1. Electricity and Ohm’s Law
The flow of electrons is called electric current
The unit of electric current is the ampere or amp for short
The symbol for amp is A
An amp is the measure of how many electrons flow per second
Electricity and Ohm’s Law
Completing the Circuit

2. Electricity and Ohm’s Law
For electricity to flow in a circuit it must have:
A complete pathway for the electrons to flow through eg connected wires
An energy supplier eg a battery
An energy user eg a lamp
Electricity and Ohm’s Law
Completing the Circuit 2

3. Electricity and Ohm’s Law
Draw the 3 circuits
Under each one say why you think they will work or won’t work. A B
Electricity and Ohm’s Law
Completing the Circuit C 3

4. Electricity and Ohm’s Law
Voltage (Potential Difference)
A potential difference occurs when there are more electric charges in one location than in another
The unit of potential difference is a volt
The symbol for volt is V
Example: excess electrons will be attracted to the positive plate because of the potential difference
Electricity and Ohm’s Law
Completing the Circuit e- 4

5. A More Relevant Example
In a household electric outlet, the potential difference (voltage) between the two slots is 240 volts
Because of a power plant many miles away, electrons are under “pressure” to move from one slot to the next

6. Electrical Circuits
An electrical circuit is formed when electrons are given a path to move across a potential difference
Generally, conducting (metal) wires are used to “complete” the circuit 6

7. Electrical Circuits
A circuit like this where the electrons have only one path is called a series circuit. 7

8. Bomb Circuit
Pretend a bulb is a bomb and a switch is a key to detonate it.
Draw a circuit where the key will set off the bomb

9. What type of Circuit is it?
A series circuit

10. Emergency
Now the evil powers of the world rewired the circuit so two keys would detonate the bomb
Draw this circuit

11. What Type of Circuit is this?
A parallel circuit

12. Electrical Components
An electric current is a movement of charged particles.
Draw, name and give the main function of the following components + -
Name: cell Function: source of electrical energy
Name: Lamp Function: turns electrical energy to light
Name: resistor Function: to slow current flow
Name: fuse Function: to melt when short circuit occurs or too much power is drawn

13. Copy out and complete the following
Name Function:
ammeter
to measure current
Ammeters must be placed in circuits in
series V
Name Function:
voltmeter
To measure voltage
Voltmeters must be placed in circuits in
parallel
Name Function:
diode
lets current flow in one
direction only

14. Draw the following in symbol form
3 cells joined in series
power pack
variable power pack
open switch

15. Electrical Components
An electric current is a movement of charged particles.
Draw, name and give the function of the following components
Name: Rheostat Function: to vary resistance
Or variable resistor

16. - A V Draw a series circuit with a 6volt cell 2 lamps and two ammeters+ - A V
If both lamps are the same comment on their brightness
Draw voltmeters to measure the voltage of each lamp
If 4 amps was applied to the circuit, what would each ammeter read 16

17. - V V Now make the circuit with the gear provided.
Use the multimeters to: V V
Measure the voltage across each lamp
Lamp 1 =
Lamp 2 =
Measure the current at 2 different places in the circuit
A1 =
A2 = A1 - + A2 17

18. Voltage Voltage (also called potential difference) measures:
1. The energy lost across a component
2. Energy supplied by the power supply or battery
The unit of voltage, the volt (symbol V) represents the energy change each second for every amp of current
If a lamp uses 1.5 volts this means that for every amp of current 1.5 joules of energy is lost every second.
Voltage is measured with a voltmeter.
Voltmeters must always be connected in parallel to the component for which the voltage is being measured

19. Current in a Series Circuits
In a series circuit the current flow is along the same path
Current in a series circuit is the same at any point of a series circuit
Current is measured by an ammeter and must always be added to a circuit in series A1 A1 - - + + A2 A2
If 4 amps is supplied to the series circuit
A1 = 4 amps
A2 = 4 amps

20. Voltage in a Series Circuit
If there is only one component in a circuit the voltage across the power supply is equal to the voltage across the components + - V 6V
In a series circuit the voltage across the components add up to the supply voltage

21. Voltage in a Series Circuits+ -
In a series circuit the voltage across the components add up to the supply voltage, what is the voltage at V1?
What does the voltage drop over each component depend on? V1
4V + 2V= 6V
The resistance of the component which is related to the energy usage

22. Voltage in a Series Circuits
What’s an advantage of a series circuit?
What’s a disadvantage of a series circuit?
Where are series circuits used?

