physics 5 Flashcards
Why is the current provided by a cell called a direct current (d.c.)?
only flows in one direction
What is an alternating current (a.c.)?
current that repeatedly reverses direction
What kind of current is supplied by mains electricity?
alternating current
What colours are the live, neutral, and earth wires in a three-core cable?
live = brown
neutral = blue
earth = green and yellow
stripes
What is the function of the live wire in a three-core
cable?
carries the alternating potential difference from the supply
What is the function of the neutral wire in a three-core cable?
completes the circuit
What is the function of the earth wire in a three-core cable?
safety wire to stop the appliance becoming live
When is there a current in the earth wire?
when there is a fault
Why is the live wire dangerous?
provides a large p.d. that would cause a large current to
flow through a person if they touched it
What is the National Grid?
nationwide network of cables and transformers that link power stations to customers
What are step-up transformers used for in the National Grid?
increase the p.d. from the power station to the transmission cables
What are step-down transformers used for in the
National Grid?
decrease the p.d. from the transmission cables to the
mains supply in buildings so that it is safe to use
How does having a large potential difference in the transmission cables help to make the National
Grid an efficient way to transfer energy?
large p.d. means a small current is needed to transfer the same amount of power, small current in the transmission cables means less electrical power is wasted due to heating
What two things does energy transfer to an appliance depend on?
power of appliance, time it is switched on for
What are the units for power, current, potential difference, and resistance?
watts (W), amps (A), volts (V), ohms (Ω)
