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KS4 Electricity – Resistance. Teacher’s Notes. A slide contains teacher’s notes wherever this icon is displayed - To access these notes go to ‘Notes Page View’ (PowerPoint 97) or ‘Normal View’ (PowerPoint 2000). Notes Page View. Normal View. Flash Files. - PowerPoint PPT Presentation
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© Boardworks Ltd 2003
KS4 Electricity – Resistance
© Boardworks Ltd 2003
A slide contains teacher’s notes wherever this icon is displayed -
To access these notes go to ‘Notes Page View’ (PowerPoint 97) or ‘Normal View’ (PowerPoint 2000).
Normal ViewNotes Page View
Teacher’s Notes
Flash Files
A flash file has been embedded into the PowerPoint slide wherever this icon is displayed –
These files are not editable.
© Boardworks Ltd 2003
Use a textbook or other resource to fill in the table below:
Component Circuit symbol
Diode
Light Dependent Resistor
Variable Resistor
Thermistor
© Boardworks Ltd 2003
How are current and voltage related for a resistor?
V
A
Set up the circuit as shown below:
Slowly move the variable resistor and for each setting record the current in amps and the voltage in volts.
Plot a graph of your results.
P.D.(V)
I(A)
00.51.01.52.02.53.03.5
00.61.11.82.53.03.54.2
© Boardworks Ltd 2003
Ohm’s Law:
The current flowing through a wire is proportional to the potential difference (voltage) across it provided the temperature remains constant. Georg Ohm 1826
Curre
nt, I
/ a
mps
Potential difference / V
What does proportional mean?
Current and voltage
If you double the voltage then the current doubles.x
xx
x
x xx
x
Plot your points on the graph.Draw a line of best fit for your graph.What does your graph look like?If you get a straight line it means that the two quantities current and voltage are proportional.
This fact was put into a law:
© Boardworks Ltd 2003
Electron flow and Resistance
Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire they collide with the metal atoms. These collision make the atoms vibrate more…which makes the metal hotter.
Resistance is a measure of how much a material tries to stop electricity passing through it.
© Boardworks Ltd 2003
V=IR
We can express Ohm’s Law mathematically using the equation:
Voltage = Current x Resistance
V=IR
Voltage measured in Volts (V)
Current measured in Amps (A)
Resistance measured in Ohms ()
© Boardworks Ltd 2003
Formula triangles
V
I R
x
Formula triangles help you to rearrange formula, the triangle for the Ohm’s Law is shown below:
Whatever quantity you are trying to find cover it up and it will leave you with the calculation required.
So if you were trying to find current, I…..
…you would cover I up…
…and you are left with the sum…
I = V
R
© Boardworks Ltd 2003
Formula triangles
© Boardworks Ltd 2003
Resistance for a bulb
If you have a filament bulb and it has a current of 20A running through it, with a potential difference of 100V across it, what is the resistance of the bulb?
V = IR
R = V/I
R = 100V/20A
R = 5
© Boardworks Ltd 2003
Current for a diode
A diode has a current of 5A running through it, and a resistance of 5. What is the potential difference across the diode?
V = IR
V = 5A x 5
V = 25V
© Boardworks Ltd 2003
Resistors in series
4 2
Total resistance = R1 + R2
What is the total resistance for the circuit shown?
Total resistance = R1 + R2
Total resistance = 4 + 2
Total resistance = 6
© Boardworks Ltd 2003
6 34
What is the total resistance for the circuit shown?
Total resistance = R1 + R2
Total resistance = 6 + 34
Total resistance = 40
Resistors in series
© Boardworks Ltd 2003
Resistors in parallel
2
4
Total resistance = R1xR2
R1+ R2
What is the total resistance for the circuit shown?
Total resistance = 4 x 2 4 + 2
= 1.33
© Boardworks Ltd 2003
6
8
What is the total resistance for the circuit shown?
Total resistance = 8 x 6 8 + 6
= 3.4
Resistors in parallel
© Boardworks Ltd 2003
Voltage/current graphs
1. A wire or resistor.2. A filament lamp.3. Wires of different materials. 4. A diode.
Which of the components obeys Ohms Law?
I
V
I
V
I
V
I
V
1 2 3 4
..partly
x
© Boardworks Ltd 2003
P=IV
We can express the relationship between current, voltage and power mathematically using the equation:
Power = Current x Voltage
P=IV
Voltage measured in Volts (V)
Current measured in Amps (A)
Power measured in Watts (W)
© Boardworks Ltd 2003
Formula triangles
P
I V
x
Formula triangles help you to rearrange formula, the triangle for the Power Law is shown below:
Whatever quantity you are trying to find cover it up and it will leave you with the calculation required.
So if you were trying to find current, I…..
…you would cover I up…
…and you are left with the sum…
I = PV
© Boardworks Ltd 2003
Power calculations
If you have a filament bulb and it has a potential difference of 200V across it and a current of 0.2A running through it. At what power is the bulb operating at?
P = IV
P = 0.2A x 200V
P = 40W
© Boardworks Ltd 2003
If you have a filament bulb and it operates at a power of 60W and it has a potential difference of 240V across it, what is the current running through the bulb?
P = IV
I = P/V
I = 60W / 240V
I = 0.25A
Power calculations
© Boardworks Ltd 2003
kV, kJ, kW
1 kV = 1000 V
1 kJ = 1000 J
1 kW = 1000 W
How many Volts in 6kV? _________ V
How many Joules in 12.3kJ? _________ J
How many Watts in 0.6kW? _________ W
6 000
12 300
600
© Boardworks Ltd 2003
1 kV = 1000 V
1 kJ = 1000 J
1 kW = 1000 W
How many kiloVolts in 9 000V? _________ kV
How many kiloJoules in 23 500J? _________ kJ
How many kiloWatts in 325W? _________ kW
9.0
23.5
0.325
kV, kJ, kW
© Boardworks Ltd 2003
What are the units of resistance?
A. Amps
B. Ohms
C. Volts
D. Watts
© Boardworks Ltd 2003
What does the circuit symbol shown represent?
A. Voltmeter
B. Variable resistor
C. Light dependent resistor
D. Thermistor
© Boardworks Ltd 2003
What causes resistance?
A. Inertia
B. Friction
C. Buoyancy
D. Viscosity
© Boardworks Ltd 2003
If two resistors of 6 and 4 are placed in parallel, what is their total resistance in the circuit?
A. 10
B. 2
C. 2.4
D. 24
© Boardworks Ltd 2003
If a resistor that obeys Ohm’s Law has a potential difference of 10V across it and a current of 5A running through it. What is its resistance?
A. 50
B. 2
C. 0.5
D. 15