Resistance

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Lesson notes

What is Resistance?

Resistance is a measure of how hard it is for current to flow through a component.
Resistance is measured in ohms (Ω).
Conductors (e.g. metals) have low resistance; insulators (e.g. plastic) have high resistance.

Materials form a range from good conductors like copper and iron to good insulators like glass and rubber.

The Resistance Equation

The equation linking potential difference (V), current (I) and resistance (R) is: V = I × R.
Rearranged: R = V ÷ I and I = V ÷ R.
Resistance is the ratio of potential difference to current.

Using the Equation

To find potential difference: V = I × R (units: V = A × Ω).
To find resistance: R = V ÷ I (units: Ω = V ÷ A).
Always write the equation, substitute values, then calculate with units.

Example Calculations

Example 1: A lamp with R = 20 Ω and I = 0.5 A → V = 0.5 × 20 = 10 V.
Example 2: A resistor with V = 5 V and I = 2 A → R = 5 ÷ 2 = 2.5 Ω.

Converting Kiloohms

Resistance can be given in kiloohms (kΩ).
1 kΩ = 1000 Ω.
To convert kΩ to Ω: multiply by 1000.

A series circuit showing a cell, resistor, ammeter and lamp. The ammeter measures current.

Slides

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Practice questions

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1.State the equation that links potential difference, current and resistance.
Easy
- AV = I × R
- BV = I ÷ R
- CV = R ÷ I
- DV = I + R
2.State the unit of resistance.
Easy
- AOhms (Ω)
- BVolts (V)
- CAmps (A)
- DWatts (W)
3.Which of the following best describes the difference between an electrical conductor and an insulator?
Easy
- AA conductor has low resistance; an insulator has high resistance.
- BA conductor has high resistance; an insulator has low resistance.
- CA conductor allows current to flow easily; an insulator does not allow current to flow at all.
- DA conductor is made of plastic; an insulator is made of metal.
4.A lamp has a resistance of 15 Ω and a current of 2.0 A flowing through it. Calculate the potential difference across the lamp.
Medium
- A30 V
- B7.5 V
- C0.13 V
- D17 V
5.A resistor has a resistance of 500 Ω and a current of 0.10 A flowing through it. Calculate the potential difference across the resistor.
Medium
- A50 V
- B5000 V
- C0.0002 V
- D5 V
6.Using the information in the diagram (a series circuit with a cell, switch, lamp, and ammeter), calculate the resistance of the lamp if the voltmeter across the lamp reads 6 V and the ammeter reads 2 A.
Medium
- A3 Ω
- B12 Ω
- C0.33 Ω
- D8 Ω
7.A resistor has a resistance of 100 Ω and a current of 0.8 A flowing through it. Calculate the potential difference across the resistor.
Medium
- A80 V
- B125 V
- C0.008 V
- D100.8 V
8.A light dependent resistor (LDR) has a potential difference of 6 V across it and a current of 0.02 A flowing through it. Calculate the resistance of the LDR.
Hard
- A300 Ω
- B0.12 Ω
- C120 Ω
- D0.0033 Ω

