Physics · Simulation and virtual lab Free
Electromagnetic induction: magnet, coil and galvanometer
Push a magnet into a coil, watch the galvanometer kick, and discover what makes the induced emf bigger and which way it points.
- You will learn to
- Show that an emf is induced only when the flux linking a coil changes.
- Find how the emf depends on the speed of the magnet, the number of turns and the strength of the magnet (Faraday's law).
- Predict the direction of the induced current with Lenz's law, and observe mutual induction.
- You will measure
- Peak induced emf (mV) for different speeds (m/s), numbers of turns and magnets.
- Time needed
- About 35–45 min
- Equipment
- Nothing extra. Real version: bar magnets, coil of insulated copper wire (100–400 turns), centre-zero galvanometer, two coils on a soft-iron rod, cell, switch, 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 show that an emf is induced only when the magnetic flux linking a coil changes, to investigate how the size of the emf depends on the speed of the magnet and the number of turns (Faraday's law), and to find its direction (Lenz's law).
What to do
- Choose Magnet and coil, 200 turns, one magnet, N pole facing the coil, speed 0.50 m/s. Tap Push the magnet in. Note the direction of the kick, then tap Record the peak reading.
- Tap Pull the magnet out and record. Compare the direction of the kick with step 1. What happens when the magnet is still?
- Turn the magnet round (S pole facing the coil) and repeat steps 1 and 2. Look at the induced poles shown at the ends of the coil.
- Keep 200 turns. Push the magnet in at 0.20, 0.40, 0.60, 0.80 and 1.00 m/s, recording each peak (pull it out between pushes).
- Repeat step 4 with 400 turns, then with two magnets. Compare the gradients of the lines on the graph.
- Choose Two coils and a switch. Close the switch, wait, then open it, recording each kick. Compare an air core with the soft-iron rod.
Results
Questions
When is an emf induced in the coil?
Only while the magnetic flux linking the coil is changing: when the magnet (or the coil) is moving, or when the current in a nearby coil is changing. A still magnet, even inside the coil, gives no emf.
State Faraday's law of electromagnetic induction.
The induced emf is directly proportional to the rate of change of magnetic flux linkage: E = −N ΔΦ/Δt.
Why does the needle kick the other way when the magnet is pulled out?
The flux is now decreasing instead of increasing, so the rate of change of flux has the opposite sign and the induced current flows the other way.
State Lenz's law and use it to explain the poles shown at the end of the coil.
The induced current flows in a direction that opposes the change producing it. When an N pole approaches, the near end of the coil becomes an N pole and repels it; when it is pulled away, the near end becomes an S pole and attracts it.
Give three ways to increase the induced emf.
Move the magnet faster, use a coil with more turns, use a stronger magnet (or two magnets), or put a soft-iron core in the coil.
Why must a transformer be supplied with alternating current?
A steady current makes a steady flux, so no emf is induced in the secondary coil. Alternating current changes all the time, so the flux keeps changing and an emf is induced continuously.
For teachers: expected results and how the model works
Model: flux per turn Φ(x) = Φ₀k/(1 + x²/a²)^1.5 with Φ₀ = 4.0 × 10⁻⁵ Wb, a = 6 cm and k = 1 or 2 magnets; E = −N dΦ/dx × v. Peak emf = 0.573 × N × k × v mV: 200 turns, one magnet: 0.5 m/s → 57.3 mV, 1.0 m/s → 115 mV; 400 turns → twice as much. Pushing the N pole in gives a kick to the left; pulling out or reversing the magnet gives a kick to the right. Two coils: the rise and fall of the primary current is slowed down (time constants 0.25 s and 0.20 s) so that the kick can be seen; peak emf on closing = 0.5 × N mV with the iron core and 0.1 × N mV with air (e.g. 100 mV and 20 mV for 200 turns); opening gives a slightly larger kick the other way. The needle has a red marker that stays at the peak.
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 coil of 500 turns has an area of 4.0 × 10⁻⁴ m². The magnetic flux density through it falls uniformly from 0.20 T to zero in 0.050 s. Calculate (a) the change in flux linkage and (b) the average induced emf. (4 marks)
Marking scheme: (a) ΔΦ = ΔB × A = 0.20 × 4.0 × 10⁻⁴ = 8.0 × 10⁻⁵ Wb (1); flux linkage change NΔΦ = 500 × 8.0 × 10⁻⁵ = 0.040 Wb-turns (1). (b) E = NΔΦ/Δt = 0.040 ÷ 0.050 (1) = 0.80 V (1).
