Topic 2 - Electricity

Question 1

Define

electrical current.

and give its unit

Answer 1

the rate of flow of charge

amps (A)

Question 2

What is

potential difference?

(or voltage) and give its unit

Answer 2

the driving force that pushes the charge round

volts (V)

Question 3

What is

resistance?

and give its unit

Answer 3

anything that slows the flow down

ohms (Ω)

Question 4

What equation links current, charge flow and time?

Answer 4

charge flow = current x time

Q = It

Q: coulombs, C
I: amps, A
t: seconds, s

Question 5

How would you draw a

fuse

for a circuit diagram?

Answer 5

a rectangle with the wire running through it

Question 6

How would you draw a

variable resistor

for a circuit diagram?

Answer 6

a rectangle with a diagonal arrow going through it

Question 7

How would you draw a

thermistor

for a circuit diagram?

Answer 7

a rectangle with a straight then diagonal line going through it

Question 8

How would you a draw an

LED

for a circuit diagram?

Answer 8

a diode with two arrows pointing north east out of it

Question 9

How would you drawn an

LDR

for a circuit diagram?

Answer 9

a small resistor with a circle around it with two arrows pointing into it, south east

Question 10

What equation links

current, potential difference and resistance?

Answer 10

potential difference = current x resistance

V = IR

V: volts (V)
I: amps (A)
R: ohms (Ω)

Question 11

Explain the I-V graph for an

ohmic conductor.

Answer 11

The current through an ohmic conductor (at a constant temperature) is directly proportional to potential difference so you get a straight line.

(looks like a y=x graph)

Question 12

Explain the I-V graph for a

filament lamp.

Answer 12

As the current increases, the temperature of the filament increases, so the resistance increases. This means less current can flow per unit pd, so the graph gets shallower - hence the curve.

(looks like the middle bit of a sin graph)

Question 13

Explain the I-V graph for a

diode.

Answer 13

Current will only flow through a diode in one direction, as shown. The diode has a very high resistance in the reverse direction.

Question 14

What is the relationship between resistance and light intensity in an LDR?

Answer 14

resistance is inversely proportional to light intensity

Question 15

What is the relationship between resistance and temperature in a thermistor?

Answer 15

resistance is inversely proportional to temperature

Question 16

What is the voltage from the UK mains supply?

and what kind of supply is this?

Answer 16

230V

ac

Question 17

What is the frequency of the ac mains supply?

Answer 17

50Hz

Question 18

How are most electrical appliances connected to the mains supply?

Answer 18

by three-core cables

this means that they have three wires inside them, each with a core of copper and a coloured plastic coating

Question 19

What is the purpose of the

live wire?

and what is its colour?
and pd?

Answer 19

the live wire provides the alternating potential difference from the mains supply

brown, about 230V

Question 20

What is the purpose of the

neutral wire?

and what is its colour?
and pd?

Answer 20

the neutral wire completes the circuit - when the appliance is operating normally, current flows through the live and neutral wires

blue, around 0V

Question 21

What is the purpose of the

earth wire?

and what is its colour?
and pd?

Answer 21

it is for protecting the wiring, and for safety - it stops the appliance casing from becoming live, it doesn’t usually carry a current - only when there’s a fault

green and yellow, 0V

Question 22

What would happen if you touched a

live wire?

and why?

Answer 22

a large electric shock would be caused, which could injure you or even kill you

Your body is at 0V. If you touch the live wire, a large potential difference is produced across your body and a current flows through you.

Question 23

What is the power of an appliance?

Answer 23

the energy that it transfers per second

Question 24

What equation links

charge flow, energy transferred and potential difference?

Answer 24

energy transferred = potential difference x charge flow

E = VQ

E: joules, J
V: volts, V
Q: coulombs, C

Question 25

# What equation links power, potential difference and current?

Answer 25

power = current x potential difference | P = IV ## Footnote P: watts, W I: amps, A V: volts, V

Question 26

# What equation links current, resistance and power?

Answer 26

power = current x current x resistance | P = IIR ## Footnote P: watts, W I: amps, A R: ohms, Ω

Question 27

# What is the national grid? | and what does it do?

Answer 27

a giant system of **cables** and **transformers** that covers the UK and connects **power stations** to consumers ## Footnote it transfers electrical power from **power stations** anywhere on the grid to anywhere else on the grid where it's needed

Question 28

What equation explores current and potential difference in the coils of a transformer?

Answer 28

Vs x Is = Vp x Ip

Question 29

# What equation links voltage and number of turns of wire | in a transformer?

Answer 29

Vp / Vs = Np / Ns

Question 30

How do transformers make the national grid an efficient way of transferring energy?

Answer 30

for a given **power**, increasing the pd **decreases** the **current**, which decreases the **energy lost** by heating the wires and the surroundings

Question 31

# What is static electricity?

Answer 31

charges which are not free to move

Question 32

# How are sparks created? | (2 steps)

Answer 32

1. As **electric charge** builds up on an object, the **potential difference** between the object and the earth (which is at **0V**) increases. 2. If the potential difference gets **large enough**, electrons can **jump** across the **gap** between the charged object and the earth. | (this usually happens when the gap is fairly small)

Question 33

# What are some rules about electric field lines? | (3 rules)

Answer 33

- they go from **positive** to **negative** - they're always at a **right angle** to the surface - the **closer** together the lines are, the **stronger** the field is

Question 34

# How do electric fields explain sparks? | (4 steps)

Answer 34

1. **Sparks** are caused when there is a high enough **potential difference** between a **charged object** and the earth. 2. A high potential difference causes a **strong electric field** between the **charged object** and the **earthed object**. 3. The strong electric field causes **electrons** in the **air particles** to be **removed** (known as **ionisation**). 4. **Air** is normally an **insulator**, but when it is **ionised** it is much more conductive, so a **current** can flow through it.