1. Be able to describe what resistance is and calculate it.
20 November 2018
Ohm’s Law I V
Sketch the graphs and describe what they show. I
Objectives
Be able to describe what resistance is and calculate it.
HSW: AF4 – Using investigative approaches
Used before in: Energy, Work and Momentum
Will use again in: lesson 6 – Non-Ohmic devices
PLTS: Team workers – Work collaboratively with others.
Used before in: lesson 4 – circuits
Will use again in: lesson 6 -Non-Ohmic devices I V I V
Keywords
Current (I), Amp, Voltage (V), Volts, ammeter, voltmeter, resistor, variable resistor, resistance (R), Ohm, directly proportional.

2. OUTCOMES
All students should be able to describe that resistance restricts the flow of current and understand that resistors add in series.
Most students should be able to test circuits for current and potential difference and describe Ohm’s Law and use the definition of resistance in calculations.
Some students should be able to interpret graphs that relate to current-potential difference and describe resistance in terms of ions and electrons.

3. Resistance Song

4. circuit symbol for a resistor
Resistance
a resistor
Resistance is the opposition that an electrical device has to the flow of electrical current.
All devices have some resistance. A resistor is a device that has a particular resistance.
circuit symbol for a resistor

5. Resistance equation resistance = potential difference current Where:
potential difference is in volts (V)
current is in amperes (A)
resistance is in ohms (Ω)

6. potential difference = current x resistance and:
Also:
potential difference = current x resistance
and:
current = potential difference
resistance
potential difference
current
resistance

7. Circuit used for measuring the resistance of an indicator lamp
Measuring Resistance
The resistance of a component can be found by measuring the current through, and potential difference across, the component.
Circuit used for measuring the resistance of an indicator lamp

8. Example
Calculate the resistance of a lamp if a potential difference of 12V causes a current of 3A to flow through the lamp.
resistance = potential difference
current
= 12V / 3A
resistance = 4 ohms (4Ω)

9. Complete: Answers potential difference current resistance 20 V 4 A 5 A
40 Ω
300 V
50 Ω
8 V
500 mA
3 kV
150 Ω
4 mA
30 kΩ
5 Ω
200 V
6 A
16 Ω
20 A
120 V

10. Current-potential difference graphs
These are used to show how the current through a component varies with the potential difference across it.
The circuit opposite could be used to obtain a current-potential difference graph of a wire.

11. Current-Potential Graph of a Wire/Resistor
Set up the equipment as shown:
Vary the PD and take readings of current.
Analysis
Once you have a set of results draw a graph of Current on the y-axis and PD on the x-axis and write a conclusion.
Results
Potential Difference, V (V)
Current, I (A)

12. The current-potential difference graph of a resistor at a constant temperature
The graph is a straight line through the origin.
The graph shows that the current through a resistor is directly proportional to the potential difference across the resistor.
Typical results:

13. Ohm’s Law
Ohm’s Law says that current is directly proportional to voltage when the temperature of the conductor is constant. I V
If the current is reversed then the pattern stays the same:

14. Choose appropriate words to fill in the gaps below:
An electric ________ will only flow around a circuit if there are no ______ in the circuit.
All components have __________. The greater the resistance the ________ is the current for the same applied potential difference. Resistance is measured in ______.
A current – potential difference graph for a ________ is a straight line through the _______. This shows that the current through the resistor is ___________ to the applied potential difference.
current
gaps
resistance
smaller
ohms
resistor
origin
proportional
WORD SELECTION:
smaller
resistor
proportional
gaps
origin
resistance
ohms
current

15. Tell me two things... (for each)
that you have done well this session
that you know now that you didn’t know at the start
that you could do better next time
that you would like to know more about 2