P5: Electricity in the home Flashcards
What is a power rating?
The rate at which energy is transferred
How do you calculate the total energy transferred by an electrical appliance?
E = Pt
Energy transferred (J) = Power (W) * time (s)
What is the formula for power relating potential difference and current?
P = VI
Power (W) = Voltage (V) * Current (A)
What is the formula for power relating current and resistance?
P = I²R
Power (W) = Current² (A) * Resistance (Ω)
What is direct current (DC)?
Electric current only ever travels in one direction
What is an alternating current (AC)?
Electric current is constantly changing direction
What is the benefit of using alternating current?
It’s very easy to use a transformer to increase or decrease the voltage
What is the frequency of the ac supply in the UK?
50Hz
What is the voltage of the ac supply in the UK?
230 V
What is an oscilloscope?
A device which shows the amplitude and frequency of electrical current
How do you calculate the frequency in an oscilloscope?
Read the time interval between two of the maximum points on the graph
Number of cycles/ time = frequency
1 cycle = two peaks
What is the live wire?
The brown wire, carries the potential difference from the supply - 230 V.
It is connected to a fuse in the plug
What is the neutral wire?
The blue wire, completes the circuit with the live wire
- 0V
What is the earth wire?
Green/ yellow, safety wire.
Attached to metal casing, connected to the ground with a metal rod.
Why could the live wire be dangerous?
Can be fatal if touched, first part still has a p.d. of 230V, if touched, a current would flow through them and into the earth and they would be electrocuted
Why can appliances with metal casing be dangerous?
If the live wire comes loose and touches the metal case, it can become live and give someone an electric case
What happens when a metal case becomes live?
A huge current flows to the earth, the fuse melts and shuts off the current, preventing someone being electrocuted
What is the national grid?
The system which transports energy from power stations to homes - a system of transformers and high voltage cables
How can energy loss from the cables be reduced?
Build power stations closer to homes
What is a step-up transformer?
Increases potential difference, so less energy wasted in the power cables when p.d. is high
What is a step-down transformer?
Reduces p.d. to 230V, so it can be safely transported to homes
Why are metals good conductors?
Electrons can easily flow through them
Describe the electrical field of a positively charged object
Goes outwards
Describe the electrical field of a negatively charged object
Goes inwards
