Electrical Circuits COPY - mock revision list Flashcards
What is an electric current
An electric current is a flow of electric charge
What do cells and batteries supply
Cells and batteries supply direct current (DC)
What is Direct current (D.C)
Current that always passes in the same direction
What is alternating current (A.C)
Current that is constantly changing direction
What type of supply is mains electricity
Alternating current (A.C) In the UK it has a frequency of 50 cycles per second (50Hz) and is about 230V
What can a diode be used for
A diode may be used for half wave rectification of a.c
Most electrical appliances are connected to the mains using………
A cable and a three pin plug
What are the colours of the different wires in a cable
Neutral wire = bLue (goes Left)
Live wire= bRown (goes Right)
Earth wire = green and yellow sTripes (goes to the Top
What is the case, cable grip and cable insulation of a three pin plug made of and why
The case is made of tough plastic or rubber, because these materials are good electrical insulators, and flexible too
What are the three pins from a three pin plug made from and why
The three pins are made from copper or brass, which are very good conductors of electricity
What happens if an electrical fault causes too great a current to flow
If an electrical fault causes to great a current to flow, the circuit is disconnected by a fuse or a circuit breaker in the live wire
What happens when the current in a fuse wire exceeds the rating of the fuse
It will melt, breaking the circuit
The thicker the wire in the fuse, …………
The higher the fuse’s rating .
This is because the thinner wire cannot take such a big load as the thicker one can; the thinner one will just melt at the same voltage and current that the thicker one would be able to take
What are some circuits protected by
Residual current circuit breakers (RCCBs)
Which is faster, a fuse or RCCBs
RCCBs operate much faster than a fuse
How do RCCBs operate
RCCBs operate by detecting a difference in the current between the live wire and the neutral wires
Appliances with metal cases are usually …….
Earthed
In what case would there be no need for an earth wire connection
If the appliances are double insulated
The ……………. And the ………. Together protect the wiring of a circuit
The EARTH WIRE and FUSE together protect the wiring of a circuit
What substances can electrical charges move easily through
Metals
What is the relationship between current, charge and time
current = charge / time
I = Q / t
What is the relationship between potential difference, energy transferred and charge
potential difference (or voltage) = energy transferred / charge
V = E / Q
What is a thermistor
A thermistor is a temperature - dependent resistor
Where do we use thermistors and light-dependent resistors
In sensor circuits
Whats an example of a use of a LDR
An LDR is used in a sensor circuit that switches an electrical light on and off.
For example, switching lights on when it gets dark
Whats an example of a use of a thermistor
A thermistor may be used in a thermostat to control temperature
What causes the resistance of a thermistor to decrease
The resistance decreases if its temperature increases
What causes the resistance of a LDR to decrease
The resistance decreases if the light intensity on it increases
How can you find the resistance of a component
The resistance of a component can be found by measuring the current through and potential difference across, the component
What does the current through a component depend on
The current through a component depends on its resistance
The greater the resistance, the ………… the current for a given potential difference across the component
The greater the resistance, the SMALLER the current for a given potential difference across the component
What is the relationship between potential difference, current and resistance
potential difference = current x resistance
V = I x R
The current through a resistor (at a constant temperature) is ……………………………… to the potential difference across the resistor
The current through a resistor (at a constant temperature) is DIRECTLY PROPORTIONAL to the potential difference across the resistor
What are current-potential difference graphs used to show
Current-potential difference graphs are used to show how the current through a component varies with the potential difference across it
What is the relationship between the resistance if a filament bulb and the temperature of the filament
The resistance of a filament bulb increases as the temperature of the filament increases
Explain change in resistance in terms of ions and electrons
When an electrical charge flows through a resistor, some of the electrical energy is transferred to heat energy and the resistor gets hot.
