Electricity Up to level 6

Question 1

What is the current at points 1 and 2 on this circuit. Explain why you have put this answer

Answer 1

The current at 1 is 3A and the current at 2 is 3A.

This is becuase current is the same at all points in a series circuit

Question 2

What is the potential difference across the resistor in this diagram. Explain why this is the case.

Answer 2

The potential difference across the resistor is 5V.

This is becuase the potential differences across the components in a series circuit must add up to make the potential difference across the resistor

This is the formula for potential differences in series

V_T = V₁ + V₂ + V₃….

Question 3

how is electric current calculated?

Example: What is the current when 12C of charge takes 4s to flow past a point?

Answer 3

Current is found from this formula I=Q/t

I=Current (A - Amps)

Q = Charge (C - coulombs)

t = time (s - seconds)

For example if there are 12C of charge and it takes 4s to flow

I = 12/4 = 3 A

Question 4

What is potential difference?

How do you calculatre potentaion difference?

Example: What is the potential difference when 6C of charge has 9J of energy

Answer 4

Potential difference is the amount of energy that each coulomb of charge looses or gains when it moves through the component.

You can think of it as how hard the electricity is pushed

Potential difference can be found by this formula

V = E/Q

V = potential difference (V - volts)

E = Energy (J - Joules)

Q = charge (C - Coulombs)

For example if there is 6C of charge with 9J of energy

V = 9/6 = 1.5V

Question 5

What is this the circuit symbol for and what does it do

How is it connected in a circuit?

Answer 5

This is an ammeter and it measures the current through a component.

It is connected in series with the component it is measuring

Question 6

What component is this the graph of?

What does the graph tell you about the component?

Answer 6

This is an ohmic resistor

The graph shows that it has a constant resistance becuase it is a straight line.

Question 7

Which component is this the V-I graph for?

What does the graph tell us about this component?

Answer 7

This is a graph for a filament lamp.

The graph shows that the resistance of the graph increases at a higher potential difference. This is shown becuase the flatter the gradient of the graph the higher the resistance.

The graph is flatter at a higher potential difference, so the resistance must be higher

Question 8

Which component is this the graph of?

What does it tell us about the component?

Answer 8

This is the graph of a diode

It shows that a diode only allows the current to flow in one direction. It shows this becuase it has a high current when the potential difference flows in one direction and no current when the potential difference flows in the other direction.

Question 9

How do you calculate the resistance of resistors in series.

Example: What is the resistance of this series of resistors?

Answer 9

When reisitors are in series, the total resistance can be found by adding up the individual resistances.

In the example the resistance = 5 + 10 + 14 = 29Ω

Question 10

What does resistance do in a circuit?

What is resistance mearued in?

Answer 10

Resistance slows down the current in a circuit.

Resistance is measured in Ohms (Ω)

Question 11

What does potential difference do to the current in a circuit?

What is potential difference measured in?

Answer 11

Potential difference speeds up the current in a circuit

Potentail difference is measured in volts (V)

Question 12

What is the definition of current?

What carries the charge in a metal wire?

Answer 12

Current is the rate of flow of charge in a circuit. This means it is how fast the electric charge is travelling through a circuit.

Electrons carry the charge in a circuit.

Question 13

What is ohms law?

Answer 13

Ohm’s law states that the resistance in a circuit is found from the potential difference divided by the current

R = V/I

R = Resistance(Ω - ohms)

V = Potential difference (V - Volts)

I = Current (A - Amps)

Question 14

How do you calculate the resistance of a component?

For example, what is the resistance of a lamp with a current of 4A and a potential difference of 16V

Answer 14

Resistance is calculated from ohms law. The equation for ohms law is

R = V/I

R = Resistance (Ω - ohms)

V = Potential difference (V - Volts)

I = Current (A - Amps)

Example: Resistance = V/I = 16/4 = 4Ω

Question 15

Explain how to investigate how the length of a piece of wire affects its resistance

Answer 15

To investigate how the length of a piece of wire affects its resistance, set up a circuit with the wire a cell, an ammeter and a switch in series. Then attach a voltmeter across the crocodile clips and place the clips 10cm apart.

