Electricity Up to level 6 Flashcards
This is P2 Electricity from the AQA syllabus up to level 6
What is the current at points 1 and 2 on this circuit. Explain why you have put this answer
The current at 1 is 3A and the current at 2 is 3A.
This is becuase current is the same at all points in a series circuit
What is the potential difference across the resistor in this diagram. Explain why this is the case.
The potential difference across the resistor is 5V.
This is becuase the potential differences across the components in a series circuit must add up to make the potential difference across the resistor
This is the formula for potential differences in series
VT = V1 + V2 + V3….
how is electric current calculated?
Example: What is the current when 12C of charge takes 4s to flow past a point?
Current is found from this formula I=Q/t
I=Current (A - Amps)
Q = Charge (C - coulombs)
t = time (s - seconds)
For example if there are 12C of charge and it takes 4s to flow
I = 12/4 = 3 A
What is potential difference?
How do you calculatre potentaion difference?
Example: What is the potential difference when 6C of charge has 9J of energy
Potential difference is the amount of energy that each coulomb of charge looses or gains when it moves through the component.
You can think of it as how hard the electricity is pushed
Potential difference can be found by this formula
V = E/Q
V = potential difference (V - volts)
E = Energy (J - Joules)
Q = charge (C - Coulombs)
For example if there is 6C of charge with 9J of energy
V = 9/6 = 1.5V
What is this the circuit symbol for and what does it do
How is it connected in a circuit?
This is an ammeter and it measures the current through a component.
It is connected in series with the component it is measuring
What component is this the graph of?
What does the graph tell you about the component?
This is an ohmic resistor
The graph shows that it has a constant resistance becuase it is a straight line.
Which component is this the V-I graph for?
What does the graph tell us about this component?
This is a graph for a filament lamp.
The graph shows that the resistance of the graph increases at a higher potential difference. This is shown becuase the flatter the gradient of the graph the higher the resistance.
The graph is flatter at a higher potential difference, so the resistance must be higher
Which component is this the graph of?
What does it tell us about the component?
This is the graph of a diode
It shows that a diode only allows the current to flow in one direction. It shows this becuase it has a high current when the potential difference flows in one direction and no current when the potential difference flows in the other direction.
How do you calculate the resistance of resistors in series.
Example: What is the resistance of this series of resistors?
When reisitors are in series, the total resistance can be found by adding up the individual resistances.
In the example the resistance = 5 + 10 + 14 = 29Ω
What does resistance do in a circuit?
What is resistance mearued in?
Resistance slows down the current in a circuit.
Resistance is measured in Ohms (Ω)
What does potential difference do to the current in a circuit?
What is potential difference measured in?
Potential difference speeds up the current in a circuit
Potentail difference is measured in volts (V)
What is the definition of current?
What carries the charge in a metal wire?
Current is the rate of flow of charge in a circuit. This means it is how fast the electric charge is travelling through a circuit.
Electrons carry the charge in a circuit.
What is ohms law?
Ohm’s law states that the resistance in a circuit is found from the potential difference divided by the current
R = V/I
R = Resistance(Ω - ohms)
V = Potential difference (V - Volts)
I = Current (A - Amps)
How do you calculate the resistance of a component?
For example, what is the resistance of a lamp with a current of 4A and a potential difference of 16V
Resistance is calculated from ohms law. The equation for ohms law is
R = V/I
R = Resistance (Ω - ohms)
V = Potential difference (V - Volts)
I = Current (A - Amps)
Example: Resistance = V/I = 16/4 = 4Ω
Explain how to investigate how the length of a piece of wire affects its resistance
To investigate how the length of a piece of wire affects its resistance, set up a circuit with the wire a cell, an ammeter and a switch in series. Then attach a voltmeter across the crocodile clips and place the clips 10cm apart.
Close the switch and read the potential difference and current in the circuit. use this to calculate the resistance
Repeat the experiment where the length of the wire is different each time, going up in intervals of 10cm.
Then plot a graph of length against resistance
Explain what a variable resistor does and how it works
A variable resistor is a component where you can change its resistance
You can do this by sliding the slider, which makes the length of the resistance wire longer or shorter depending on which way you slide it
The longer the resistance wire the higher the resistance
How do you calculate the charge transferred by a current.
For example, what is the charge transferred by a 3.5A current which flows for 30 seconds
The charge transferred by a current can be found from the formula Q=It
Q = charge (C - Coulombs)
I = current (A - Amps)
t = time (s - seconds)
Example: If 3.5A carries a current for 30 seconds, charge = 3.5 x 30 = 105C
What is the rule for working out the potential difference of components that are in parallel with each other?
Example: What is the potential difference across the unknown resistor in the parallel circuit?
Resistors in paralell always have the same potential difference across them. The formula for this is:
VT = V1 + V2 + V3
Example: This resistor has 9V across it becuase there is 9V accross the cell that it is in parallel with
What carries the charge in a metal wire?
