TOPIC 2 - ELECTRICITY

What is the frequency and the potential difference of the mains electricity supply in the UK?

50Hz; 230V

Describe a method the student could use to obtain these results for current vs pd

• ammeter in series with filament lamp 2 measure current
• voltmeter in parallel with lamp 2 measure pd
• variable resistor used to vary current + pd across the lamp
• range of pd 0-6V; interval of 1V
• reverse connections to power supply to obtain negative values
• repeat readings + calculate mean + discard anomalies

What colour is the insulation covering the live wire in a three-core cable?

brown

What happens to the power output as you add more resistors in a parallel circuits?

the total resistance decreases; so the current increases; which increase the power output

Explain why the student s hair stands up on end when touching the Van de Graaf?

• electrons are transferred to the student
• so her hair is negatively charged
• and like charges repel

What is an electric field?

the region around a charged object where another charged object experiences a force

How does the electric field strength vary as the distance from the charged metal dome increases?

the field strength decreases

How would a decreases electrical resistance of air affect the distance a spark can jump between the dome and the earthed conductor?

it would increase

Explain why the cloth became positively charged when rubbed with a plastic rod?

electrons are transferred from the cloth to the rod; electrons are negatively charged; so there are more positive charges than negative charges on the cloth

Explain why the reading on the balance increases when two negatively charged things interact?

there is an additional downard force on the balance; since both are negatively charge; and like charges repel

Explain why a spark jumps between the negatively charged rod and the earthed conductor

the large potential difference between the two objects; causes negative electrons to move through the air; to the conduct (there is a current in the air between the object for that time)

Explain how closing the switch would affect the current in the variable resistor (in a parallel circuit where a switch on a branch is unconnected)

• current would be almost zero; because the switch has zero resistance/ pd across the variable resistor is zero

Explain the advantage of transmitting electricity at a very high potential difference

• very high pd means very low currents
• so less thermal energy is transferred to the surroundings
• which increases the efficiency of power transmission

The person could receive a fatal electric shock if the kite was very close to, but not touching the power cable - why?

• the electric field is very high
• causing the air to become ionised
• the kite conducts charge to the person

Explain why the current in the steel wire is different to the current in a single aluminium wire

the pd across the wires is the same; but the resistance of the steel wire is greater (so less current in the steel)

Explain how the resistance of a filament lamp changes as the potential difference across it increases

• the current increases (when the pd increases)
• which causes the temperature of the filament to increase
• so the resistance increases

Describe the force that acts on the student’s hair when near the balloon?

non-contact force; of attraction (electrostatic)

Explain why adding resistors in parallel decreases the total resistance

multiple paths for charge/ electrons to flow; so total current is greater (for the same pd when more resistors are added)

The electrician would have received an electric shock unless the circuit was disconnected from the mains supply

one wire in the switch is live; the electrican is earthed/ at earth potential; so there will be a large potential difference between the live wire + the electrican if touche