More questions

9.A thermistor has a resistance of 1.5 kΩ and a current of 0.5 A flowing through it. Calculate the potential difference across the thermistor.
Hard
- A750 V
- B3 V
- C0.75 V
- D7500 V
10.A resistor has a resistance of 2.0 kΩ and a potential difference of 10 V across it. Calculate the current flowing through the resistor.
Hard
- A0.005 A
- B0.02 A
- C200 A
- D5 A
11.What is the unit of potential difference?
Easy
- AVolt (V)
- BOhm (Ω)
- CAmpere (A)
- DWatt (W)
12.In the circuit diagram shown, which component is used to measure current?
Medium
- AAmmeter
- BVoltmeter
- CResistor
- DCell
13.A student sets up a series circuit with a 6 V battery and a resistor. The ammeter reads 0.3 A. What is the resistance of the resistor?
Medium
- A20 Ω
- B1.8 Ω
- C18 Ω
- D0.05 Ω
14.A lamp has a potential difference of 12 V across it and a current of 0.25 A flowing through it. What is the resistance of the lamp?
Medium
- A48 Ω
- B3 Ω
- C0.021 Ω
- D0.48 Ω
15.A thermistor has a resistance of 2.0 kΩ. A current of 0.005 A flows through it. What is the potential difference across the thermistor?
Hard
- A10 V
- B0.01 V
- C400 V
- D10000 V
16.A resistor has a resistance of 0.5 kΩ and a potential difference of 15 V across it. What is the current through the resistor?
Hard
- A0.03 A
- B30 A
- C7.5 A
- D0.075 A
17.Which of the following is the correct unit for resistance?
Easy
- AOhm (Ω)
- BVolt (V)
- CAmpere (A)
- DWatt (W)
18.Which of the following materials would be expected to have the lowest resistance?
Easy
- ACopper
- BPlastic
- CRubber
- DWood
19.A light dependent resistor (LDR) has a potential difference of 9 V across it and a current of 0.015 A. What is its resistance?
Medium
- A600 Ω
- B0.135 Ω
- C135 Ω
- D6000 Ω
20.A student measures the current through a resistor at different potential differences. The results show that when V = 2 V, I = 0.4 A; when V = 4 V, I = 0.8 A; when V = 6 V, I = 1.2 A. What is the resistance of the resistor?
Medium
- A5 Ω
- B0.8 Ω
- C1.25 Ω
- D20 Ω
21.A component has a resistance of 3 kΩ. If the current through it is 2 mA, what is the potential difference across it?
Hard
- A6 V
- B1.5 V
- C0.006 V
- D6000 V
22.In a series circuit, which component is used to measure current?
Medium
- AAmmeter
- BVoltmeter
- CResistor
- DCell
23.A student connects a voltmeter across a lamp in a series circuit. The voltmeter reads 3 V and the ammeter reads 0.6 A. What is the resistance of the lamp?
Medium
- A5 Ω
- B1.8 Ω
- C0.2 Ω
- D18 Ω
24.A wire has a resistance of 10 Ω. A second wire of the same material is twice as long and has half the cross-sectional area. What is the resistance of the second wire?
Hard
- A40 Ω
- B20 Ω
- C5 Ω
- D10 Ω
25.A student wants to determine the resistance of a lamp. She sets up a series circuit with a cell, the lamp, an ammeter, and a voltmeter. Which diagram shows the correct positions of the ammeter and voltmeter?
Medium
- AAmmeter in series with the lamp, voltmeter in parallel with the lamp
- BAmmeter in parallel with the lamp, voltmeter in series with the lamp
- CBoth ammeter and voltmeter in series with the lamp
- DBoth ammeter and voltmeter in parallel with the lamp
26.A student uses the circuit shown to measure the current through and voltage across a resistor. The ammeter reads 0.25 A and the voltmeter reads 5.0 V. What is the resistance of the resistor?
Medium
- A20 Ω
- B1.25 Ω
- C0.05 Ω
- D5.25 Ω
27.A student investigates how the resistance of a wire depends on its length. She uses wires of the same material and cross-sectional area but different lengths. She measures the current through each wire for a fixed potential difference of 6.0 V. Her results are shown in the table. Which additional piece of apparatus is essential for this experiment?
Hard
- AA micrometer to measure the diameter of the wire
- BA ruler to measure the length of the wire
- CA balance to measure the mass of the wire
- DA thermometer to measure the temperature of the wire
28.In an experiment to determine the resistance of a component, a student records the current at different potential differences. The graph of current against potential difference is a straight line through the origin. What does this indicate about the component?
Hard
- AIt is an ohmic conductor
- BIt is a non-ohmic conductor
- CIts resistance increases with current
- DIts resistance decreases with current
29.A student connects a variable resistor in series with a fixed resistor and a battery. She adjusts the variable resistor and records the current and the potential difference across the fixed resistor. Which graph shows the relationship between the current through the fixed resistor and the potential difference across it?
Medium
- AA straight line through the origin
- BA curve that flattens as current increases
- CA horizontal line at constant voltage
- DA vertical line at constant current
30.A student measures the resistance of a light-dependent resistor (LDR) in bright light and in darkness. Which of the following is likely to be correct?
Medium
- AResistance is higher in bright light than in darkness
- BResistance is lower in bright light than in darkness
- CResistance is the same in both conditions
- DResistance is zero in bright light
31.A thermistor is placed in a circuit with a 6.0 V battery and a fixed resistor. As the temperature increases, the current in the circuit increases. What can be concluded about the thermistor?
Medium
- AIts resistance decreases as temperature increases
- BIts resistance increases as temperature increases
- CIts resistance is constant
- DIts resistance becomes zero
32.A student has a 10 Ω resistor and wants to create a circuit that has a total resistance of 30 Ω. She has another resistor of unknown resistance. How should she connect the two resistors?
Hard
- AIn series
- BIn parallel
- CEither series or parallel
- DShe cannot achieve 30 Ω with one additional resistor
33.A wire of length L and cross-sectional area A has resistance R. Which of the following wires of the same material will have resistance 2R?
Hard
- ALength 2L, area A
- BLength L, area 2A
- CLength 2L, area 2A
- DLength L/2, area A
34.Which of the following is the correct unit for potential difference?
Easy
- AVolts (V)
- BAmps (A)
- COhms (Ω)
- DWatts (W)
35.What is the unit of resistance?
Easy
- AOhm (Ω)
- BVolt (V)
- CAmpere (A)
- DWatt (W)
36.Which of the following best describes a material with low resistance?
Easy
- AA conductor
- BAn insulator
- CA semiconductor
- DA superconductor
37.A current of 2 A flows through a resistor with a potential difference of 12 V across it. What is the resistance?
Medium
- A6 Ω
- B0.17 Ω
- C24 Ω
- D10 Ω
38.In the circuit diagram shown, what is the resistance of the lamp if the voltmeter reads 3 V and the ammeter reads 0.6 A?
Medium
- A5 Ω
- B1.8 Ω
- C0.2 Ω
- D3.6 Ω
39.A light dependent resistor (LDR) has a potential difference of 6 V across it and a current of 0.02 A flowing through it. What is the resistance of the LDR?
Medium
- A300 Ω
- B0.12 Ω
- C120 Ω
- D0.0033 Ω
40.A thermistor has a resistance of 1.5 kΩ and a current of 0.5 A flowing through it. What is the potential difference across the thermistor?
Medium
- A750 V
- B0.75 V
- C3.0 V
- D3000 V

