9 i energy & electricity

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20051 of 34

KS3 Physics

9I Energy and Electricity



Contents

Measuring current

Measuring voltage

Energy in circuits

Summary activities


The unit of measure for current is the amp, which has the symbol A.

A

Measuring current

AA

Current is measured using a device called an ammeter. In a circuit diagram, an ammeter is shown by the symbol .

When measuring the current through a component, the ammeter is always connected in series (in the same loop) with that component.


1

2

1. Set up the circuit as shown above.

R1

A

A

Experiment 1: Current in series circuit

Circuit 1

2. Measure the current using the ammeter at positions 1 and 2.


3

1. Add another resistor into the circuit (R2) and another ammeter after it.

R1

A

A

A

R2

Experiment 1: Current in a series circuit

Circuit 2

1

2

2. Now measure the current using the ammeter at positions 1, 2 and 3.


Circuit 2 results:

Current at position 1 =



Circuit 1 results:



Experiment 1: Current in a series circuit – results

R1A

A

Circuit 1 Circuit 2

R1 R2A

AA1

2

1

2

3


Experiment 1: Current in a series circuit – summary

Circuit 1The current at different positions in the circuit, before and after the resistor, was the _______.

Current is _____ used up by the components in the circuit.

Circuit 2Increasing the number of components in the circuit ________ the current.

The current at all points in a series circuit is the _______.

same / same / decreased / not

Circuit 1 Circuit 2

R1A

A R1 R2A

AA


What is a parallel circuit?

A B

This means that there is more than one path around the circuit.

A parallel circuit is one which contains a point (a junction) where the current can split (point A) or join (point B).


A1

A2

A3

A4

1. Place the ammeter, in turn, at positions 1, 2, 3 and 4.

Measuring current in a parallel circuit

Ammeter Current (A)

A1

A2

A3

A4

2. Record the ammeter readings in the table.


For a parallel circuit, the current that leaves the cell is the same as the current that returns to the cell.

Current in a parallel circuit

The current does not get used up by the circuit, just the energy that the electrons are carrying.

A1

A2

A3

A4

A1 = A4


The current splits up at the first junction and then joins together at the second junction.

Current in a parallel circuit

If the bulbs are identical then the current will split evenly.

The following is always true for this type of parallel circuit:A1

A2

A3

A4

A1 = (A2 + A3) = A4

If the bulbs are not identical, then the current will not split evenly.



Contents

Measuring current

Measuring voltage

Energy in circuits

Summary activities


When measuring the voltage across a component, the voltmeter is always connected in parallel with (or across) the component.

Voltage is the amount of push and is measured in ‘volts’ which has the symbol V.

This is still a series circuit.

Measuring voltage

The voltage supplied by the battery is shared between all the components in a series circuit.

V3

V2

V1

VVoltage is measured using a device called a voltmeter. In a circuit diagram, a voltmeter is given the symbol .



R1

V

V

Circuit 1

Experiment 2: Voltage in a series circuit

2. Connect the voltmeter across the power supply (battery) and measure the supply voltage.

3. Then connect the voltmeter across the resistance (R) and measure this voltage.


1. Add another resistor (R2) to the circuit as shown.

V2

Experiment 2: Voltage in a series circuit

Circuit 2

R1 R2

V

V1

2. Connect the voltmeter across the power supply (battery) and measure the supply voltage.

3. Then measure the voltage across each of the resistor.


Circuit 1 results:

Voltage (supply) = V

Voltage (R1) = V

R1 R2

V

V1 V2

R1

V

V

Circuit 1 Circuit 2

Experiment 2: Voltage in a series circuit – results

Circuit 2 results:

Voltage (supply) = V

Voltage (R1) = V

Voltage (R2) = V


Experiment 2: Voltage in a series circuit – summary

The current is the _______ of electricity around the circuit. The _________ is the amount of push.

When two components were put into Circuit 2, the voltage of the supply was the _______ as Circuit 1. However, the voltage across R1 __________ .

The voltage across both components in Circuit 2 added to be equal to the ________ voltage.

R1 R2

V

V1 V2

R

V

V

Circuit 1 Circuit 2

supply / decreased / voltage / flow / same


Make your own series circuit


V2

Connect up this circuit and measure, in turn, the voltage at V1, V2 and V3. Record your results in the table.

Measuring voltage in a parallel circuit

What do you notice about the results?

How can you explain this?

Voltmeter Voltage(V)

V1

V2

V3

V1

V3


Make your own parallel circuit



V

V

R A

Experiment 3: Cells in a series circuit

Circuit 1

2. Connect the voltmeter across the power supply (battery) and measure the supply voltage. Then measure the voltage across the resistance. Also measure the current.


1. Add an additional battery to the circuit.

V

V

R A

Experiment 3: Cells in a series circuit

Circuit 2

2. Connect the voltmeter across the power supply and measure the supply voltage. Then measure the voltage across the resistance. Also measure the current.


Circuit 1 results:

Supply voltage =

Voltage R =

Current =

Circuit 2 results:

Supply voltage =

Voltage R =

Current =

Experiment 3: Cells in a series circuit – results

R

V

V

A R

V

V

A

Circuit 1 Circuit 2


Experiment 3: Cells in a series circuit – summary

Delete the wrong answer:

Increasing the number of cells increases/decreases the current that flows in the circuit.

The current/voltage depends on the current/voltage.

Circuit 1 Circuit 2

R

V

V

A R

V

V

A


Current and voltage – key ideas

Current

In a series circuit, the current is the same in all parts of the circuit.

In a parallel circuit, the current splits up and recombines when the branches of the circuit meet up. (The sum of the current in the branches equals the total current.)

The current depends on the voltage in any circuit.

Voltage

In a series circuit, the supply voltage is shared between the components. (The sum of the voltage across each component is the same as the total supply voltage.)

In a parallel circuit, the voltage across each component is the same as the supply voltage.



Contents

Measuring current

Measuring voltage

Energy in circuits

Summary activities


Energy transfer in circuits

Energy cannot be created or destroyed.

In all devices and machines, including electric circuits, energy is transferred from one type to another.

When this circuit is connected, chemical energy stored in the battery is transferred via electrical energy to heat and light energy in the bulbs.

The total amount of heat and light energy is the same as the amount of chemical energy lost from the battery.


Most of the energy from the battery does not produce light – most of it is wasted as heat!

chemical energy from battery (e.g. 100J)

heat energy of bulb

95 J transferred to

5 J transferred to bulb as light energy

Energy transfer in electrical circuits


The efficiency of an energy transfer can be calculated using this formula:

total energy input

useful energy output %Efficiency = x 100

Calculating energy efficiency

This bulb converts 200 J of chemical energy form battery into 10 J of useful light energy:

Efficiency of bulb = 10 200

= 5%

x 100( )


Batteries can power many electrical devices.

What sort of energy is electrical energy transferred into in these electrical devices?

What’s the energy transfer?



Contents

Measuring current

Measuring voltage

Energy in circuits

Summary activities


Glossary

current – The flow of electricity, measured in amps (A).

efficiency – A measure of how much energy is changed from one form to another.

potential difference – The amount of ‘push’ or electrical energy there is in a circuit, measured in volts (V).

power – The amount of energy that an electrical device uses per second, measured in watts (W).

power station – A place where an energy resource is transformed into electrical energy.

transfer – To move energy from one place to another.

transform – To change energy from one type to another.

voltage – Another name for ‘potential difference’.


Anagrams


Multiple-choice quiz

Business

9 i energy & electricity