9.3 Resistance

• Physics
• Secondary Education
• Age 12,13

22 October 2025

Unit 9.3 Resistance

Learning Targets

• I can explain the effect of resistance to current in a circuit.
• I can state the equation that links voltage, current, and resistance (Ohm's Law).
• I can use Ohm's law to calculate resistance from voltage and current.

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Resistance

• a measure of how easy or difficult it is for electrons to move through a material
• Symbol: R
• measured in units called "ohms" or Ω 

Which of the following is a conductor?

Which of the following has the lowest resistance?

copper

plastic

wood

Examples

• A 100 m length of copper can have a resistance of 0.5 - 1.0 Ω.

• A 1-cm length of plastic can have a resistance of 1,000,000,000,000 Ω.

What does resistance do?

1. slows down the flow of electrons
2. makes current higher
3. speeds up the flow of electrons
4. makes current unchanged

Why are these wires covered with plastic?

Why are these wires covered with plastic?

Conductors in light bulbs

• Some conductors are designed to have high resistance. WHY?

Conductors in light bulbs

• Some conductors are designed to have high resistance. WHY?

filament

As resistance makes it difficult for current to flow, ____________.

the greater the resistance in a circuit

the lesser the resistance in a circuit

The greater the resistance in a circuit, the __________ the current flow.

smaller

bigger

low

high

easy

difficult

What is the unit for resistance?

1. ohm (Ω)
2. hertz (Hz)
3. amps (A)
4. volts (V)

Ohm's Law

• Georg Simon Ohm
• link between voltage, current, and resistance
• Resistance applies to many electrical components, not only wires.

What does Ohm's law state?

1. voltage = current / resistance
2. voltage = current + resistance
3. voltage = current x resistance
4. voltage = current - resistance

Using an equation, what does Ohm's Law say?

Using an equation, what does Ohm's Law say?

Resistors

• a type of electrical component designed to have a known resistance
• colours on these resistors to indicate the resistance value in ohms.

Do this in your notebook.

Learner's Book, page 322 

Questions 2a to 2c.

Questions 3a to 3c.

Questions 4a and 4b.