A bar magnet is dropped, N pole first, through a vertical coil connected to a sensitive voltmeter. (a) Sketch how the emf changes with time. (b) Explain why the second peak is larger than the first and opposite in sign. (5 marks)
Marking scheme: (a) A peak of one sign as the magnet enters (1), zero when it is in the middle (1), a peak of opposite sign as it leaves (1). (b) The magnet is moving faster as it leaves (it accelerates under gravity), so the rate of change of flux is greater (1); the flux is decreasing instead of increasing, so the emf has the opposite sign (1).
Use Lenz's law to explain why a magnet falls more slowly through a vertical copper tube than through a plastic tube. (3 marks)
Marking scheme: The moving magnet changes the flux through the copper, inducing eddy currents (1); by Lenz's law these currents create a magnetic field that opposes the motion of the magnet (1); so there is an upward force on the magnet and it falls slowly. Plastic is an insulator: no currents, no opposing force (1).
Two coils are wound on the same soft-iron rod. The first is connected to a cell and a switch, the second to a galvanometer. Describe and explain what the galvanometer shows when the switch is closed, kept closed and then opened. (4 marks)
Marking scheme: On closing: a brief kick (1) because the growing current makes the flux in the second coil increase (1). While closed: zero, because the current and flux are steady (1). On opening: a brief kick in the opposite direction, because the flux decreases (1).
Explain how the energy of a moving magnet is converted to electrical energy, using the idea of work. (2 marks)
Marking scheme: The induced current produces a field that opposes the motion (Lenz) (1), so work must be done to push the magnet; this work becomes the electrical energy of the induced current (energy is conserved) (1).
Donat Sciences and MathsPhysics · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Electromagnetic induction: magnet, coil and galvanometer
Aim: To show that an emf is induced only when the magnetic flux linking a coil changes, to investigate how the size of the emf depends on the speed of the magnet and the number of turns (Faraday's law), and to find its direction (Lenz's law).
Apparatus (real laboratory): bar magnets, coil of insulated copper wire (100–400 turns), centre-zero galvanometer, two coils on a soft-iron rod, cell, switch, wires.
What to do (with the simulation on www.donatstem.co.rw)
- Choose Magnet and coil, 200 turns, one magnet, N pole facing the coil, speed 0.50 m/s. Tap Push the magnet in. Note the direction of the kick, then tap Record the peak reading.
- Tap Pull the magnet out and record. Compare the direction of the kick with step 1. What happens when the magnet is still?
- Turn the magnet round (S pole facing the coil) and repeat steps 1 and 2. Look at the induced poles shown at the ends of the coil.
- Keep 200 turns. Push the magnet in at 0.20, 0.40, 0.60, 0.80 and 1.00 m/s, recording each peak (pull it out between pushes).
- Repeat step 4 with 400 turns, then with two magnets. Compare the gradients of the lines on the graph.
- Choose Two coils and a switch. Close the switch, wait, then open it, recording each kick. Compare an air core with the soft-iron rod.
Results
| # | What happened | Turns N | Set-up | v (m/s) | Peak emf (mV) | Needle |
|---|
| 1 | | | | | | |
| 2 | | | | | | |
| 3 | | | | | | |
| 4 | | | | | | |
| 5 | | | | | | |
| 6 | | | | | | |
| 7 | | | | | | |
| 8 | | | | | | |
| 9 | | | | | | |
| 10 | | | | | | |
Graph
Plot the peak emf (mV) on the vertical axis against the speed v (m/s) for each set-up. Draw the best straight line through the origin for each and compare the gradients.
