1. Chapter 4: Electrical Circuits

2. Chapter 4: Electrical Circuits
How do objects become charged?
What are electric fields?
What goes on in a circuit?
Circuit symbols
What is resistance?
Series and Parallel Circuits.
Current –Voltage Graphs
Characteristics of thermistors and LDRs

3. Can you????

5. How do objects become charged?

6. What is static electricity?
Static electricity is due to the build up of electric charge.
It is called ‘static’ electricity because the charge is unable to flow.
The build up of electric charge can cause dangerous sparks.
Sometimes, after walking on a carpet and then touching a metal object, such as a door knob, you might get a small shock. This is caused by static electricity.

7. What causes static electricity?
Static electricity is due to electric charge that builds up on the surface of an insulator, such as a plastic comb.
The charge that has built up cannot easily flow away from the insulator, which is why it is called static electricity.

8. Where does static charge come from?
All materials are made of atoms, which contain electric charges.
Around the outside of an
atom are electrons, which
have a negative charge.
The nucleus at the centre of
an atom contains protons, which have a positive charge.
An atom has equal amounts of negative and positive charges, which balance each other, so the atom has no overall charge.
Electrons do not always stay attached to atoms and can sometimes be removed by rubbing.

9. How does static charge build up?
Static charge can build up when two insulating materials are rubbed together, such as a plastic comb moving through hair.
Friction between the materials causes electrons to be transferred from one material to the other:
One material ends up with more electrons, so it now has an overall negative charge.
One material ends up with fewer electrons, so it now has an overall positive charge.

10. How can static charge be created?
Friction can be used to create a static charge.
If an insulator is rubbed with a cloth, it can become charged in one of two ways: OR
Electrons move from
the cloth to the insulator.
Electrons move from
the insulator to the cloth.
The insulator ends up with an overall positive charge.
The insulator ends up with an overall negative charge.

11. Static charge – true or false?
Teacher notes
This true-or-false activity could be used as a plenary or revision exercise on static charge, or at the start of a lesson to gauge students’ existing knowledge of the subject matter. Coloured traffic light cards (red = false, yellow = don’t know, green = true) could be used to make this a whole class exercise.

12. What are the forces between charges?
The forces between charges can be investigated using rods made of insulating materials.
acetate rods
What happens when two positively-charged acetate rods are placed near each other?
The rods repel each other because they have the same overall charge.
rods repel
each other
What will happen if one rod is replaced with a charged polythene rod?

13. How do opposite charges behave?
When a charged acetate
rod is placed near a
charged polythene rod,
the rods attract each other.
polythene rod
acetate rod
Why does this happen?
The polythene rod has an overall positive charge and the acetate rod has an overall negative charge.
The overall charges of these rods are opposite and so they attract each other.
rods attract
each other
CONTENTS

14. What are electric fields?

15. Charges and electric fields
Boardworks KS3 Science 2014
Static Electricity
All charged objects are surrounded by a region called an electric field.
An electric field is a region where a charged particle will experience a force.
Electric fields can be represented by electric field lines.
The direction of the lines represents the direction of the field – it shows the direction a positive charge would move in the field.
Teacher notes
Students could compare the definition of an electric field with that of a gravitational field.
The distance between lines represents the strength of the field – the closer the lines, the stronger the field.
© Boardworks 2014 15

16. Drawing electric fields
Boardworks KS3 Science 2014
Static Electricity
The field consists of a family of lines extending evenly in all directions to or from the charge.
The rules of drawing electric fields are:
1. The lines start on positive charges and finish on negative charges.
2. The number of lines drawn is proportional to the strength of the charge.
3. No two field lines ever cross.
© Boardworks 2014

17. The shape of an electric field
Boardworks KS3 Science 2014
Static Electricity
The shape of an electric field
© Boardworks 2014 17

18. How big is the electric field?
Boardworks KS3 Science 2014
Static Electricity
The electric field extends to infinity – the field goes on forever.
You can draw the field lines as long as you want. But remember that the longer the field lines are, the further the ends of the lines are from each other.
What happens to the strength of the electric field as you get further away?
As the distance from the electric field increases, the strength of the electric field decreases.
What is the effect on the force between charges?
© Boardworks 2014

19. Effect of distance and charge on force
Boardworks KS3 Science 2014
Static Electricity
Effect of distance and charge on force
© Boardworks 2014 19

20. Placing a charge in an electric field
CONTENTS 20

21. What goes on in a circuit?

22. What goes on in a circuit?
The movement of charge around a circuit is the current.
These charges have no energy but they are still moving, so there is still a current but the voltage is zero
These charges carry energy around the circuit. The amount of energy each charge carries is called the voltage or potential difference.

23. What is current? A
An ammeter must be connected in series as it measures the current flowing through it.
Current is a measure of the rate of flow of electric charge in a circuit.
Current is measured in amps (A) using an ammeter.
Teacher notes
It may help some students to conceptualize the idea of electrons moving around a circuit by introducing the idea of electron drift velocity. This is a measure of how much electrons move in a material. For example, when a 1 ampere current travels through a copper wire with a 1mm2 cross section, the drift velocity is less than 0.1 mm per second.
More information about drift velocity is available at

24. What is potential difference?
Potential difference is a measure of the difference in electrical potential energy between two points in a circuit. Potential difference is also called voltage.
A voltmeter must be connected in parallel as it measures the potential difference across the component. V
Voltage is measured in volts (V) using a voltmeter.
Teacher notes
It is important to highlight to students that although wiring the voltmeter in parallel forms a potential short circuit, voltmeters are designed to have a large resistance so that they take as little current from the circuit as possible. This means that the bulb will still work.
CONTENTS

25. Circuit Symbols Teacher notes
This matching components to symbols activity can be used to test students’ knowledge of circuit symbols. It can be used as a plenary activity or for revision purposes.