23. Home Wiring: Series Circuits
If lights or appliances are connected in series they will not experience the same voltage
Since most lights and appliances are designed to work at a specific voltage, they cannot be connected in series
The resistance of lights or appliances connected in series is the sum of their resistances
Fuse
Voltage
Lights
Switch

24. - V V Parallel Circuits +
These are circuits where the lamps are in parallel with each other in separate branches
6 V V
All the lamps glow with same brightness.
Why?
Because the voltage over each lamp is the same as the supply voltage
6 V V - +
6 V 24

25. - A3 A2 A4 A1 + Current in Parallel Circuits
Current splits over each branch of a parallel circuit A3
If the current supplied to the parallel circuit at ammeter A1 = 6 amps
then:
A1 = ____amps
A2 = ____ amps
A3= ____ amps
A4= ____amps A2 A4 6 A1 - + 3 3 6
6 V 25

26. Home Wiring: Parallel Circuits
A single switch can be used to turn on multiple lights or appliances if they are connected in parallel
The voltage is the same for each light or appliance, although the current increases with each new addition
A fuse is designed to break if the current gets too large
Fuse
Voltage
Lights
Switch

27. Starter – draw the grid then make a statement
Current
Voltage
Current stays the same at all points in a series circuit
Voltage drops over each component in a series circuit
Series circuit
Parallel circuit
Current splits over each branch in a parallel circuit
Voltage in a parallel circuit stays the same as the supply voltage

28. How can two switches work lights in a hallway?

29. Conventional Current
Conventional current is the flow of positive charges from the positive terminal of a power supply to the negative terminal of a power supply –
we will use conventional current when looking at the direction of current flow in a circuit
Electron flow moves this way through the wire
Conventional current (I)
This is important when looking at diodes!

30. Resistance
Resistance is anything that slows the flow of charged particles
Resistance is measured in ohms (symbol )

31. Investigating voltage – current relationships
We will investigate the R = V/I relationship in a circuit to see if resistance changes when voltage and current is changed at constant temperature.
Method
Set up a series circuit with a heat sync resistor (designed to dissipate heat) a variable voltage supply and an ammeter.
Vary the voltage supply, read current and fill in the chart below.
Voltage
Current
Now plot your results on the provided graph paper
Voltage on the ‘Y” axis and Current on the “X” axis 31

32. Investigating voltage – current relationships
Once you have plotted the points draw a straight line graph and find the gradient of the line
The gradient of the line gave us the r________ of the circuit which = ____ ( )?
Ohmic conductors
Because the resistance did not change when both current and voltage were changed the resistor is an ohmic conductor
This is because the temperature of the heat sync resistor remained constant
Non Ohmic Conductors (conductors whose temperature increases with voltage and current)
Now plot the figures from a circuit with a lamp (a non Ohmic conductor) on page 167
What do you notice? 32

33. Investigating voltage – current relationships
The gradient of the ohmic voltage vs current graph gave us the ________ of the circuit.
Ohmic conductors
Because the resistance did not change when both current and voltage were changed the resistor is an ohmic conductor
This is because the temperature of the heat sync resistor remained constant V I 33

34. Examples of non ohmic conductors are lamps and resistors
NonOhmic Conductors (conductors whose temperature and therefore resistance increases with increasing voltage and current)
Examples of non ohmic conductors are lamps and resistors V I 34

35. Ohm’s Law
Voltage, current and resistance in an electric circuit can be related using Ohm’s Law
Ohm’s law states that
V = I x R
where
V is the voltage (in volts)
I is the current (in amps)
R is the resistance (in ohms)

36. V I R X
Task: Rearrange the formula (V= I x R) using ohms law to find the formula for:
I =
V =
R =

37. Using Ohms law Example
If a lightbulb in a 240volt circuit is connected and has a resistance of 60 , what current is flowing through the bulb?
Answer:
Rearrange V = I x R

38. Example 1
If 4 amps of current are flowing through a toaster connected to a 120 V outlet, what is the resistance of the toaster?
Answer: Rearrange V = I x R

39. Level one Questions – draw the following circuit
With both switches closed the current in L1 is 2A
1. Draw the symbol for the component that could measure the voltage across L2
2. What would the device measure?
12 V L1 L2 L3
3. Calculate the resistance of lamp L1 show your working and state the unit
4. Describe the brightness of lamp L2 compared with the brightness of lamp L1 and lamp L3 39

40. Power
In an electrical circuit power (P) is the rate at which electrical energy is :
Used by a component eg a lamp or …
Supplied by a power source eg battery

41. Power continued Power is measured in Watts (W)
One watt = 1 joule of energy used per second
Larger amounts of energy use the unit kilowatts
One kilowattt =
1000 watts

42. Examples
A 100 W lamp uses 100 joules of energy per second.
A motor which uses 2,500 joules of electrical energy each second has a power rating of 2,500W or 2.5 Kw