Revision notes

## Resistance - **Resistance** measures how hard it is for current to flow. Unit: **Ohms (Ω)**. - **Conductors** (e.g., metals) have low resistance; **insulators** (e.g., plastic) have high resistance. - Key formula: **V = I × R** (Potential difference = Current × Resistance) - Rearranged: **R = V ÷ I** - Units: V in volts (V), I in amps (A), R in ohms (Ω). - **kiloohms (kΩ)**: 1 kΩ = 1000 Ω. Convert to ohms before calculations (multiply by 1000). ### Example Calculations 1. **Find V**: Lamp with R = 20 Ω, I = 0.5 A → V = 0.5 × 20 = 10 V 2. **Find R**: Resistor with V = 5 V, I = 2 A → R = 5 ÷ 2 = 2.5 Ω

Resistance: A measure of how hard it is for current to flow through a component, measured in ohms (Ω).
Ohm (Ω): The unit of resistance.
Potential difference: The voltage across a component, measured in volts (V).
Current: The flow of electric charge, measured in amps (A).
Conductor: A material with low resistance that allows current to flow easily (e.g., metals).
Insulator: A material with high resistance that does not allow current to flow easily (e.g., plastic).
kiloohm (kΩ): A unit of resistance equal to 1000 ohms.

Flashcards

What is resistance?

Resistance is a measure of how hard it is for current to flow through an electrical component.

What is the unit of resistance?

Ohms (Ω).

Do electrical conductors have high or low resistance?

Electrical conductors (like metals) have low resistance.

Do insulators have high or low resistance?

Insulators (like plastic) have high resistance.

Write the equation linking potential difference, current and resistance.

Potential difference = Current × Resistance, or V = I × R.

Write the rearranged equation to calculate resistance from potential difference and current.

Resistance = Potential difference ÷ Current, or R = V ÷ I.

What are the units of potential difference and current?

Potential difference is in volts (V) and current is in amps (A).

A lamp has a resistance of 20 Ω and a current of 0.5 A. Calculate the potential difference.

V = I × R = 0.5 × 20 = 10 V.

A resistor has a potential difference of 5 V and a current of 2 A. Calculate the resistance.

R = V ÷ I = 5 ÷ 2 = 2.5 Ω.

What is a kiloohm?

One kiloohm (kΩ) equals one thousand ohms (Ω).

How do you convert kiloohms to ohms?

Multiply the number of kiloohms by 1000: kΩ × 1000 → Ω.

A lamp has a resistance of 15 Ω and a current of 2.0 A. Calculate the potential difference.

V = 2.0 A × 15 Ω = 30 V.

A resistor has a resistance of 500 Ω and a current of 0.10 A. Calculate the potential difference.

V = 0.10 A × 500 Ω = 50 V.

From a circuit diagram with V=5 V and I=2 A, calculate the resistance of the lamp.

R = 5 V ÷ 2 A = 2.5 Ω.

A resistor has a resistance of 100 Ω and a current of 0.8 A. Calculate the potential difference.

V = 0.8 A × 100 Ω = 80 V.

An LDR has a potential difference of 6 V and a current of 0.02 A. Calculate its resistance.

R = 6 V ÷ 0.02 A = 300 Ω.

A thermistor has a resistance of 1.5 kΩ and a current of 0.5 A. Calculate the potential difference.

First convert: 1.5 kΩ = 1500 Ω. Then V = 0.5 A × 1500 Ω = 750 V.

A resistor has a resistance of 2.0 kΩ and a potential difference of 10 V. Calculate the current.

First convert: 2.0 kΩ = 2000 Ω. Then I = V ÷ R = 10 V ÷ 2000 Ω = 0.005 A.

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