P5 - Electricity in the home Flashcards
Describe the properties of the domestic electricity supply:
-50Hz AC
-230 V
This is for the UK, it may be different for other countries
What is AC?
-direction of current/polarity swaps back and forth at a consistent rate
-produced when an alternating PD is used
What is DC?
-constant direction of current/polarity
-produced when a direct PD is used
What does an oscilloscope do?
measures PD in a circuit, and produces a PD vs time graph
What is a 3 core cable?
insulated cable with 3 individually insulated cables inside it (live, neutral, earth) that have copper wires inside each
What is the colour is the insulation on each of the 3 core cable’s wires?
live - brown
neutral - blue
earth - green and yellow stripes
Describe the 3 wires in a 3 core cable:
-live, 230V, carries the alternating PD from mains supply
-neutral, 0V, completes the circuit by carrying away current
-earth, 0V (when there isn’t a fault), safety wire connected to appliance casing, stops it from becoming live
When does the earth wire have to be used? Describe what happens after:
-live wire touches metal casing
-large current flows to the earth rather than a person
-causes fuse to melt, cuts off current
-prevents electric shock
Where (give examples) might a 2 core cable be used?
A 2 core cable is one that only has a live and neutral wire
appliances with plastic/double insulated casing (hairdryers, phone chargers, radios)
What can cause a short circuit?
-if a live wire inside the appliance touches a neutral wire, a large current flows between them at the point of contact
-fuse blows, current is cut off
Why would you get an electric shock from touching a live wire?
-potential of live wire is 230V
-potential of a person is 0V
-there is a large PD between live wire and person
-current passes through body easily
Same reason why current from live can flow across to neutral/earth
Describe the structure of a 3 pin plug:
-cable grip stops 3 core cable being pulled out
-earth (top), longest pin so goes in first
-neutral (left)
-live (right)
-fuse protects the circuit, between live wire and pin
What is a surge, and when might one happen?
sudden increase in supplied current that can damage an appliance (causing fire/electric shock)
-turning an appliance on/off, or if there is a fault
What is power, and give its equations:
P = E/t (and P=IV, P = I²R, P=V²/R)
rate of energy transferred in joules per second (W)
Energy transferred, or work done - either is fine
What is current, and give its equation:
Rate of flow of electric charge supplied by electrons
(Q=It, or I = Q/t to show the definition)
What is charge, and give its equations:
measure of the total current that flowed in a certain period of time
(Q=It, E=QV)
Why do resistors get hot?
-as charge flows through a component, work is done against its resistance
-electrons collide with ions in the lattice of the resistor
-as the electrons bump into the ions, energy is transferred causing the ions to vibrate
-some energy dissipated as thermal energy, so its temperature increases
What is the National Grid?
network of transformers, wires and pylons across the UK to distribute electrical power from power stations to consumers
What is a step-up transformer and why are they used?
-increases PD and decreases current at same time to maintain same power (P=IV)
-decreasing current minimises energy loss by heat during transmission (from resistance in wires), and increases the efficiency of the transmission
P=I²R is the power lost by heating, and decreases the current will decrease the energy lost by its square
What is a step-down transformer and why are they used?
-decreases PD and increases current
-lower voltages are safer and wouldn’t blow our devices
What is the equation for efficiency?
useful power out / total power in
What is an electrical appliance?
something that transfers electrical energy to another energy store
How does a fuse stop the wires overheating?
-current goes above appliance rating
-fuse wire melts
-cuts off current flowing in circuit of appliance
-stops current increasing further, reducing heat loss
What is double insulation?
-outer casing is plastic
-wires inside are insulated in plastic
Why might someone flying a kite near a electricity pylon be shocked?
-electric field strength is very high
-causes air to become ionised, and can conduct charge
-the kite’s string would conduct the charge to the person/earth
From November 2021, Paper 1, Q10.2
