electricity exam q Flashcards
an ammeter and a variable resistor is connected in one parallel loop of a circuit. in the second loop there is a switch. explain what would happen to current in the variable resistor if the switch was closed (2)
the current would be almost zero. because the switch has zero resistance.
Explain the advantage of the national grid transmitting electricity at a very high potential difference (3)
high p.d. means low current, which means less thermal energy is transferred to the surroundings, which increase the efficiency of the power transmission
It is dangerous for a person to fly a kite near an overhead power cable. The person could receive a fatal electric shock if the kite was very close to, but not
touching the power cable. explain why (3)
electric field strength is very high
causing the air to become ionised
string conducts charge to the person
in a power cable, The individual wires behave as if they are resistors connected in parallel.
Explain why the current in the steel wire is different to the current in a single aluminium
wire. (2)
the p.d. across the wires is the same. but the resistance of the steel wire is greater so the current is smaller
Explain how the resistance of a filament lamp changes as the potential difference across it
increases (3)
the current increases (when the potential difference increases)
(which) causes the temperature of the filament to increase
(so) the resistance increases
What does more energy efficient mean?
a high input of energy is usefully transferred
What is the frequency of the mains electricity supply in the UK?
50 herts
in a lawn mower circuit both handle’s switches have to pressed for to to be turned on. explain why (2)
both switches have to be closed to complete the series circuit
The coffee machine has a metal case.
Why would it be dangerous for the live wire of the electric cable to touch the metal case?
risk of electric shock (if someone touched the case)
a student rubs a balloon on his head. Describe the force that acts on the student’s hair (2)
a non-contact force of attraction between the hair and the balloon
3 readings were 14.8, 15.2, 14.9. The student thought that taking a fourth reading would improve the precision of the results.
The fourth reading was 16.2 Ω.
Explain why the student was wrong. (2)
precise results show little variation
the 4th result was further away from the mean than the other values
Explain why adding resistors in parallel decreases the total resistance. (2)
multiple paths for charge / electrons to flow
so total current is greater (
Give two environmental advantages of using a gas-fired power station to generate
electricity compared with using a coal-fired power station. (2)
no sulfur dioxide released
* doesn’t cause acid rain
no particulates released
* doesn’t cause global dimming
An electrician replaced the switch.
The electrician would have received an electric shock unless the circuit was disconnected
from the mains supply.
Explain why. (3)
one wire in the switch is live
the electrician is earthed
the electrician is earthed
The UK mains frequency is 50 Hz.
Explain why it would be safer if the UK mains frequency was not 50 Hz (2)
50 Hz has the lowest (maximum) let-go current
1
a higher / lower / different frequency would allow people to let go at a greater current
