power in the secondary coil?
equal to
why does a moving charge transfer energy?
the charge does work against the resistance of the circuit (work done is the same as energy transferred)
what are electrical appliances designed to do?
electrical appliances transfer energy between stores electrically - they are designed to transfer energy to components in the circuit when a current flows
give the equation to work out the amount of energy transferred by electrical work
energy transferred (J) = Power (W) x Time (s) (E = Pt)
how does a power source work?
energy is supplied to the charge at the power source to ‘raise’ it through a potential. The charge then gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit. A battery with a bigger pd will supply more energy to the circuit which flows round it, because the charge is raised up ‘higher’ at the start
what is the equation to work out energy transferred using charge flow and potential difference?
Energy transferred (J) = charge flow (C) x potential difference (V) (E = QV)
give the formula to find power using potential difference and current. Include units
Power (W) = Potential difference (V) x Current (A)
P = VI
give the equation to find power using current and resistance
power (W) = current (A)^2 x resistance (ohms)
P = I^2 x R
how is electricity distributed around the UK?
through the national grid
what does the national grid do?
the national grid transfers electrical power from power stations anywhere on the grid (the supply) to anywhere else on the grid where it’s needed (the demand) - e.g. homes and industry
what are step-down transformers used for?
to increase the current and decrease the potential difference of electricity transferred over the national grid in order to make it safe for the local consumer
describe an experiment to find a component’s I-V characteristic
- set up a circuit with an ammeter, a variable resister, and the component you want to test in series, and a voltmeter connected in parallel around the component.
- Begin to vary the variable resister. This alters the current flowing through the circuit and the potential difference across the component
- take several pairs of readings from the ammeter and voltmeter to see how the potential difference across the component varies as the current changes. Repeat each reading twice more to get the average pd at each current
- swap over the wires connected to the cell, so the direction of the current is reversed, and repeat the experiment with a negative current
- plot a graph of the current against voltage for the component.
how can you calculate the resistance at any point on an I-V characteristic?
since V = IR, you can calculate the resistance at any point on an I-V characteristic by calculating R = V/I
