What is DC ?
- direct current
- current where electrons only flow in one direction
- p.d of DC is CONSTANT
- DC is often used in cells and in appliances
What is AC ?
- alternating current
- current changes direction constantly
- used for mains electricity
[] in the UK, frequency of mains electricity is 50 Hz
[] UK, max. p.d of mains AC is 230V
How can you calculate the frequency of AC from an oscilloscope trace ?
- find the time between one peak and the next (one full cycle) on the x-axis
- frequency = 1/time
What is a three-core cable ?
a cable with three wires inside that connects to the mains electricity supply
Why is PURE copper used for electrical wiring ?
- good conductor of electricity
- has to be pure, as impurities may restrict electron flow through the wires
- bends easily, so is good for wires
- ductile (easy to make into wires)
What are the 3 wires in a 3-core cable and what are they used for (also state their colours) ?
- live wire (brown)
[] carries the AC and has an average p.d of 230V in the UK
[] in a plug, is connected to a fuse (safety device)
[] can be fatal if touched due to high voltage - neutral wire (blue)
[] completes the circuit with the live wire
[] is constantly kept at 0V - earth wire (green and yellow STRIPED)
[] is earthed, so has a p.d of 0 volts and is there as a safety mechanism
[] connected to the ground
[] if current escapes live wire into a metal case (making the case live), it will travel through the earth wire to the ground rather than a person or animal due to there being a greater difference in voltage between the live wire and the earth wire as opposed to the live wire and a living being
Why are wires encased in plastic ?
plastic is a good insulator, so stops the wires with current touching other metal and making it live etc.
- STOPS PEOPLE GETTING ELECTRIC SHOCKS
Which appliances have an earth wire, and which don’t ?
have = appliances with metal casing that could be live if the live wire frayed + touched the casing
don’t have = appliances with cases made of insulators (eg. plastic)
[] these use 2-core cables
What are the pins on a plug made out of and why ?
- brass
- not as good a conductor as copper but is harder
- doesn’t rust
- doesn’t oxidise
Describe the significance of the internal shape of a plug
shaped so that the wires cannot touch each other when fitted
What is the live wire connected to in a three pin plug ?
- live pin (right)
- fuse
What is the neutral wire connected to in a three pin plug ?
neutral pin (left)
What is the earth wire connected to in a three pin plug ?
earth pin (longest pin at the top)
- automatically earths the appliance
Why are thicker cables sometimes used ?
when carrying a larger current, so that the wires maintain a low(er) resistance and don’t heat up quickly
What are sockets made out of and why ?
stiff plastic
- doesn’t melt if wires become heated
- is a good insulator
What is the National Grid ?
nationwide system of cables and transformers that transfer electricity from power stations to people’s homes
Why is energy ALWAYS lost in the National Grid’s power cables ?
resistance of wires
How is electricity transferred to homes in the UK in the most energy efficient way ?
- sent through step-up transformer
[] gives the electricity higher p.d (132 000 V)
[] higher p.d = less current needed to transfer same amount of electricity
[] thus power loss due to resistance by heating is reduced, so less energy is lost overall - travels through cables
- when reaches near houses, step-down transformer
[] lowers p.d to 230V before sending to houses
Give the equation for power with units
power (W) = energy transferred (J)/time (s)
Give the equation for power SUPPLIED/power of a component with units
power supplied (W) = current (A) x p.d (V)
Give the equation for power using current and resistance
power = current^2 (A) x resistance (ohms)
Give the equation for charge flow
charge flow (C) = current (A) x time (s)
Why does a resistor become hotter when charge flow goes through it ?
(kinetic) energy is transferred to the resistor as thermal energy
Give the equation for energy transferred to a component using charge flow and voltage
energy transferred (J) = p.d (V) x charge flow (C)