43. Starter – Question
Where are parallel circuits used and why?

44. Calculating Power P V I X
Power can be calculated using the voltage and current in the circuit. Using the following formula.
P = V x I Where: P = power (Watts)
V = volts
I = current P V I X
Can you put these in a triangle to help remember them

45. A V Calculate the following power supplied to the lamp P = V I
= 12v x 5A
= 60 W A 5A V
12V

46. Use the following circuit to answer the questionsV
1. If each of the cells is rated at 2.5 volts what would the voltmeter read? A1
2.5v + 2.5v + 2.5v = 7.5v (A)
2. If each lamp had a resistance of 1.25 ohms calculate the total resistance in the circuit A4 A2 A3
1.25   = 3.75  (A)
3. How much power would the circuit use? (M/E)

47. 3 Amps 8 V The resistors are identical 4V I= 1.5A
1. What is the voltage across the bulb? 4V
2. What is the current through the bulb?
3 Amps

48. What is the purpose of a fuse?
Draw the circuit
240 V AC
Fuse 10A
What is the purpose of a fuse?
0.416A
Radio (100 W)
To break the circuit if too much current is drawn
0.83A
Fridge (200 W)
2.08A
TV (500 W)
b. If all the appliances are turned on at once will the fuse blow?
4.16A
Heater (1000 W)
No the fuse would not blow

49. Read Page 168
Complete questions – 3 , 4 a & 4b on page 169 These are important questions

50. R= 3 
What is the current through the resistor? V 3V A
0.5 A V 3V
A car battery supplies a current of 200A to a starter motor at a voltage of 10V. What is the resistance of the starter motor
What is the resistance of a bulb if a 240V supply causes a current of 2A to flow through it when it is heated to constant temperature.

51. QUESTION THREE: ELECTRICITY
Amanda connected a light bulb in series to a variable power
supply and an ammeter. A voltmeter was connected in parallel
to the light bulb.
(a) Draw a circuit diagram of this set-up in the space below.
Comment on the positions of the ammeter and
voltmeter in the circuit.

52. The power supply voltage was gradually increased and the readings on
the voltmeter and ammeter were recorded in the chart below.
(c) Draw a graph of these results on the grid below.
Voltage (V) 2 4 6 8 10 12
Current (I)
0.2
0.4
0.6
0.8
0.9
0.95

53. (d) Use the equation V = I x R to calculate the resistance of the light bulb when it is operating on 6 Volts.
R =V/I = 6/ = 10 Ohms
(e) Describe what happens to the resistance of the light bulb at higher voltages, and give a reason for this behaviour.
The resistance of the bulb increases at higher voltages. At higher voltages the temperature of the filament increases. As the temperature increases so does the resistance of the circuit.

54. (f) (i) Show that the light bulb uses 90J of electrical energy when it is operated at 10V for 10 seconds.
One of the two steps attempted.
P =V x I = 10 x 0.9 = 9W
E = P x t = 9 x 10 = 90J

55. (f) (ii)
Comment on whether you think the light bulb will
produce 90J of light energy in 10 seconds at this voltage setting.
It will produce less than 90J of light energy. As energy is lost as heat energy

56. Task 1 : Use the wires and bulbs to make a series circuit with one bulb
Transformer on 6 volts _ +

57. Task 1 : Use the wires and bulbs to make a series circuit with one bulb57

58. Task 2 : Use the wires and bulbs to make a series circuit with two bulbs
Transformer on 6 volts _ +
Write down how the brightness of the bulb in Task 1 compares the brightness of the bulbs inTask 2

59. When the current flowed through just one bulb the lamp was b________.
Bulbs in series
When the current flowed through just one bulb the lamp was b________.
When another lamp was added both lamps went d________.
This showed us that the voltage dropped each time it went through a lamp. 59

60. Adding Voltmeters
Voltmeters are always added to a circuit in parallel
Task
Now make a circuit with one lamp and measure the voltage over it using the voltmeter (remember voltmeters always go in parallel as I showed you) V1
Draw the diagram
The voltmeter reading
V1 = ______ volts - + 60

61. - Now draw the series circuit below V1 V2 +
The 2 voltmeter readings are
V1 = ______ volts V2 = _____ volts
Read parallel and series circuits page ____ in text 61

62. Read parallel and series circuits page ____ in text
Write Heading in book
Current in Series circuits 62

63. - Now draw the series circuit below A1 A2 +
Use an ammeter to measure the current at A1 and A2
A1 = ______ amps A2 = _____ amps 63

64. - Now draw the series circuit below and copy the text A1 A2 +
Ammeters are always added in series (that is one behind each other) to all circuits A1 A2 - +
Complete the following
Current in a series circuit s____ the s_____ at all points 64