Close the switch and read the potential difference and current in the circuit. use this to calculate the resistance

Repeat the experiment where the length of the wire is different each time, going up in intervals of 10cm.

Then plot a graph of length against resistance

Question 16

Explain what a variable resistor does and how it works

Answer 16

A variable resistor is a component where you can change its resistance

You can do this by sliding the slider, which makes the length of the resistance wire longer or shorter depending on which way you slide it

The longer the resistance wire the higher the resistance

Question 17

How do you calculate the charge transferred by a current.

For example, what is the charge transferred by a 3.5A current which flows for 30 seconds

Answer 17

The charge transferred by a current can be found from the formula Q=It

Q = charge (C - Coulombs)

I = current (A - Amps)

t = time (s - seconds)

Example: If 3.5A carries a current for 30 seconds, charge = 3.5 x 30 = 105C

Question 18

What is the rule for working out the potential difference of components that are in parallel with each other?

Example: What is the potential difference across the unknown resistor in the parallel circuit?

Answer 18

Resistors in paralell always have the same potential difference across them. The formula for this is:

V_T = V₁ + V₂ + V₃

Example: This resistor has 9V across it becuase there is 9V accross the cell that it is in parallel with

Question 19

What carries the charge in a metal wire?

Answer 19

In a metal wire, electrons carry the charge around the circuit.

Question 20

How does energy get into and out of an electric circuit?

Explain the energy transfers in each case

Answer 20

Energy is supplied to an electric circuit by a power source. This transfers energy into electrical potential energy.

For example a cell transfers chemical energy into electrical potential energy

Energy is transferred away from a circuit by a resistor or another component with resistance. This transfers energy out of the electrical potential energy store.

For example, a resistor transfers electrical potential energy into thermal energy

Question 21

Explain how to calculate the energy that is transferred to a component when charge passes across a potential difference

For example, what is the energy dissipated when 2C of charge passes across a potential difference of 3V

Answer 21

The energy transferred by a coulomb of charge can be found from this formula

E = VQ

E = energy (J - Joules)

V = Potential difference (V - Volts)

Q = charge (C - coulombs)

Example: When 2C of charge passes across a PD of 3V the energy transferred is 2 x 3 = 6J of energy

Question 22

Explain how the law of conservation of energy applies to an electric circuit

Answer 22

The law of conservation of energy states that “energy cannot be created or destroyed, it can only be transferred from one store to another”

The cell transfers chemical energy into electrical potential energy. This energy is transported round the circuit to the component, which transfers it into other forms.

Due to conservation of energy, the total energy supplied by the power source must be equal to the total energy transferred out by all of the components combined.

Question 23

How do you work out how much energy has been transferred by an appliance in Kilowatt-hours?

Example: A 2kW heater runs for 4 hours, what is the energy transferred in kWh

Answer 23

To work out the energy in kilowatt hours you multiply the power in kilowatts by the time in hours

Example: Energy in kWh = 2 x 4 = 8kWh

Question 24

What does the word efficiency mean?

How do you express an efficiency of 35% as a decimal?

Answer 24

Efficiency is the proportion of the energy that you put into an appliance that is transferred into a useful energy output

To convert an efficiency of 35% into a decimal you divide it by 100 - 35/100 = 0.35

Question 25

how do you calculate the power of a device when you know how long it has been left on for?

Example: what is the power of TV which converts 7500J of energy in 20 seconds?

Answer 25

The power of a device can be found from this equation:

P = E/t

P = Power (W - Watts)

E = Energy (J - Joules)

t = time (s - seconds)

A TV that converts 7500J of energy in 20 seconds has a power of 7500/20 = 375W

Question 26

How do you calculate the energy transfer of an appliance in Joules when you are told how long it is left on for?

Example: How much energy is transferred by a 60W lightbulb that is left on for 800 seconds?

