Electricity

Question 0

Define the kilowatt hour (kWh)?

Answer 0

A unit of energy equal to 3.6MJ or 1kW for 1hour.

Question 1

What is meant by a ‘230V, 25W’ for an appliance?

Answer 1

When connected to the 230V supply, the power (energy per second) is 25W.

Question 2

Define intensity?

Answer 2

Intensity is the incident energy per unit area per second.

Question 3

What is a current?

Answer 3

A flow of charge.
In a metal due to flow of electrons.
In an electrolyte due to flow of +ve and -ve ions.

Question 4

Direction of conventional current?

Answer 4

Positive to negative

Question 5

Direction of electron flow?

Answer 5

Negative to positive

Question 6

What is a coulomb?

Answer 6

One coulomb is the charge which passes a point when a current of 1A flows for 1s

Question 7

Charge =

Answer 7

Current x time

Question 8

What is the elementary charge?

Answer 8

Minimum charge something can have - 1.6x10^-19C

Question 9

What is does Kirchhoffs first law state?

Answer 9

That the sum of the current entering any point in a circuit is equal to the sum of the currents leaving that same point.

Question 10

Current in a conductor:

I = ?

Answer 10

Anev

A is area
n is the number density
e is elementary charge
v is mean drift velocity

Question 11

How should you attach an ammeter?

Answer 11

In series

Question 12

How should you connect a voltmeter?

Answer 12

In parallel

Question 13

What is 2.00mm^2 in metres?

Answer 13

2x10^-6

Question 14

What is the shape of a diode?

Answer 14

Triangle with flat line on pointy end

Question 15

If a thermistor heats up, what happens to the resistance through it?

Answer 15

Temperature rises, resistance falls, current rises

After x amount of time, resistance will become constant because thermal equilibrium will be reached

Question 16

How do LDR’s work?

Answer 16

As light intensity increases, resistance falls

Question 17

Define emf?

Answer 17

The energy transferred per unit charge when another type of energy is converted into electrical energy

Question 18

How do you work out the power dissipated by a resistor?

Answer 18

Current (through resistor) x voltage (across resistor)

Question 19

What is mean drift velocity?

Answer 19

The average velocity that an electron attains when an electric current flows.

Question 20

Giving examples, what are the differences between conductors, insulators and semi conductors?

Answer 20

Conductors (eg. Copper) contain a high number density of conducting electrons.

Insulators (eg. Rubber) contain very few or no conducting a electrons

Semiconductors (eg. Silicon) materials that have increasing conductivity with temperature, so a medium number density

Question 21

Define potential difference?

Answer 21

The electrical energy transferred per unit charge when electrical energy is converted into another form of energy

Question 22

Define the volt?

Answer 22

One volt is 1joule per coulomb

Question 23

W=

Answer 23

Voltage x charge

W is work done

Question 24

Define resistance?

Answer 24

The opposition to a flow of charge

Question 25

Define the ohm?

Answer 25

The resistance when 1volt produces 1amp of current

Question 26

What is ohms law and how can you tell if it is obeyed in a circuit?

Answer 26

Electrical current is proportional to voltage. | Straight line graph through origin

Question 27

Benefits of LEDs?

Answer 27

Long lifespan Energy efficient Only works one way round (positively biased) Has a threshold voltage Lots of different colours that require different voltages

Question 28

What happens to the resistance of s material when it's temperature increases?

Answer 28

Resistance increases

Question 29

How is an NTC thermistor affected by temperature?

Answer 29

When temperature increases, resistance decreases.

Question 30

How does a fuse work?

Answer 30

A fuse is fitted in an electrical circuit to protect wiring from excessive current (and therefore helps to prevent excessive heat and fires). Inside the fuse cartridge is a thin wire which gets hot and melts when the current gets too high, thus breaking the circuit. Each fuse has a max current it will permit.

Question 31

What is Kirchhoffs second law?

Answer 31

The sum of all of the emfs around a circuit are equal to the sum of all the pds around the same circuit.

Question 32

What is terminal pd?

Answer 32

The pd across the external resistor connected to an emf source.