Electricity

Question 1

Define current and state an equation that links current, charge and time, with units for each.

Answer 1

Current is the flow of electric charge, C=It
Charge-C
Current-A
Time-s

Question 2

What is meant by potential difference and resistance in a circuit?

Answer 2

Potential difference is the driving force that pushes the charge around
Resistance is anything that slows the flow down

Question 3

What is the equation that links potential difference, current and resistance?

Answer 3

V=IR

Question 4

what is an ohmic conducter

Answer 4

Something that has a constant resistance that doesn’t change with the current

Question 5

Name one linear component and one non-linear component

Answer 5

Linear-A fixed restistor (have I-V characteristics that’s a straight line)
Non-linearA filament lamp or a diode (have curved I-V characteristics)

Question 6

Explain how the resistance of an LDR varies with light intensity

Answer 6

As light intensity increases resistance falls

Question 7

What happens to the resistance of a thermistor as it gets hotter

Answer 7

As temperature increases resistance falls

Question 8

Is potential difference shared between components in a series circuit

Answer 8

Yes the potential difference round a circuit always adds up to equal the source pd

Question 9

How does the current through each component vary in a series circuit

Answer 9

In a series circuit the same current flows through all components

Question 10

How does potential difference vary between components connected in parallel

Answer 10

In parallel circuits all components get the full source pd so the potential difference is the same across all components

Question 11

Explain why adding resistors in parallel decreases the total resistance of a circuit but adding them in series increases the total resistance

Answer 11

In parallel all resistors have the same potential difference across them. By adding another loop, the current has more than one direction to go in therefore increasing the total current that can flow through the ciruit. An increase in currents means a decrease in total resistance of the circuit. In series they are simply added up.

Question 12

What current is mains electricity

Answer 12

Alternating current

Question 13

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

Answer 13

230V 50Hz

Question 14

Name and give the colours of the three wires in a three core cable. Why are they colour coded?

Answer 14

Neutral wire-blue
Live wire-Brown
Earth wire-Green and yellow
The colour shows it’s purpose

Question 15

Give the potential differences for the three wires in a three core mains cable

Answer 15

Neutral wire-0V
Live wire-230V
Earth wire-0V

Question 16

Explain why touching a live wire is dangerous

Answer 16

Your body is at 0V therefore if you touch a live wire a large potential difference is produced across your body and a current flows through you, causing a large electric shock which could injure or kill you.

Question 17

State the three equations that can be used to calculate electrical power

Answer 17

Energy transferred(J)=Power(W)xTime(s)
Power(W)=Potential difference(V)xCurrent(A)
Power(W)=Current^2(A)xResistance(Ohms)

Question 18

What is the power rating of an appliance

Answer 18

It tells you the maximum amount of energy transferred between stores per second when the appliance is in use.

Question 19

Explain why electricity is transferred by the national grid at a high pd but low current

Answer 19

It’s cheaper to boost the pd up really high. For a given power, increasing the pd decreases the current which means less energy is lost by the heating of the wires

Question 20

What are the functions of step up and step down transformers

Answer 20

Step up transformers are used to increase potential difference. Step down transformers are used to reduce potential difference for domestic use

Question 21

How does rubbing of materials cause static electricity to build up

Answer 21

When certain insulating materials rub together, negatively charged electrons will be scraped off one and dumped on the other. This will leave the materials electrically charged.

Question 22

In which direction do the arrows on an electric field lines point?

Answer 22

From positive to negative

Question 23

Using the concept of electric fields explain how a build up of static electricity can cause a spark

Answer 23

As electric charge builds up on an object the potential difference between the object and the earth (0V) increases. A high potential difference causes a strong electrical field between the charged object and the earthed object. the strong electric field ionises air particals. Air is normally an insulator, but when ionised is much more conductive so current can flow through it. This is the spark.