Answer 26

To calculate the energy transfer in joules of an appliance you use this formula

E = Pt

E = Energy (J - Joules)

P = Power (W - Watts)

t = time (s - seconds)

When a 60W light bulb is left on for 800 seconds the energy transferred is 60 x 800 = 48000J

Question 27

What are the main factors that affect how much an appliance costs when you use it?

Answer 27

The main factors that affect the cost of an appliance are:

The power of the appliance - the more powerful it is the more energy it uses

The time it is used for - the more time it is used for the more energy it uses

The electricity tariff - the more each unit of electricity costs, the higher the bill

Question 28

Describe the mains electricity supply in the UK

Answer 28

The mains electricity supply in the UK is an alternating current supply.

It has a potential difference of 230V and a frequency of 50Hz (it goes up and down 50 times each second)

Question 29

What is the difference between an AC and a DC power supply

Answer 29

AC means alternating current. In an AC circuit the current moves backwards and forwards. This is becuase the potential difference is constantly chaning, from positive to negative and back again. AC current works with transformers, so the potential difference can be altered to stop energy lost when the electricity is in the transmission lines.

DC means direct current. In a DC circuit the current always flows one way. This is becuase it has a constant potential difference. It is a lot easier to build a motor using a DC supply rather than an AC supply

Question 30

How does changing the frequency or amplitude of an AC current affect the trance on an oscilloscope.

Answer 30

When the potential difference (amplitude) is changed the wave changes height (high P.D = tall height)

When the frequency is changed the wave becomes shorter becuase more cycles are happening in the same time.

Question 31

What are the colours of the three different wires in a plug?

What does each wire do?

Answer 31

Brown = live wire. This takes electricity to the component

Blue = Neutral wire. This takes electricity away from the component

Green/Yellow = Earth wire. Electricity only travels through this if the case comes into contact with the live wire

Question 32

What are the common hazards that can be found on electricl wiring?

Answer 32

The common hazards are

• loose wiring
• broken wire sheathes exposing the insides of the wires.
• Missing cable grip meansing the wires could be pulled out easily
• Water near electrical appliances

Question 33

Compare the amplitude and frequency of these two AC waves

Answer 33

The wave at the top has a higher frequency. This is seen becuase the waves are closer toghether.

The wave at the bottom has a larger amplitude, this is seen because this wave is taller than the wave at the top

Question 34

Explain what a short circuit in an appliance is and why it could cause a problem.

What are the safety features of a plug that can prevent this from being a problem?

Answer 34

A short circuit occurs when a live wire touches the metal casing of an appliance, this means that the electric current bypasses the appliance and flows through the case.

This is dangerous because the case is now charged to 230V. If a person touches the case they will get electrocuted.

There are two safety features that stop this from being a problem.The earth wire will take the current away from the case, this neutralises the case. When there is a short circuit the current gets very high. This causes the fuse to blow which breaks the circuit and stops any more current.

Question 35

Which material are each of these pieces of a plug made of?

Casing

Pins

Wires

Sheath

For each one explain why it is made of this material

Answer 35

The casing is made of plastic. This is becuase it is an insulator, protecting the user from being electrocuted, and is strong and rigid, preventing it from being damaged.

The pins are made of brass. This is becuase brass conducts electricity. Although it is not as good as copper at conducting electricity it is preferred for the pins becuase it is strong and does not bend

The wires are made of copper. Copper conducts electricity very well and is flexible.

The sheath is made of rubber. This is becuase it is flexible and is an insulator.

Question 36

What is the definition of power?

What is it measured in?

Answer 36

Power is how quickly the energy is transferred from one form into another

Power is measured in Watts

1 Watt = 1 Joule per second

Question 37

What factors affect the power that is transferred through an appliance

Answer 37

The power transferred by an appliance depends on the current and the potential difference. It can be found from this formula

P = IV

P = Power (W - Watts)

I = current (A - Amps)

V = Potential difference (V - Volts)

When there is a higher potential difference, each coulomb of charge that passes through an appliance transfers more energy, so the energy is transfered faster

When there is a higher current more coulombs of charge pass through the appliance each second. This means the energy is transferred faster so power goes up.