65. - + Starter : On the sheet of paper place your name
then draw a series circuit with a cell two lamps and a switch between the two lamps
Now mark it
1 mark each for
Lamp symbol
Switch symbol
Cell symbol
All wires connected
Give mark out of 5 - + 65

66. - Current in a series circuit A1 A2 +
Ammeters are always added in series (that is one behind each other) in all circuits A1 A2 - +
If current at A1 = 3 Amps then the current at A2= 3 amps
Current in a series circuit stays the same at all places
If the current was changed so A1 = 4 Amps what would A2 = 4 Amps 66

67. - Voltage in a Series circuit V1 V2 + 6 V
If the battery supplies 6 volts to the circuit and the voltage drops evenly over both bulbs then the voltmeter readings are
V1 = ___ and V2 = _____
In a series circuit voltage drops over each lamp
6 V 67

68. Read parallel and series circuits page ____ in text68

69. - V1 V2 + Voltage in Parallel Circuits
These are circuits where the lamps are in parallel with each other in separate branches V1
All the lamps glow with same brightness because the voltage over each lamp in a parallel circuit is the same as the cell voltage V2
This means if the supply voltage is 6 volts then:
V1= ____ volts
V2= ____ volts - +
6 V 69

70. Summary – remember this
Current
Voltage
Current stays the same at all points in a series circuit
Voltage drops over each component in a series circuit
Series circuit
Parallel circuit
Current splits over each branch in a parallel circuit
Voltage in a parallel circuit stays the same as the supply voltage 70

71. Draw the circuit diagram
Task 1 : Make a series circuit with 2 bulbs and a switch that turns both bulbs off.
Draw the circuit diagram
Task 2 : Make a parallel circuit with two bulbs in it Draw the circuit diagram
Task 3 : Place a switch in the circuit that turns off one lamp at a time
place a variable resistor in it to dim the lights 71

73. Electric Current The flow of electrons is called electric current
The unit of electric current is the ampere or amp for short
The symbol for amp is A
An amp is the measure of how much charge flows by per second

74. The structure of the atom
19/04/2017
ELECTRON – negative, mass nearly nothing
PROTON – positive, same mass as neutron (“1”)
NEUTRON – neutral, same mass as proton (“1”) 74

76. Electrical Circuits
An electrical circuit is formed when electrons are given a path to move across a potential difference
Generally, conducting (metal) wires are used to “complete” the circuit

77. Electrical Resistance
Electrical resistance is the measure of the resistance of part of a circuit to the flow of electricity (current)
Electrical resistance is measured in ohms
Ohms are often represented by the symbol omega ()
Example: the filament of a 100 W lightbulb presents a resistance of about 140  to the flow of electricity

78. The Water Pressure Model
A helpful way of thinking about current, voltage and resistance is to compare electricity to the flow of water
voltage (i.e. potential difference) is similiar to the water pressure
current is similar to the flow of water
resistance is similar to the diameter of the pipe (e.g. a small pipe allows much less water to flow than a large pipe)

79. Ohm’s Law
Voltage, current and resistance in an electric circuit can be related using Ohm’s Law
Ohm’s law states that
V = I x R
where
V is the voltage (in volts)
I is the current (in amps)
R is the resistance (in ohms)

80. Example 1
If 4 amps of current are flowing through a toaster connected to a 120 V outlet, what is the resistance of the toaster?
Answer: Rearrange V = I x R to get R = V/I
Thus, R = (120 V)/(4 A) = 30 

81. Example 2
If a lightbulb connected to an outlet has a resistance of 144 , what current is flowing through the bulb?
Answer: Rearrange V = I x R to get I = V/R
Thus,
I = (120 V)/(144 )
= 0.83 A

82. Home Wiring: Parallel Circuits
A single switch can be used to turn on multiple lights or appliances if they are connected in parallel
The voltage is the same for each light or appliance, although the current increases with each new addition
A fuse is designed to break if the current gets too large
Fuse
Voltage
Lights
Switch

83. Home Wiring: Series Circuits
If a lights or appliances are connected in series they will not experience the same voltage
Since most lights and appliances are designed to work at a specific voltage, they cannot be connected in series
The resistance of lights or appliances connected in series is the sum of their resistances
Fuse
Voltage
Lights
Switch

84. Example
What is the total resistance of three 100 W light bulbs (each 144 ) in series?
Answer: = 432 
What is the total current through three 100 W light bulbs?
Answer: I = (120 V)/(432 ) = 0.27 A
Note: this is significantly less than the 0.83 A that will flow through a single bulb (i.e. the bulbs will be dimmer)