Energy Transfers (AQA GCSE Physics): Flashcards
Exam code: 8463
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Define power.
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Cards in this collection (40)
Define power.
Power is the rate of energy transfer, or the amount of energy transferred per second.
What two quantities does the power of an electrical component depend on?
The voltage of the component and the current through it.
What equation links power, potential difference and current?
P = V × I, where P is power in watts, V is potential difference in volts, and I is current in amperes.
The unit of power is the watt (W), which is the same as a ______ per second.
The unit of power is the watt (W), which is the same as a joule per second.
What equation links power, current and resistance?
P = I² × R, where P is power in watts, I is current in amperes, and R is resistance in ohms.
True or False?
The equation P = I²R can be derived by combining P = VI with V = IR.
True.
Substituting V = IR into P = VI gives P = I²R.
Calculate the power dissipated by a 20 Ω resistor carrying a current of 6 A.
P = I² × R = 6² × 20 = 720 W
What two factors determine the amount of energy transferred to or from an appliance?
The time the appliance is switched on for and the power of the appliance.
A 1 kW iron is switched on for 1 hour. For how long must a 2 kW iron be switched on to transfer the same amount of energy?
30 minutes, since the 2 kW iron transfers energy at twice the rate of the 1 kW iron.
As electrons pass through each component, they transfer ______ to the component.
As electrons pass through each component, they transfer energy to the component.
Which energy store does a remote control transfer energy from?
The chemical store of its DC cells and batteries.
Name three appliances that transfer energy to the kinetic energy store of an electric motor.
Vacuum cleaners, washing machines (or tumble dryers), and refrigerators.
Name three household appliances that transfer energy to a thermal store by heating.
Toasters, kettles, and radiators.
True or False?
Most larger household appliances transfer energy electrically from DC cells and batteries.
False.
Most larger household appliances transfer energy electrically from the AC mains; DC cells and batteries are used by smaller devices such as remote controls.
What is work done in a circuit equal to?
Energy transferred.
What three quantities does the energy transferred by electrical work depend on?
The current, the potential difference, and the time the component is used for.
Write the equation linking energy transferred, power and time.
E = P × t
Write the equation linking energy transferred to charge and potential difference.
E = Q × V
Since P = IV, the equation E = Pt can also be written as E = ______.
Since P = IV, the equation E = Pt can also be written as E = I × V × t.
Calculate the energy transferred in 1 minute when a current of 0.7 A passes through a potential difference of 4 V.
E = I × V × t = 0.7 × 4 × 60 = 168 J
True or False?
When using energy equations, time must be converted into minutes before substituting into the equation.
False.
Time must be converted into seconds before substituting into any of the energy equations.
Define power rating.
The power rating of an appliance tells you how much energy it needs to work; it is the amount of energy the appliance transfers per second.
What three pieces of information are usually included on a domestic appliance's power label?
The potential difference required, the frequency of the supply, and the power rating in watts.
A kettle has a power rating of 2000 W. How much energy does it transfer every second?
2000 J of energy every second.
The higher the power rating, the ______ the energy is transferred.
The higher the power rating, the faster the energy is transferred.
True or False?
It is safe to plug an appliance into a mains supply with a voltage much higher than the value stated on its label.
False.
Using a supply voltage much higher than stated could cause the appliance to fuse, catch fire, or become damaged.
What is the standard mains potential difference and frequency in the UK?
230 V and 50 Hz.
Define the National Grid.
A system of cables and transformers that links power stations to consumers, used to distribute electricity across the UK.
What effect does a step-up transformer have on voltage and current?
It increases the voltage and reduces the current.
What effect does a step-down transformer have on voltage and current?
It decreases the voltage and increases the current.
Why does transmitting electricity over large distances at a high current cause energy to be wasted?
The resistance in the transmission wires causes heating, which wastes energy to the thermal store of the surroundings.
By increasing the potential difference at which current is transmitted, the same amount of power can be transferred using a much ______ current.
By increasing the potential difference at which current is transmitted, the same amount of power can be transferred using a much smaller current.
True or False?
A high potential difference during transmission means a high current is needed for the same power, resulting in more energy dissipated.
False.
A high potential difference means a low current is needed for the same power, which results in less energy dissipated.
Why is the National Grid an efficient way to transfer energy?
It uses step-up and step-down transformers to transmit electricity at a high potential difference (and low current), which reduces the energy wasted through heating in the cables.
What type of current is produced by power stations and transmitted via the National Grid?
Alternating current (AC).
Define a transformer.
A device made of two coils of wire, the primary and secondary coils, wound around a magnetic iron core, used to increase or decrease the potential difference of a current.
In a step-up transformer, which coil has more turns?
The secondary coil has more turns than the primary coil.
In a step-down transformer, which coil has more turns?
The primary coil has more turns than the secondary coil.
Step-down transformers are used to decrease the potential difference to a much ______ value from transmission cables for domestic use.
Step-down transformers are used to decrease the potential difference to a much lower value from transmission cables for domestic use.
True or False?
Step-up transformers are used to decrease the potential difference from the power station to the transmission cables.
False.
Step-up transformers increase the potential difference from the power station to the transmission cables.
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