Question 38

Explain why different appliances have different sized fuses

What are the problems that could occur if the wrong fuse was put into an appliance?

Answer 38

Different appliances need to have different sized fuses because they have different currents running through them.

If you put in a fuse that is too small the fuse will blow even in normal operation. If you put in a fuse that is too large it will blow in normal operation.

Question 39

Explain how to select which fuse to use for an appliance

For example what fuse would you use for an appliance that draws 3.6 A of current in normal operation

Answer 39

Fuses come in 3A, 5A and 13A sizes. to select the right size of fuse to use you need to pick the smallest fuse that is larger than the normal current of the appliance

For a current that normally draws 3.6A of current you would choose a 5A fuse, because this is the smallest fuse that is larger than the current.

Question 40

Explain how current acts in parallel branches.

What is the unknown current in this circuit?

Answer 40

Current in a parallel circuit splits becuase the charge can only flow down one branch or the other

This means that the current in the branches add up to make the total current. This is given by the formula

I_T = I₁ + I₂ + I₃….

Example: in this circuit the total current is 5.6A and the current in one of the branches is 3.9A. this means the current in the other branch must be 5.6-3.9 = 1.7A

Question 41

Explain how you calculate the energy transferred by an appliance from time, potential differnce and current

Example: What is the energy transferred by a kettle that draws 9A of current on the mains electricity supply for 120 seconds

Answer 41

The energy of an electrical applaince can be found from this formula:

E = IVt

E = Energy (J - Joules)

I = Current (A - Amps)

t = time (s - seconds)

Example: for a 9A kettle on mains electricity (230V) for 120 seconds the energy is 9 x 230 x 120 = 248,400 J

Question 42

Complete this diagram to show the different parts of a plug

Answer 42

Question 43

What is this component called and what does it do?

Answer 43

This is a thermistor

The resistance goes down when it is heated up.

The resistance goes up when it is cooled

Question 44

What is this component called and what does it do?

Answer 44

This is a lamp, it converts electrical energy into light energy

Question 45

What is this component called and what does it do?

Answer 45

This is a fuse

It is a safety feature that breaks the circuit if the current gets too high

Question 46

What is this component called and what does it do?

Answer 46

This is a light dependent resistor (LDR)

The resistance goes up when it is dark.

The resistance goes down when it is light

Question 47

What is this component called and what does it do?

Answer 47

This is a switch.

It breaks the circuit to stop the current from flowing and completes the circuit to start the current flowing

Question 48

What is this component called and what does it do?

Answer 48

This is a cell

It converts chemical potential energy into electrical potential energy

This makes the current move

Question 49

What is this component called and what does it do?

Answer 49

This is a resistor

It slows down the current in the circuit

When it does this it converts electrical potential energy into thermal energy, so it gets hotter

Question 50

What is this component called and what does it do?

Answer 50

This is a variable resistor

It allows a person to change the resistance of the resistor

Question 51

What is this component called and what does it do?

Answer 51

This is a diode

It only allows current to flow in one direction

Question 52

What happens to the potential difference in a circuit when cells are connected in series

Example: what is the total potential difference of these 3V cells?

Answer 52

The total potential difference is found by adding up the potential differences of each of the cells

Example: this battery contains 6 3V cells so the total potential difference is 6x3 = 18V

Question 53

What is the potential difference across each of these resistors?

Explain why this is the case

Answer 53

The potential difference across each of these resistors is 18V. This is because the potential difference across each branch of a parallel circuit is the same as the potential difference across the cell.

Question 54

Explain why this circuit will not work

Answer 54

Diodes will only allow the current to flow one way. In electrical circuit symbols, the current flows from the long part of the cell symbol to the short part of the cell symbol. To make the circuit work you need to turn the diode round the other way.

Question 55

Explain why the resistance of this circuit is 30Ω and not 50Ω

Answer 55

One of the 20Ω resistors has be short circuited. This means that the resistance is not counted in the total resistance as the electrons bypass it and travel through the wire instead