Physics · Simulation and virtual lab Free
Ohm's law: test resistors and a torch lamp
Build the circuit, change the voltage and read the meters. Find out which components obey Ohm’s law.
- You will learn to
- Measure current and voltage with an ammeter and a voltmeter.
- Plot an I–V graph and decide whether a component obeys Ohm’s law.
- Find a resistance from the gradient, including series and parallel pairs.
- You will measure
- Current I (A) for voltages up to 6 V; then resistance R in Ω.
- Time needed
- About 30–40 min
- Equipment
- Nothing extra. Real version: cells, resistors, torch bulb, ammeter, voltmeter, wires.
- Price
- Free: no payment, no account.
Virtual lab
Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.
Aim
To investigate how the current through a component depends on the voltage across it, and to find the resistance of an unknown resistor.
What to do
- Look at the circuit: the ammeter is in series, the voltmeter is connected across the resistor with crocodile clips. Tap the switch to close it: current flows.
- Choose Resistor 10 Ω. Set the voltage to 1.0 V and tap Record V and I.
- Increase the voltage by 1.0 V at a time up to 6.0 V, recording each time.
- Look at the graph: a straight line through the origin means the component obeys Ohm’s law. Read its resistance from the box.
- Repeat with the Unknown resistor X and find its resistance.
- Test Two 10 Ω in series and Two 10 Ω in parallel. Compare their resistances with one 10 Ω resistor.
- Test the Torch lamp from 0.5 V to 6.0 V. Describe the shape of its graph. (Do not go above 9 V!) Open the switch at the end, as you would in a real laboratory.
Results
Questions
State Ohm’s law.
The current through a metallic conductor is directly proportional to the potential difference across it, provided the temperature and other physical conditions stay the same.
A 10 Ω resistor has 6.0 V across it. What current flows?
I = V/R = 6.0 ÷ 10 = 0.60 A.
Why does the graph for the torch lamp curve?
As the current increases, the filament gets hotter. The resistance of a metal increases with temperature, so V/I increases and the current rises less and less: the lamp is non-ohmic.
Two 10 Ω resistors are joined in series, then in parallel. Find the total resistance each time.
Series: R = 10 + 10 = 20 Ω. Parallel: 1/R = 1/10 + 1/10, so R = 5 Ω.
Why must the ammeter be in series and the voltmeter in parallel?
The ammeter must carry the same current as the component, so it is in series. The voltmeter measures the potential difference between two points, so it is connected across the component.
The lamp burnt out at 10 V. Explain using power.
P = VI. At 10 V the power is much greater than the 1.8 W the lamp is designed for, so the filament gets so hot that it melts.
For teachers: expected results and how the model works
Unknown resistor X is 33 Ω. Learners should get gradients of 0.1 A/V (10 Ω), 0.03 A/V (X), 0.05 A/V (series) and 0.2 A/V (parallel). The lamp is modelled with a resistance rising from about 4 Ω (cold) to 20 Ω at 6 V (0.30 A). Use the burn-out to discuss fuses and power ratings.
Exam practice Exam set
National-exam style questions on this simulation (18 marks). Try each one on paper first, then open the marking scheme.
A resistor has a current of 0.25 A when the voltage across it is 3.0 V. (a) Calculate its resistance. (b) Calculate the current when the voltage is 4.5 V, if it obeys Ohm's law. (4 marks)
Marking scheme: (a) R = V ÷ I = 3.0 ÷ 0.25 (1) = 12 Ω (1). (b) I = V ÷ R = 4.5 ÷ 12 (1) = 0.375 A (1).
Draw the circuit used to obtain an I–V graph for a resistor. Show how the ammeter and voltmeter are connected. (4 marks)
Marking scheme: Cell(s) and variable resistor (or variable supply) in series with the resistor (1); ammeter in series (1); voltmeter in parallel across the resistor (1); correct symbols (1).
Sketch the I–V graphs of a metal resistor at constant temperature and of a filament lamp. Explain the shape of the lamp graph. (5 marks)
Marking scheme: Resistor: straight line through the origin (1). Lamp: curve that gets less steep as V increases (1). As the current increases the filament gets hotter (1); the metal ions vibrate more and collide more with electrons (1), so the resistance V/I increases (1).
Two 6 Ω resistors are connected (a) in series and (b) in parallel. Calculate the total resistance in each case. (3 marks)
Marking scheme: (a) R = 6 + 6 = 12 Ω (1). (b) 1/R = 1/6 + 1/6 = 1/3 (1), R = 3 Ω (1).
State Ohm's law, including the condition under which it applies. (2 marks)
Marking scheme: The current through a metallic conductor is directly proportional to the voltage across it (1), provided the temperature and other physical conditions stay constant (1).
Donat Sciences and MathsPhysics · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Ohm's law: test resistors and a torch lamp
Aim: To investigate how the current through a component depends on the voltage across it, and to find the resistance of an unknown resistor.
Apparatus (real laboratory): cells, resistors, torch bulb, ammeter, voltmeter, wires.
What to do (with the simulation on www.donatstem.co.rw)
- Look at the circuit: the ammeter is in series, the voltmeter is connected across the resistor with crocodile clips. Tap the switch to close it: current flows.
