Section 4 - Electricity Flashcards
What is current?
The flow of electric charge around a circuit.
What is the unit for current?
Ampere (A)
When will a current flow through a component?
If there is a potential difference across a component.
What is potential difference?
- The driving force that pushes the current round a circuit.
- It is also the energy transferred per coulomb of charge that passes between 2 points
What is the unit for potential difference?
Volts (V)
What is resistance?
Anything that slows down the flow in a circuit.
What is the unit for resistance?
Ohms (Omega)
Given a potential difference for a component, how are the resistance and current related?
The greater the resistance, the smaller the current.
What is the unit for charge?
Coulombs (C)
What is the equation for charge (in terms of current and time)?
Charge (C) = Current (A) x Time (s)
Q = I x t
What is the potential difference across a component?
The amount of energy transferred / work done by that electrical component per coulomb of charge.
What is the equation for potential difference (in terms of energy transferred and charge)?
Potential Difference (V) = Energy Transferred (J) / Charge (C)
V = E / Q
Remember to learn circuit symbols.
Pg 68 of revision guide.
Describe the standard test circuit used for finding out the resistance of a component.
- Battery
- Variable resistor
- Ammeter
- Component
- Voltmeter in parallel to component
(See diagram pg 68 of revision guide)
In the standard test circuit, what happens when the resistance of the resistor is increased?
The current through the circuit decreases.
Which way does conventional current flow?
From positive to negative terminals.
Which way do electrons flow in a circuit?
From negative to positive terminals.
In exams, which way is current presumed to flow in a circuit?
From positive to negative terminals (conventional current).
Which sides of a battery symbol are the positive and negative side?
The longer line is positive.
The shorter line is negative.
In an I-V graph, what goes along each axis?
x axis - Potential difference (V)
y axis - Current (I)
Describe the I-V graph for a fixed resistor.
- Straight line going from bottom left of graph, through origin, to the top right of graph
- I is directly proportional to V
(See diagram pg 69 of revision guide)
Describe the I-V graph for a filament lamp.
- Straight line at first
- After a while, as potential difference increases, current increases at a decreasing rate
- This results in an S-shaped curve
(See diagram pg 69 of revision guide)
Describe the I-V graph for a diode.
- Line along the x-axis on left side of graph
- Suddenly there is a spike upwards
(See diagram pg 69 of revision guide)
Give reasons for the shape the I-V graph of a filament lamp.
As the temperature of the filament increases, so does the resistance, which causes the curve.
Give reasons for the shape of the I-V graph of a diode.
Current only flows through a diode in one direction since the diode has very high resistance the other way.
What happens when the temperature of a component increases and why?
The resistance increases because the ions in the component vibrate more, making it more difficult for electrons to pass.
What is the equation for potential difference (in terms of current and resistance)?
Potential Difference = Current x Resistance
V = I x R
How can the resistance of a component be calculated from its I-V graph?
By looking at the gradient or 1/gradient (depending on what is on the x and y axis).
What are diodes used for?
- Converting AC into DC in a circuit
What is a light-emitting diode (LED)?
A diode which emits light when current flows through it in one direction.
What are diodes made from?
A semiconductor, such as silicon.
What are the advantages of LEDs?
The use a much smaller current than other forms of lighting.
What are LEDs used for?
- Traffic lights, digital clocks, etc.
- Showing that current is flowing (e.g. to show that the TV is on
How is an AC supply converted to DC in a circuit and what is this called?
A diode is used - half wave rectification.
What is a light dependent resistor (LDR)?
A resistor that is dependent on the intenisty of light hitting it.
How does the resistance of a LDR change?
- In bright light -> Low resistance
- In darkness -> High resistance
What are LDRs used for?
- Automatic night lights
- Outdoor lighting
- Burglar alarms
What is a thermistor?
A resistor that is dependent on the temperature.
How does the resistance of a thermistor change?
- In hot -> Low resistance
- In cold -> High resistance
What are thermistors used for?
- Temperature detectors (e.g. car engine temperature sensors)
What is a series circuit?
When all of the components are connected in a line from the positive to negative terminal.
What happens to potential difference in a series circuit?
It is shared between various components.
VT = V1 + V2
What happens to current in a series circuit?
It is the same everywhere.
AT = A1 = A2
How is current in a series circuit calculated?
I = V/R
V is the total potential difference of the cells.
