Resistance and Ohm's lawIB MYP Physics: Revision notes
Section 1
Resistance
Resistance is the opposition to the flow of electric current. It is measured in ohms (Ω).
R = V ÷ I, which can be rearranged to V = I × R and I = V ÷ R.
- R is resistance in ohms (Ω)
- V is potential difference in volts (V)
- I is current in amperes (A)
Worked example: a resistor has 12 V across it and carries 0.60 A. R = 12 ÷ 0.60 = 20 Ω.
For the same p.d., a higher resistance means a smaller current.
Do not mix up the rearrangements. Cover the quantity you want in the triangle V, I, R: V is on top, I and R are below.
Section 2
Ohm's law
Ohm's law says that the current through an ohmic conductor is directly proportional to the potential difference across it, as long as the temperature stays constant.
This means that if you double the p.d., the current doubles and the resistance stays the same. A metal wire at constant temperature and a fixed resistor are ohmic conductors.
If the temperature changes, the resistance can change and the conductor is no longer ohmic.
Section 3
I-V graphs
An I–V graph plots current (y-axis) against potential difference (x-axis).
- Fixed resistor: a straight line through the origin. Current is directly proportional to p.d., so the resistance is constant.
- Filament lamp: a curve that gets less steep as p.d. rises. The filament gets hotter, so its resistance increases.
- Diode: current flows in one direction only. In the forward direction the current rises after a small p.d. In the reverse direction almost no current flows.
A steeper line on an I–V graph means a lower resistance.
Section 4
Factors affecting resistance
The resistance of a wire depends on four things.
- Length: a longer wire has a higher resistance, because the electrons collide with more atoms. Resistance is directly proportional to length.
- Cross-sectional area: a thicker wire has a lower resistance, because there is more room for charge to flow.
- Material: copper has a low resistance, while nichrome has a high resistance.
- Temperature: for a metal, a higher temperature gives more vibration of the atoms and a higher resistance.
In an investigation, change one of these and control the others.
Section 5
Variable resistors
A variable resistor is a resistor whose resistance can be changed, for example by sliding a contact along a resistance wire. It changes the current in a circuit.
Increasing the resistance reduces the current, and decreasing it increases the current. Variable resistors are used in dimmer switches for lights and volume controls.
Section 6
LDRs and thermistors
A light-dependent resistor (LDR) has a resistance that decreases as light intensity increases. It is used in automatic street lights, which come on when it gets dark, and in light meters.
A thermistor has a resistance that decreases as temperature increases. It is used in thermostats, fire alarms and engine temperature sensors.
Both are used as sensors in circuits that switch other devices on or off.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Resistance and Ohm's law
- A technician in a workshop tests a resistor from a heating unit. She connects it to a power supply with a voltmeter and an ammeter. The voltmeter reads 12 V and the ammeter reads 0.60 A. The resistor stays at a constant temperature throughout the test.The technician doubles the potential difference to 24 V. Calculate the new current, assuming the resistor is ohmic at constant temperature.2 marks
- A student plots current against potential difference for three components in turn: a fixed resistor, a filament lamp and a diode. She increases the potential difference from zero in small steps and then reverses the connections to test the diode in the opposite direction.Describe the shape of the current–potential difference graph for the fixed resistor and state what this shows about its resistance.2 marks
- A group of students investigates how the length of a piece of nichrome wire affects its resistance. They connect the wire to a cell, with an ammeter and a voltmeter, and use a metre rule to change the length of wire in the circuit.Describe how the students can find the resistance of the wire at each length, and state one safety precaution.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).