In a metal wire, electrons carry the charge around the circuit.
How does energy get into and out of an electric circuit?
Explain the energy transfers in each case
Energy is supplied to an electric circuit by a power source. This transfers energy into electrical potential energy.
For example a cell transfers chemical energy into electrical potential energy
Energy is transferred away from a circuit by a resistor or another component with resistance. This transfers energy out of the electrical potential energy store.
For example, a resistor transfers electrical potential energy into thermal energy
Explain how to calculate the energy that is transferred to a component when charge passes across a potential difference
For example, what is the energy dissipated when 2C of charge passes across a potential difference of 3V
The energy transferred by a coulomb of charge can be found from this formula
E = VQ
E = energy (J - Joules)
V = Potential difference (V - Volts)
Q = charge (C - coulombs)
Example: When 2C of charge passes across a PD of 3V the energy transferred is 2 x 3 = 6J of energy
Explain how the law of conservation of energy applies to an electric circuit
The law of conservation of energy states that “energy cannot be created or destroyed, it can only be transferred from one store to another”
The cell transfers chemical energy into electrical potential energy. This energy is transported round the circuit to the component, which transfers it into other forms.
Due to conservation of energy, the total energy supplied by the power source must be equal to the total energy transferred out by all of the components combined.
How do you work out how much energy has been transferred by an appliance in Kilowatt-hours?
Example: A 2kW heater runs for 4 hours, what is the energy transferred in kWh
To work out the energy in kilowatt hours you multiply the power in kilowatts by the time in hours
Example: Energy in kWh = 2 x 4 = 8kWh
What does the word efficiency mean?
How do you express an efficiency of 35% as a decimal?
Efficiency is the proportion of the energy that you put into an appliance that is transferred into a useful energy output
To convert an efficiency of 35% into a decimal you divide it by 100 - 35/100 = 0.35
how do you calculate the power of a device when you know how long it has been left on for?
Example: what is the power of TV which converts 7500J of energy in 20 seconds?
The power of a device can be found from this equation:
P = E/t
P = Power (W - Watts)
E = Energy (J - Joules)
t = time (s - seconds)
A TV that converts 7500J of energy in 20 seconds has a power of 7500/20 = 375W
How do you calculate the energy transfer of an appliance in Joules when you are told how long it is left on for?
Example: How much energy is transferred by a 60W lightbulb that is left on for 800 seconds?
To calculate the energy transfer in joules of an appliance you use this formula
E = Pt
E = Energy (J - Joules)
P = Power (W - Watts)
t = time (s - seconds)
When a 60W light bulb is left on for 800 seconds the energy transferred is 60 x 800 = 48000J
What are the main factors that affect how much an appliance costs when you use it?
The main factors that affect the cost of an appliance are:
The power of the appliance - the more powerful it is the more energy it uses
The time it is used for - the more time it is used for the more energy it uses
The electricity tariff - the more each unit of electricity costs, the higher the bill
Describe the mains electricity supply in the UK
The mains electricity supply in the UK is an alternating current supply.
It has a potential difference of 230V and a frequency of 50Hz (it goes up and down 50 times each second)
What is the difference between an AC and a DC power supply
AC means alternating current. In an AC circuit the current moves backwards and forwards. This is becuase the potential difference is constantly chaning, from positive to negative and back again. AC current works with transformers, so the potential difference can be altered to stop energy lost when the electricity is in the transmission lines.
DC means direct current. In a DC circuit the current always flows one way. This is becuase it has a constant potential difference. It is a lot easier to build a motor using a DC supply rather than an AC supply
How does changing the frequency or amplitude of an AC current affect the trance on an oscilloscope.
When the potential difference (amplitude) is changed the wave changes height (high P.D = tall height)
When the frequency is changed the wave becomes shorter becuase more cycles are happening in the same time.
What are the colours of the three different wires in a plug?
What does each wire do?
Brown = live wire. This takes electricity to the component
Blue = Neutral wire. This takes electricity away from the component
Green/Yellow = Earth wire. Electricity only travels through this if the case comes into contact with the live wire
What are the common hazards that can be found on electricl wiring?
The common hazards are
- loose wiring
- broken wire sheathes exposing the insides of the wires.
- Missing cable grip meansing the wires could be pulled out easily
- Water near electrical appliances
Compare the amplitude and frequency of these two AC waves
The wave at the top has a higher frequency. This is seen becuase the waves are closer toghether.
The wave at the bottom has a larger amplitude, this is seen because this wave is taller than the wave at the top
Explain what a short circuit in an appliance is and why it could cause a problem.
What are the safety features of a plug that can prevent this from being a problem?
A short circuit occurs when a live wire touches the metal casing of an appliance, this means that the electric current bypasses the appliance and flows through the case.
This is dangerous because the case is now charged to 230V. If a person touches the case they will get electrocuted.