R is the total resistance of the components.
How is total resistance calculated in a series circuit?
It is the sum of the resistances of the components.
R = R1 + R2 + R3
In a series circuit, what determines each component’s share of the cell’s potential difference?
The bigger the resistance of a component, the bigger its share of potential difference.
What happens when two cells are connected in series?
Their potential difference adds up (if they are connected in the same direction).
What is a parallel circuit?
When each component is connected seperately to the power supply.
What happens to potential difference in a parallel circuit?
The potential difference is the same on each branch.
VT = V1 = V2
What happens to current in a parallel circuit?
It shared between each branch.
AT = A1 + A2
How is total resistance calculated in a parallel circuit?
1/RT = 1/R1 + 1 /R2 + 1/R3
How are ammeters connected?
In series.
How are voltmeters connected?
In parallel.
What is the voltage of UK mains supply?
230V
Is mains supply AC or DC?
AC
What is the frequency of the UK mains supply?
50 Hz
Do batteries supply AC or DC?
DC
What device is used to display the voltage of a power supply?
Cathode ray oscilloscope (CRO)
On an oscilloscope used to show electricty supply, what is on the x and y axis?
x axis - Time
y axis - Potential Difference
On an oscilloscope used to show electricty supply, what does the GAIN dial do?
Controls how many volts each centimetre division represents on the vertical axis.
On an oscilloscope used to show electricty supply, what does the TIMEBASE dial do?
Controls how many milliseconds each division represents on the horizontal axis.
How many seconds is 1 millisecond?
0.001 seconds
What does the trace of an AC current look like on an oscillosocpe?
Regularly repeating wave
What does the trace of a DC current look like on an oscillosocpe?
Straight line parallel to x axis
Give some examples of electrical hazards in the home.
- Long cables
- Frayed cables
- Cables in contacts with heat or moisture
- Water near sockets
- Pushing things into sockets
- Damaged plugs
- Too many plugs in one socket
- Lighting sockets withou bulbs on
- Appliances without their covers on
How many wires are inside the electrical cables of most appliances?
Three:
- Live
- Neutral
- Earth
Describe the structure of most electrical wires.
Three copper wires with a coloured plastic coating inside an insulating sheath.
What does the live wire do?
Carries current to the appliance.
What does the neutral wire do?
Carries current away from the appliance.
What does the earth wire do?
It protects against the user from electricity.
How does the earth wire protect the user?
- It is connected to the metal casin of the appliance.
- It carries the elecricity to the earth (away from the user) in case something goes wrong or the live or neutral wires touch the case
What is the voltage of the live wire?
It alternates between a negative and positive voltage.
What is the voltage of the neutral wire?
0V
What colour is the live wire?
Brown
What colour is the neutral wire?
Blue
What colour is the earth wire?
Yellow and green
Remember to learn the wiring of a plug.
Diagram pg 74 of revision guide
In a plug, which pin is the earth wire connected to?
The top one
In a plug, which pin is the live wire connected to?
The right one
In a plug, which pin is the neutral wire connected to?
The left one
What are wires in an electric cable made of?
Copper
What things have to be done correctly when wiring a plug?
- Connect each wire to the correct pin and screw it in firmly
- No bare wires inside the plug
- Cable grip fastened over cable outer layer
What are the pins of a plug made from?
Brass - because it is hard, is a conductor and doesn’t rust
What are the case, cable grip and cable insulation of a plug made from?
Rubber or plastic - because they are insulators and flexible
Why are different electrical cables of different thicknesses?
Those with more current flowing through need to be thicker to stop the wire heating up.
How do fuses and the earth wire work together?
- A fault develops so that the live wire touches the metal case
- Since the case is earthed, too great a current flows through the live wir, through the case and down the earth wire
- This surge in current melts the fuse (or circuit breaker), which cuts of the live supply
- This isolates the appliance, preventing electric shock or a fire caused by the heating effect of a large current
What is the main use of a fuse?
To protect the appliances and wires in the event of a current surge.
What rating should a fuse be?
Just higher than the normal operating current.
What is double insulation?
An appliance which has a plastic case and no metal parts showing.
When is the only time an appliance may not need an earth wire?
When the appliance is double insulated.
What is a circuit breaker?
An electrical safety device which can be used instead of a fuse.
What are some advantages of a circuit breaker over a fuse?