This heat energy causes the ions in the resistor to vibrate more. The increase in movement of the ions makes it more difficult for the charge-bearing electrons to get through the resistor - the current can’t flow as easily and the resistance increases
Why does the current through a diode flow in one direction only
As the diode has a very high resistance in the reverse direction
What is the potential difference provided by cells connected in series equal to
The potential difference provided by cells connected in series is the sum of the potential difference of each cell (depending on the direction in which they are connected)
What three things are true for components connected in series
- the total resistance is the sum of the resistance of each component
- there is the same current through each component
- the total potential difference of the supply is shared between the components
What two things are true for components connected in parallel
- the potential difference across each component is the same
- the total current through the whole circuit is the sum of the currents through the separate components
When does an LED emit light
An LED emits light when a current flows through it in the forward direction
Why is the use of LEDs for lighting increasing
The use of LEDs for lighting is increasing because they use a much smaller current than other forms of lighting
What happens to a resistor when electrical charge flows through it
When electrical charge flows through a resistor, the resistor gets hot
What type of bulbs waste a lot of energy
A lot of energy is wasted in filament bulbs by heating
What type of bulbs waste less energy
Less energy is wasted in power saving lamps such as Compact Fluorescent Lamps (CFLs)
How to determine the period of an AC supply from an oscilloscope trace
The period of an AC supply is the time taken for one complete oscillation.
You can find this by looking at the time between one peak and the next, between one trough and the next, or between any two identical places on adjacent oscillations. (a bit like wavelength)
How to determine the frequency of an AC supply from an oscilloscope trace
The frequency of an AC supply is the number of oscillations per second.
You can find it from the period:
Frequency = 1 ÷ period
Always remember to put the period into seconds before performing the calculation
How do you determine the potential difference of an AC supply from an oscilloscope trace
The vertical height of the AC trace at any point shows the input voltage at that point.
By measuring the height of the trace you can find out the potential difference of the AC supply
How do you determine the potential difference of a DC supply from an oscilloscope trace
The voltage is the distance from the straight line trace to the centre line
What is the power
The rate at which energy is transferred by an appliance
What is the relationship between power, energy transferred and time
Power = energy transferred / time P = E / t
What is the relationship between power, current and potential difference
Power = current x potential difference P = I x V
What is resistance measured in
Ohms, Ω
How should you determine the size of the fuse needed
The fuse should be rated at a slightly higher current than the device needs.
e.g If the device works at 3 A, use a 5 A fuse
What is the relationship between energy transferred, potential difference and charge
Energy transferred = potential difference x charge
E = V x Q
What are examples of electrical appliances that bring about energy transfers
Many electrical appliances used at home are lamps or heaters.
In a lamp, electrical energy is transferred as light and heat.
In a toaster, electrical energy is transferred as heat.
In a hair dryer, electrical energy is transferred to kinetic energy, heat energy, and sound energy
What does the amount of energy an appliance transfers depend on
The amount of energy an appliance transfers depends on how long the appliance is switched on for and its power
What is the relationship between energy transferred from the mains, power and time
Energy transferred from the mains = power x time
E = p x t
What is a kilowatt-hour
A kilowatt-hour is the amount of electrical energy used buy a 1 kW appliance left on for 1 hour
What is the no. of units (kWh) used equal to
No. of UNITS (kWh) used = POWER (in kW) x TIME (in hours)
What is the cost (of electricity) equal to
COST = no. of UNITS x PRICE per unit
Reading electricity meters
If you look at a domestic electricity bill for a 3 month period, you will see that there are two meter readings. One is the present reading and the other is the previous (last) reading.
Subtracting the previous reading from the present reading gives the number of units used in that 3 month period.
Electricity is priced in pence per unit.
Multiplying the number of units used by the price per unit
gives the cost of electricity for that period.
What is electricity distributed from power stations to consumers along
Electricity is distributed from power stations to consumers along the National Grid
What are the essential parts of the national grid
Power station –> Step-up transformer –> Step-down transformer –> Consumers
For a given power, increasing the voltage …….. the current required
For a given power, increasing the voltage REDUCES the current required
Why is it better to have a high voltage rather than a high current (in order to get the huge amount of power needed by consumers)
The problem with a high current is that you lose lots if energy through heat in the cables.
Keeping the current very low, means less wasted energy because the heating of the of the cables is reduced.
THIS REDUCES THE ENERGY LOSSES IN THE CABLES
What is a transformer
A transformer is an electrical device that changes the voltage of an alternating current (ac) supply, such as the mains electrical supply.
What is a step-up transformer
A transformer that increases the voltage
What is a step-down transformer
A transformer that decreases the voltage
Why do we have transformers as part of the national grid
The transformers habe to step the voltage up at one end, for efficient transmission, and then bring it back down to safe, usable levels at the other end