- Choose Resistor 10 Ω. Set the voltage to 1.0 V and tap Record V and I.
- Increase the voltage by 1.0 V at a time up to 6.0 V, recording each time.
- Look at the graph: a straight line through the origin means the component obeys Ohm’s law. Read its resistance from the box.
- Repeat with the Unknown resistor X and find its resistance.
- Test Two 10 Ω in series and Two 10 Ω in parallel. Compare their resistances with one 10 Ω resistor.
- Test the Torch lamp from 0.5 V to 6.0 V. Describe the shape of its graph. (Do not go above 9 V!) Open the switch at the end, as you would in a real laboratory.
Results
| # | Component | V (V) | I (A) | V/I (Ω) |
|---|
| 1 | | | | |
| 2 | | | | |
| 3 | | | | |
| 4 | | | | |
| 5 | | | | |
| 6 | | | | |
| 7 | | | | |
| 8 | | | | |
| 9 | | | | |
| 10 | | | | |
Graph
Plot I (A) on the vertical axis against V (V) for each component. Use the gradient of a straight line to find R = 1 ÷ gradient.
Questions
- State Ohm’s law. [2]
- A 10 Ω resistor has 6.0 V across it. What current flows? [2]
- Why does the graph for the torch lamp curve? [2]
- Two 10 Ω resistors are joined in series, then in parallel. Find the total resistance each time. [2]
- Why must the ammeter be in series and the voltmeter in parallel? [2]
- The lamp burnt out at 10 V. Explain using power. [2]
- A resistor has a current of 0.25 A when the voltage across it is 3.0 V. (a) Calculate its resistance. (b) Calculate the current when the voltage is 4.5 V, if it obeys Ohm's law. [4]
- Draw the circuit used to obtain an I–V graph for a resistor. Show how the ammeter and voltmeter are connected. [4]
- Sketch the I–V graphs of a metal resistor at constant temperature and of a filament lamp. Explain the shape of the lamp graph. [5]
- Two 6 Ω resistors are connected (a) in series and (b) in parallel. Calculate the total resistance in each case. [3]
- State Ohm's law, including the condition under which it applies. [2]
Donat Sciences and MathsPhysics · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: Ohm's law: test resistors and a torch lamp
Expected results
Unknown resistor X is 33 Ω. Learners should get gradients of 0.1 A/V (10 Ω), 0.03 A/V (X), 0.05 A/V (series) and 0.2 A/V (parallel). The lamp is modelled with a resistance rising from about 4 Ω (cold) to 20 Ω at 6 V (0.30 A). Use the burn-out to discuss fuses and power ratings.
Answers
- State Ohm’s law. [2]
The current through a metallic conductor is directly proportional to the potential difference across it, provided the temperature and other physical conditions stay the same.
- A 10 Ω resistor has 6.0 V across it. What current flows? [2]
I = V/R = 6.0 ÷ 10 = 0.60 A.
- Why does the graph for the torch lamp curve? [2]
As the current increases, the filament gets hotter. The resistance of a metal increases with temperature, so V/I increases and the current rises less and less: the lamp is non-ohmic.
- Two 10 Ω resistors are joined in series, then in parallel. Find the total resistance each time. [2]
Series: R = 10 + 10 = 20 Ω. Parallel: 1/R = 1/10 + 1/10, so R = 5 Ω.
- Why must the ammeter be in series and the voltmeter in parallel? [2]
The ammeter must carry the same current as the component, so it is in series. The voltmeter measures the potential difference between two points, so it is connected across the component.
- The lamp burnt out at 10 V. Explain using power. [2]
P = VI. At 10 V the power is much greater than the 1.8 W the lamp is designed for, so the filament gets so hot that it melts.
- A resistor has a current of 0.25 A when the voltage across it is 3.0 V. (a) Calculate its resistance. (b) Calculate the current when the voltage is 4.5 V, if it obeys Ohm's law. [4]
(a) R = V ÷ I = 3.0 ÷ 0.25 (1) = 12 Ω (1). (b) I = V ÷ R = 4.5 ÷ 12 (1) = 0.375 A (1).
- Draw the circuit used to obtain an I–V graph for a resistor. Show how the ammeter and voltmeter are connected. [4]
Cell(s) and variable resistor (or variable supply) in series with the resistor (1); ammeter in series (1); voltmeter in parallel across the resistor (1); correct symbols (1).
- Sketch the I–V graphs of a metal resistor at constant temperature and of a filament lamp. Explain the shape of the lamp graph. [5]
Resistor: straight line through the origin (1). Lamp: curve that gets less steep as V increases (1). As the current increases the filament gets hotter (1); the metal ions vibrate more and collide more with electrons (1), so the resistance V/I increases (1).
- Two 6 Ω resistors are connected (a) in series and (b) in parallel. Calculate the total resistance in each case. [3]
(a) R = 6 + 6 = 12 Ω (1). (b) 1/R = 1/6 + 1/6 = 1/3 (1), R = 3 Ω (1).
- State Ohm's law, including the condition under which it applies. [2]
The current through a metallic conductor is directly proportional to the voltage across it (1), provided the temperature and other physical conditions stay constant (1).