26. Symbols you should know
CONTENTS

27. Series Circuits
The current is the same throughout a series circuit because the speed the charges move with does not change.
NB: It doesn’t matter which component the charge flows through first. That doesn’t effect how much energy each bulb receives!
The voltage is shared between the components.
If these bulbs were identical, they would each receive half the total voltage from the battery

28. Parallel Circuits
Each branch must be thought of as it’s own “mini” circuit
6V here!
But each charge is carrying the total 6V so each bulb receives the same voltage as the battery supplies
Any charge can only take one particular route.
Current splits at junction
6V here!

29. Comparing circuits Teacher notes
Appropriately coloured voting cards could be used with this activity to increase class participation.

30. Series Circuits
The same current passes through components in series with each other
If the current through the lamps is 0.12A, what is the current through the cell?
0.12A
0.12A
0.12A
0.12A

31. Series Circuits
The total potential difference of the voltage supply in a series circuit is shared between the components.
If the potential difference of the cell is 1.2V and the potential difference of across one lamp is 0.8V, what is the potential difference across the other lamp?
0.4V
1.2V
0.8V
0.4V

32. Series Circuits
The total resistance of components in series is equal to the sum of their separate resistances.
What is the total resistance if one lamp in series has a resistance of 2 and the other has a resistance of 3? 5 2 3

33. Parallel Circuits
The total current through the whole circuit is the sum of the currents through the separate components.
0.4A
If ammeter A1 reads 0.4A and A2 reads 0.1A, what would A3 read?
0.3A A1 A2
0.1A A3
0.3A

34. Parallel Circuits
For components in parallel, the potential difference across each component is the same. 6V
If the potential difference of the cell is 6V, the potential difference across each lamp will also be 6V. 6V 6V
CONTENTS

35. Resistance Resistance = Voltage R = V Current I
Resistance is the opposition of a component to the flow of electrical current.
Resistance = Voltage R = V
Current I
It is measured in ohms ()

36. Test yourself Teacher notes
This calculations activity could be used as a plenary exercise to check students’ ability to use Ohm’s law.

37. Resistance is measured in ohms (Ω)

38. How is resistance affected in a series circuit?
Boardworks GCSE Additional Science: Physics
Resistance and Resistors
How is resistance affected in a series circuit?
When two (or more) resistors are connected in series, the combined resistance is higher than the individual resistors.
4 
2 
There is only one path for the current to travel, which means that it flows through the resistors one after the other.
Total resistance
= R1 + R2
This has the same effect as using a longer piece of wire.
= 4  + 2 
= 6 
total resistance in series = R1 + R2

39. Calculating resistance in series
Boardworks GCSE Additional Science: Physics
Resistance and Resistors
Calculating resistance in series
What is the total resistance for each of these circuits?
Total resistance = R1 + R2
= 6  + 34 
6 
34 
= 40 
Total resistance = R1 + R2
= 15  + 5 
= 20 
15 
5 

40. How is resistance affected in a parallel circuit?
Boardworks GCSE Additional Science: Physics
Resistance and Resistors
When two (or more) resistors are connected in parallel, the current splits at the branches and does not go through each resistor.
2 
4 
This means the total combined resistance is less than any of the individual resistors.
This has the same effect as using a thicker piece of wire.
Example
In the diagram above, if the supply voltage was 6V, what would be the current passing through each resistor?
CONTENTS

41. Ban ana
Answer:
Banana Split

42. MEAL
Answer:
Well Balanced Meal

43. MEAL
MEAL
MEAL
MEAL
Answer:
Square Meal

44. Current-Voltage Graphs
You should know how some electrical components behave when the voltage across them is increased.
Normally we expect current through them to increase. How the current changes depends on the device.
Plotting a current-voltage graph allows you to find out how the current changes

45. I-V Graph for a resistor or wire.
This is what we call an Ohmic Component
It obeys “Ohm’s Law”
The resistance is constant.
We can see this from the graph because the gradient is constant.

46. Ohm’s Law
The current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor. So… If you double the voltage, the current also doubles. Summarised by V = IR

47. I-V Graph for a filament lamp.
As pd increases the bulb heats up.
Metal ions in filament vibrate more.
They then “get in the way of” the moving charges.
The resistance increases.

48. I-V Graph for a Diode The current only flows one way.
Resistance in the reverse direction (“reverse bias”) is very high.
Certain pd required to allow current to flow (“Threshold pd”)
CONTENTS

49. Thermistors and LDRs You should be able to recognise the
circuit symbols for the thermistor and
the LDR (light-dependent resistor), and
know how the resistance of these
components can be changed.

50. LDRs
A light dependent resistor is a resistor that depends on the amount of light.

51. Thermistors
A thermistors is a resistor that depends on temperature. Their resistance decreases as the temperature increases.
CONTENTS

52. R G
Roses I N
Answer:
Ring around the Roses

53. johns
Answer:
Long Johns

54. BIRD
Answer:
Big Bird

55. Who can make the best plasticine fish?
5 minutes
Who can make the best plasticine fish?