- Easy to reset
- Does not need to be replaced each time
- Quicker than a fuse
- Work with a smaller current overload, which may not melt a fuse
What are some disadvantages of a circuit breaker over a fuse?
- Expensive
Give an example of a circuit breaker.
Residual Current Circuit Breaker (RCCB)
How does an RCCB (Residual Current Circuit Breaker) work?
- Usually live and neutral wires carry same current
- When there is a fault, some current will flow through the earth wire, reducing the current in the neutral wire
- The RCCB detects this difference and quickly cuts off the power by opening a switch
How does a fuse work?
It contains a thin wire which metls if too much current flows through it.
On which wire is the fuse always fitted on?
The live wire
How does a circuit breaker work?
It is an electromagnet in series with a switch. When the current gets too large, the magnetic field is strong enough to pull open the switch. The switch stays open after this.
Anything that is supplying electricity is also supplying…
… energy.
Give some examples of how electricty is transferred as energy to components in a circuit.
Motion: Motors
Light: Light bulbs
Heat: Hair dryers / Kettles
Sound: Speakers
Do all resistors produce heat when current flows through them?
Yes
What two factors increase the heat prodcued by a resistor?
- Greater current
- Bigger voltage
Why does a greater voltage increase the heat produced by a resistor?
Because more current is being pushed through.
How do filament bulbs work?
Current is passed through a thin wire until it glows.
What is the power of an appliance?
The rate at which it transfers energy.
What is the equation for power (in terms of energy transferred and time)?
Power (W) = Energy Transferred (J) / Time (s)
P = E x t
What is the unit for power?
Watt (W)
What is the equation for power (in terms of current and potential difference)?
Power (W) = Current (A) x Potential Difference (V)
P = I x V
What are the power rating and voltage rating of an appliance?
Ratings which tell you the maximum power and voltage at which an appliance can operate.
How can the fuse required for an appliance be calculated when the voltage and power is given?
Use the equation “P = I x V” to calculate the current. Then choose the fuse with a rating just above this.
What is the equation for energy transferred (in terms of charge and potential difference)?
Energy transferred (J) = Charge (C) x Potential Difference (V)
E = Q x V
What happens in terms of energy at every stage in a circuit?
- The battery gives the charge energy as it is ‘raised’ through a potential
- The charge gives up its energy when it ‘falls’ through any potential drop in components in the circuit
What determines how much energy is transferred per charge at a component?
The potential difference across it.
What are the most common fuses?
3A, 5A and 13A
When an appliance is used, what things affect how much energy it uses?
- How long it is on for
- Its power (the rate at which it uses energy)
What is 1 Watt?
1 Joule transferred per second by an appliance
When making household electricity calculations, what units are used for energy, power and time?
Energy - Kilowatt-hours (kWh)
Power - Kilowatts (kW)
Time - Hours (h)
What is a kilowatt-hour?
The amount of electrical energy used by a 1kW appliance left on for 1 hour.
What are ‘units’ when referring to the cost of household electricity?
A unit is usually 1 kWh. A price is usually charged per unit.
What is the equation for the units of electricity used?
No. of Units (kWh) = Power (kW) x Time (h)
What is the equation for the cost of electricity (in terms of units)?
Cost = No. of Units x Price per Unit
What is the National Grid?
The network of pylons and cables that covers the whole of Britain, getting electricity everywhere.
Where does the National Grid take electricity from and to?
From power stations to homes and industry.
What are the two ways in which a large amount of power can be transmitted across the country?
- High current
* High voltage
Why is a high current not used to transmit a large amount of power across the National Grid?
A large current would heat up the wires, causing energy loss.
What is the voltage used in the National Grid across the country?
400,000V
Why is a high voltage used in the National Grid?
It keeps the current low, meaning less energy is wasted by heating of the wires.
What are the costs involved with using a high voltage in the National Grid?
Transformers and big pylons with huge insulators.
What are transformers used for in the National Grid?
At first, they step up the voltage at one end, for efficient transmission, then they step the voltage down for safe, usable levels.
What is a step-up transformer?
A transformer which increases the voltage.
What is a step-down transformer?
A transformer which decreases the voltage.
What is the main reason why AC is used in the National Grid?
Transformers require AC current to work.
Describe the path of electricity through the National Grid.
Power station -> Step-up transformer -> Step-down transformer -> Consumers