There are two safety features that stop this from being a problem.The earth wire will take the current away from the case, this neutralises the case. When there is a short circuit the current gets very high. This causes the fuse to blow which breaks the circuit and stops any more current.
Which material are each of these pieces of a plug made of?
Casing
Pins
Wires
Sheath
For each one explain why it is made of this material
The casing is made of plastic. This is becuase it is an insulator, protecting the user from being electrocuted, and is strong and rigid, preventing it from being damaged.
The pins are made of brass. This is becuase brass conducts electricity. Although it is not as good as copper at conducting electricity it is preferred for the pins becuase it is strong and does not bend
The wires are made of copper. Copper conducts electricity very well and is flexible.
The sheath is made of rubber. This is becuase it is flexible and is an insulator.
What is the definition of power?
What is it measured in?
Power is how quickly the energy is transferred from one form into another
Power is measured in Watts
1 Watt = 1 Joule per second
What factors affect the power that is transferred through an appliance
The power transferred by an appliance depends on the current and the potential difference. It can be found from this formula
P = IV
P = Power (W - Watts)
I = current (A - Amps)
V = Potential difference (V - Volts)
When there is a higher potential difference, each coulomb of charge that passes through an appliance transfers more energy, so the energy is transfered faster
When there is a higher current more coulombs of charge pass through the appliance each second. This means the energy is transferred faster so power goes up.
Explain why different appliances have different sized fuses
What are the problems that could occur if the wrong fuse was put into an appliance?
Different appliances need to have different sized fuses because they have different currents running through them.
If you put in a fuse that is too small the fuse will blow even in normal operation. If you put in a fuse that is too large it will blow in normal operation.
Explain how to select which fuse to use for an appliance
For example what fuse would you use for an appliance that draws 3.6 A of current in normal operation
Fuses come in 3A, 5A and 13A sizes. to select the right size of fuse to use you need to pick the smallest fuse that is larger than the normal current of the appliance
For a current that normally draws 3.6A of current you would choose a 5A fuse, because this is the smallest fuse that is larger than the current.
Explain how current acts in parallel branches.
What is the unknown current in this circuit?
Current in a parallel circuit splits becuase the charge can only flow down one branch or the other
This means that the current in the branches add up to make the total current. This is given by the formula
IT = I1 + I2 + I3….
Example: in this circuit the total current is 5.6A and the current in one of the branches is 3.9A. this means the current in the other branch must be 5.6-3.9 = 1.7A
Explain how you calculate the energy transferred by an appliance from time, potential differnce and current
Example: What is the energy transferred by a kettle that draws 9A of current on the mains electricity supply for 120 seconds
The energy of an electrical applaince can be found from this formula:
E = IVt
E = Energy (J - Joules)
I = Current (A - Amps)
t = time (s - seconds)
Example: for a 9A kettle on mains electricity (230V) for 120 seconds the energy is 9 x 230 x 120 = 248,400 J
Complete this diagram to show the different parts of a plug
What is this component called and what does it do?
This is a thermistor
The resistance goes down when it is heated up.
The resistance goes up when it is cooled
What is this component called and what does it do?
This is a lamp, it converts electrical energy into light energy
What is this component called and what does it do?
This is a fuse
It is a safety feature that breaks the circuit if the current gets too high
What is this component called and what does it do?
This is a light dependent resistor (LDR)
The resistance goes up when it is dark.
The resistance goes down when it is light
What is this component called and what does it do?
This is a switch.
It breaks the circuit to stop the current from flowing and completes the circuit to start the current flowing
What is this component called and what does it do?
This is a cell
It converts chemical potential energy into electrical potential energy
This makes the current move
What is this component called and what does it do?
This is a resistor
It slows down the current in the circuit
When it does this it converts electrical potential energy into thermal energy, so it gets hotter
What is this component called and what does it do?
This is a variable resistor
It allows a person to change the resistance of the resistor
What is this component called and what does it do?
This is a diode
It only allows current to flow in one direction
What happens to the potential difference in a circuit when cells are connected in series
Example: what is the total potential difference of these 3V cells?
The total potential difference is found by adding up the potential differences of each of the cells
Example: this battery contains 6 3V cells so the total potential difference is 6x3 = 18V
What is the potential difference across each of these resistors?
Explain why this is the case
The potential difference across each of these resistors is 18V. This is because the potential difference across each branch of a parallel circuit is the same as the potential difference across the cell.
Explain why this circuit will not work
Diodes will only allow the current to flow one way. In electrical circuit symbols, the current flows from the long part of the cell symbol to the short part of the cell symbol. To make the circuit work you need to turn the diode round the other way.
Explain why the resistance of this circuit is 30Ω and not 50Ω
One of the 20Ω resistors has be short circuited. This means that the resistance is not counted in the total resistance as the electrons bypass it and travel through the wire instead