Questions
- When is an emf induced in the coil? [2]
- State Faraday's law of electromagnetic induction. [2]
- Why does the needle kick the other way when the magnet is pulled out? [2]
- State Lenz's law and use it to explain the poles shown at the end of the coil. [2]
- Give three ways to increase the induced emf. [2]
- Why must a transformer be supplied with alternating current? [2]
- A coil of 500 turns has an area of 4.0 × 10⁻⁴ m². The magnetic flux density through it falls uniformly from 0.20 T to zero in 0.050 s. Calculate (a) the change in flux linkage and (b) the average induced emf. [4]
- A bar magnet is dropped, N pole first, through a vertical coil connected to a sensitive voltmeter. (a) Sketch how the emf changes with time. (b) Explain why the second peak is larger than the first and opposite in sign. [5]
- Use Lenz's law to explain why a magnet falls more slowly through a vertical copper tube than through a plastic tube. [3]
- Two coils are wound on the same soft-iron rod. The first is connected to a cell and a switch, the second to a galvanometer. Describe and explain what the galvanometer shows when the switch is closed, kept closed and then opened. [4]
- Explain how the energy of a moving magnet is converted to electrical energy, using the idea of work. [2]
Donat Sciences and MathsPhysics · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: Electromagnetic induction: magnet, coil and galvanometer
Expected results
Model: flux per turn Φ(x) = Φ₀k/(1 + x²/a²)^1.5 with Φ₀ = 4.0 × 10⁻⁵ Wb, a = 6 cm and k = 1 or 2 magnets; E = −N dΦ/dx × v. Peak emf = 0.573 × N × k × v mV: 200 turns, one magnet: 0.5 m/s → 57.3 mV, 1.0 m/s → 115 mV; 400 turns → twice as much. Pushing the N pole in gives a kick to the left; pulling out or reversing the magnet gives a kick to the right. Two coils: the rise and fall of the primary current is slowed down (time constants 0.25 s and 0.20 s) so that the kick can be seen; peak emf on closing = 0.5 × N mV with the iron core and 0.1 × N mV with air (e.g. 100 mV and 20 mV for 200 turns); opening gives a slightly larger kick the other way. The needle has a red marker that stays at the peak.
Answers
- When is an emf induced in the coil? [2]
Only while the magnetic flux linking the coil is changing: when the magnet (or the coil) is moving, or when the current in a nearby coil is changing. A still magnet, even inside the coil, gives no emf.
- State Faraday's law of electromagnetic induction. [2]
The induced emf is directly proportional to the rate of change of magnetic flux linkage: E = −N ΔΦ/Δt.
- Why does the needle kick the other way when the magnet is pulled out? [2]
The flux is now decreasing instead of increasing, so the rate of change of flux has the opposite sign and the induced current flows the other way.
- State Lenz's law and use it to explain the poles shown at the end of the coil. [2]
The induced current flows in a direction that opposes the change producing it. When an N pole approaches, the near end of the coil becomes an N pole and repels it; when it is pulled away, the near end becomes an S pole and attracts it.
- Give three ways to increase the induced emf. [2]
Move the magnet faster, use a coil with more turns, use a stronger magnet (or two magnets), or put a soft-iron core in the coil.
- Why must a transformer be supplied with alternating current? [2]
A steady current makes a steady flux, so no emf is induced in the secondary coil. Alternating current changes all the time, so the flux keeps changing and an emf is induced continuously.
- A coil of 500 turns has an area of 4.0 × 10⁻⁴ m². The magnetic flux density through it falls uniformly from 0.20 T to zero in 0.050 s. Calculate (a) the change in flux linkage and (b) the average induced emf. [4]
(a) ΔΦ = ΔB × A = 0.20 × 4.0 × 10⁻⁴ = 8.0 × 10⁻⁵ Wb (1); flux linkage change NΔΦ = 500 × 8.0 × 10⁻⁵ = 0.040 Wb-turns (1). (b) E = NΔΦ/Δt = 0.040 ÷ 0.050 (1) = 0.80 V (1).
- A bar magnet is dropped, N pole first, through a vertical coil connected to a sensitive voltmeter. (a) Sketch how the emf changes with time. (b) Explain why the second peak is larger than the first and opposite in sign. [5]
(a) A peak of one sign as the magnet enters (1), zero when it is in the middle (1), a peak of opposite sign as it leaves (1). (b) The magnet is moving faster as it leaves (it accelerates under gravity), so the rate of change of flux is greater (1); the flux is decreasing instead of increasing, so the emf has the opposite sign (1).
- Use Lenz's law to explain why a magnet falls more slowly through a vertical copper tube than through a plastic tube. [3]
The moving magnet changes the flux through the copper, inducing eddy currents (1); by Lenz's law these currents create a magnetic field that opposes the motion of the magnet (1); so there is an upward force on the magnet and it falls slowly. Plastic is an insulator: no currents, no opposing force (1).
- Two coils are wound on the same soft-iron rod. The first is connected to a cell and a switch, the second to a galvanometer. Describe and explain what the galvanometer shows when the switch is closed, kept closed and then opened. [4]
On closing: a brief kick (1) because the growing current makes the flux in the second coil increase (1). While closed: zero, because the current and flux are steady (1). On opening: a brief kick in the opposite direction, because the flux decreases (1).
- Explain how the energy of a moving magnet is converted to electrical energy, using the idea of work. [2]
The induced current produces a field that opposes the motion (Lenz) (1), so work must be done to push the magnet; this work becomes the electrical energy of the induced current (energy is conserved) (1).