Topic 2 - Electricity

Question 1

Define current.

Answer 1

Current is the flow of electrical charge.

Question 2

What force pushes the current around the circuit?

Answer 2

Potential difference, also known as voltage and represented as a V.

Question 3

What is resistance?

Answer 3

What pushes the current back, measured in Ohms.

Question 4

What does the current depend on (r and pd)? Is the current the same everywhere in a closed single loop circuit?

Answer 4

The current depends on the resistance and potential difference. Yes.

Question 5

What is the formula for charge flow?

Answer 5

Q = It
Q is charge flow in coulombs, C
I is current in Amperes, A
t is time in seconds

Question 6

Draw the symbols for a cell, battery, voltmeter, ammeter, open switch, closed switch, filament lamp, fuse, LED, resistor, variable resistor, diode, LDR and thermistor.

Answer 6

Check the book to see what you got right.

Question 7

What is the equation for potential difference?

Answer 7

V = IR
V is potential difference or voltage in volts
I is current in Amperes
R is resistance in Ohms

Question 8

Describe the resistance of an ohmic resistor.

Answer 8

If temperature is constant, the resistance is also constant because of the equation V=IR.

Question 9

Why does the resistance of a filament bulb increase over time?

Answer 9

As time goes on, more energy is transferred to the thermal store of the bulb, so this heat causes resistance to increase.

Question 10

When do diodes have high resistance?

Answer 10

They have high resistance if a current flows in a certain direction. If it flows in the ‘right’ direction they have no resistance.

Question 11

What is an LDR and how does it work? What can you use them for?

Answer 11

An LDR is a Light Dependant Resistor. In bright light, the resistance falls and in darkness the resistance increases. They have applications in burglar alarms or automatic night lights.

Question 12

How does a thermistor work? What can you use them for?

Answer 12

A thermistor is temperature-dependant. If temperature increases, resistance falls. If temperature decreases the resistance increases. They can be used for temperature detectors in car engines or thermostats.

Question 13

What is a sensing circuit? Describe how a sensing circuit works using a ceiling fan as an example.

Answer 13

A sensing circuit can increase power or turn on components based on the environment. A ceiling fan has a fixed resistor and fan that share the pd in parallel. As the room heats up, the resistance of the thermistor decreases, allowing more pd to be shared between the resistor and the fan so the fan spins faster.

Question 14

Give the equations for resistance, current and potential difference in a series circuit.

Answer 14

R(total) = R1 + R2...
I1 = 12 = ...
V(total) = V1 + V2...

Question 15

Give the equations for current and potential difference in a parallel circuit.

Answer 15

I(total) = I1 + 12...
V1 = V2 = ...

Question 16

Why is the total resistance of 2 resistors in parallel less than the resistance of the smallest one?

Answer 16

In parallel, the resistors have the same pd across them. By adding another loop, the current has more places to go, so using V = IR means that the increase in total current leads to a decrease in resistance.

Question 17

Describe UK mains electricity (current direction, Hz, etc.)

Answer 17

UK mains is an ac current at around 230Hz. The ac main supply is at 50Hz.

Question 18

Describe a battery or cell (current direction)

Answer 18

Batteries supply direct current.

Question 19

In an appliance, describe the live wire.

Answer 19

It has a copper core with brown insulation. It supplies ac electricity at 230Hz.

Question 20

In an appliance, describe the neutral wire.

Answer 20

It has a copper core with blue insulation. It completes the circuit and carries current away. It is 0V.

Question 21

In an appliance, describe the earth wire.

Answer 21

It has a copper core with yellow and green insulation. It stops the appliance casing from going live and is at 0V, carrying no current unless there is an electrical fault.

Question 22

Why can the live wire give you an electric shock?

Answer 22

You are 0V, so the current would flow through you, causing a shock. If the switch was off (open), the you would provide a link between the wire and the earth and the current would flow through you.

Question 23

What would happen if the live and earth wires touch?

Answer 23

A fire can start if a low-resistance link to earth is created.

Question 24

Give the equation for energy transferred in an electrical appliance.

Answer 24

E = Pt
E is energy transferred in J
P is power in W
t is time in s

Question 25

What does the power rating on an appliance mean?

Answer 25

It shows the maximum amount of energy is transferred between stores per second. The lower the appliance rating, the less electricity use and the cheaper it is to run. Higher powers doesn’t mean the appliance transfers more energy efficiently - the higher the current, the more energy transferred to the thermal energy stores of an object.

Question 26

Give the equation for energy transferred.

Answer 26

E = QV
E is energy transferred in J
Q is charge flow in C
V is voltage (pd) in V

Question 27

Give the equation for the power of an appliance.

Answer 27

P = IV
P is power in W
I is current in A
V is pd in V

Question 28

Give the equation for power without the pd.

Answer 28

P = I(squared)R
P is power in J
I squared is current squared in A
R is resistance in Ohms.

Question 29

How does the National Grid distribute energy across the UK?

Answer 29

Using step-up and step-down transformers to keep the current down and the pd high. The pd is boosted to 400 000V because a high current means a lot of electricity would be lost to thermal energy in the surroundings.

Question 30

How is static electricity created?

Answer 30

Through friction. When insulating materials are rubbed together, negative electrons are transferred to the other. This leaves a positive static charge on one and a negative static charge on the other. An example is a polythene (negative) or acetate (positive) rod being rubbed with a cloth duster.

Question 31

Why can static cause sparks?

Answer 31

As the charge builds, the pd between the object and earth increases from 0V. If it gets large enough, the electrons can jump between them, causing a spark.They can jump to an earthed conductor, like you. 2 examples of static shock are from a car or lightning.

Question 32

What is electrostatic attraction / repulsion and what type of force is it?

Answer 32

It is when like charges repel and unlike charges attract. It is non-contact.

Question 33

Describe an electric field diagram for an isolated charged sphere.

Answer 33

The sphere has lines coming off it at right angles to the surface. It always goes positive to negative. The closer the lines, the stronger the field. If the sphere is positive, the lines face away. If the sphere is negative, the lines face towards it.

Question 34

How do electric fields interact?

Answer 34

The closer two charged objects are, the more force will act on them. If they are like charges they will repel. If they are unlike charges they will attract. If you move them away from each other the force will be weaker.

Question 35

How do electric fields explain sparking?

Answer 35

Sparks are created through a high pd between a charged object and earth. This causes a strong electric field and removes electrons from the air particles (ionising). When air is ionised, it allows current to pass through it, causing a spark.