Page 153 - Knowledge Organiser Yr9 24-25

Knowledge Base: Science P2: Electricity Year 9
9. Mains electricity and cables
9.1 What does dc stand for? Direct current
9.2 What does ac stand for? Alternating current
9.3 Describe direct current Current that is always in the same direction
9.4 Describe alternating current Current repeatedly reverses direction
9.5 Describe the potential difference of an ac supply Continuously reverses polarity
9.6 What is the frequency of the domestic ac supply 50Hz in the UK?
9.7 What does the word frequency mean in physics? Number of cycles per second
9.8 State the unit of frequency Hertz or Hz
9.9 What is the potential difference of the domestic ac 230V supply in the UK?
9.10 Are batteries ac or dc? dc
9.11 Is the mains supply ac or dc? ac
9.12 What are the 3 wires in a three-core cable? Live, neutral and earth
9.13 Which wire in a 3-core cable is brown? Live
9.14 Which wire in a 3-core cable is blue? Neutral
9.15 What colour is the earth wire in a 3-core cable? Green and yellow stripes
9.16 Which wire in a 3-core cable carries the Live alternating potential difference from the supply?
9.17 Which wire in a 3-core cable completes Neutral the circuit?
9.18 What does the earth wire do in a 3-core cable? Stop the appliance becoming live
10. The National grid
10.1
What is the purpose of the national grid?
To provide electricity to all homes and businesses across the UK.
10.2
What are the two parts of the national grid?
Cables and transformers.
10.3
What do step-up transformers do?
Increase potential difference which reduces current.
10.4
What do step down transformers do?
Decrease potential difference which increases current.
10.5
How do step up transformers increase the efficiency of energy transfers in the national grid?
Reducing current means less energy is wasted to thermal energy stores of the cables and surroundings.
10.6
Why are step down transformers needed?
To make sure potential difference is safe level to use.
10.7
What is the equation that links the current and potential difference across primary and secondary coils in a transformer?
VpIp = VsIs
10.8
If the potential difference in the primary coils is 5000V and the current is 3A, calculate the current in the secondary coils when the potential difference is reduced to 250V.
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