Fundamentals of Electromagnetic Induction
5. Practical Demonstrations of Induction
Learning outcomes
- I can explain what happens during common induction demonstrations.
- I can describe how moving magnets induce current in coils.
- I can interpret observations from induction experiments.
- I can relate demonstrations to Faraday's and Lenz's Laws.
- I can explain real-world examples that operate using electromagnetic induction.
Practical Demonstrations of Induction
Electromagnetic induction can be observed directly using relatively simple equipment. A magnet, a coil of wire, and a sensitive meter are enough to demonstrate one of the most important principles in electromagnetism.
The central idea is:
A changing magnetic flux linkage induces an EMF.
If the circuit is complete, the induced EMF can produce an induced current.
Practical demonstrations allow us to investigate not only whether induction occurs, but also the size and direction of the induced EMF.
Learning Targets
- I can explain what happens during common induction demonstrations.
- I can describe how moving magnets induce current in coils.
- I can interpret observations from induction experiments.
- I can relate demonstrations to Faraday's and Lenz's Laws.
- I can explain real-world examples that operate using electromagnetic induction.
The Magnet and Coil Demonstration
One of the simplest demonstrations of electromagnetic induction uses:
- A bar magnet
- A coil of insulated wire
- A galvanometer or sensitive voltmeter
- Connecting wires
The coil is connected to the meter.
The magnet is then moved toward, into, and away from the coil.
Several important observations can be made.
Moving a Magnet Toward a Coil
Suppose the north pole of a magnet is moved toward a coil.
As the magnet approaches, the magnetic field passing through the coil becomes stronger.
Therefore:
Magnetic flux through the coil increases.
Because the magnetic flux linkage is changing, an EMF is induced.
If the circuit is complete:
an induced current flows.
The galvanometer needle deflects to one side.
This provides experimental evidence that a current has been produced.
Holding the Magnet Still
Now suppose the magnet is held completely still inside the coil.
The magnet is still producing a magnetic field.
However, the magnetic flux through the coil is no longer changing.
Therefore:
No change in magnetic flux → No induced EMF
The galvanometer returns to:
zero
This observation is extremely important.
A magnetic field by itself does not necessarily produce an induced EMF.
There must be a change in magnetic flux linkage.
Pulling the Magnet Out
Now pull the magnet away from the coil.
The magnetic flux through the coil decreases.
An EMF is again induced.
However, something interesting happens:
The galvanometer deflects in the opposite direction.
This means that the induced current has reversed direction.
Why?
Because the change in magnetic flux is now opposite to the change that occurred when the magnet entered the coil.
A Typical Set of Observations
For a magnet and coil experiment:
Magnet moving into coil → meter deflects
Magnet stationary → meter reads zero
Magnet moving out → meter deflects in opposite direction
This simple experiment demonstrates both:
Faraday's Law
and:
Lenz's Law
Connecting the Experiment to Faraday's Law
Faraday's Law states that the magnitude of the induced EMF depends on the rate of change of magnetic flux linkage.
The magnitude can be represented as:
|ε| = N|ΔΦ/Δt|
where:
ε = induced EMF
N = number of turns
ΔΦ = change in magnetic flux
Δt = time taken
This explains several observations from the magnet and coil experiment.
Moving the Magnet Slowly
If the magnet moves slowly into the coil, the magnetic flux changes over a relatively long time.
Therefore:
Rate of flux change is small
So:
Induced EMF is relatively small
The galvanometer produces a relatively small deflection.
Moving the Magnet Quickly
If the same magnet is pushed quickly into the coil, approximately the same overall flux change occurs in a shorter time.
Therefore:
Rate of flux change increases
So:
Induced EMF increases
The galvanometer shows a larger deflection.
This provides direct experimental evidence for Faraday's Law.
Moving the Coil Instead
The magnet does not have to move.
Suppose the magnet is held still while the coil is moved toward it.
The magnetic flux through the coil still changes.
Therefore:
An EMF is induced.
Similarly, moving the coil away from the magnet reverses the change in magnetic flux.
The important factor is:
relative motion between the coil and the magnetic field
not which object happens to be moving.
Using a Stronger Magnet
The experiment can also be repeated using magnets of different strengths.
If a stronger magnet is moved through the same coil in the same way, it generally produces a larger change in magnetic flux.
Therefore:
Stronger magnet → Greater flux change → Greater induced EMF
The galvanometer should show a larger deflection.
Increasing the Number of Turns
We can also compare coils containing different numbers of turns.
For example:
Coil A = 100 turns
Coil B = 500 turns
If the same magnet moves through both coils in the same way, the 500-turn coil should produce a larger induced EMF.
Faraday's Law explains why:
More turns → Greater change in flux linkage → Greater induced EMF
Lenz's Law
Faraday's Law helps explain the size of the induced EMF.
Lenz's Law helps explain its direction.
Lenz's Law states that:
The induced current flows in a direction that opposes the change in magnetic flux that produced it.
Notice that the induced current opposes the change.
It does not simply oppose the magnetic field.
Approaching North Pole
Imagine the north pole of a magnet approaching a coil.
The magnetic flux through the coil is increasing.
According to Lenz's Law, the induced current creates a magnetic field that opposes this increase.
The near side of the coil therefore behaves like a:
north pole
The approaching north pole and induced north pole repel one another.
The induced effect resists the magnet's approach.
North Pole Moving Away
Now imagine the north pole moving away from the coil.
The magnetic flux through the coil is decreasing.
The induced current acts to oppose this decrease.
The coil therefore produces a magnetic field that tries to maintain the original flux.
The near side of the coil behaves like a:
south pole
The north pole of the magnet is attracted toward it.
The induced effect therefore resists the magnet moving away.
Lenz's Law Opposes the Change
This gives an important pattern.
When the magnet approaches:
The coil resists the approach.
When the magnet moves away:
The coil resists the separation.
In both situations, the induced current acts against the change in magnetic flux.
This is why Lenz's Law is sometimes described as nature's electromagnetic version of resistance to change.
Why Lenz's Law Is Important
Suppose the induced current helped the magnet move rather than opposing its motion.
The magnet would cause a current.
The current would then push the magnet further.
That motion would create an even greater current.
Energy would appear without any external energy input.
That would violate the law of conservation of energy.
Instead, work must be done to move the magnet against the magnetic effect produced by the induced current.
Mechanical energy is transferred into electrical energy.
Dropping a Magnet Through a Copper Tube
A dramatic demonstration of Lenz's Law uses:
- A strong magnet
- A vertical copper or aluminium tube
When an ordinary non-magnetic object is dropped through the tube, it falls normally.
When a strong magnet is dropped through the conducting tube, it falls surprisingly slowly.
The copper itself is not permanently magnetic.
So why does the magnet slow down?
Eddy Currents in the Copper Tube
As the magnet falls, its magnetic field moves relative to the copper.
The changing magnetic flux induces circular currents in the metal.
These currents are called:
eddy currents
The eddy currents produce their own magnetic fields.
According to Lenz's Law, these fields oppose the motion that caused them.
Therefore, the falling magnet experiences an upward magnetic force.
The magnet still falls, but more slowly.
Where Does the Energy Go?
The magnet begins with gravitational potential energy.
As it falls, some of this energy is transferred into:
- Kinetic energy
- Electrical energy in the eddy currents
- Thermal energy in the metal
The copper tube may become slightly warmer.
Energy has not disappeared.
It has been transferred from one energy store to another.
Moving a Magnet Through a Conducting Plate
Another demonstration uses a strong magnet moving above or near a thick metal plate.
The changing magnetic field induces eddy currents in the conductor.
These currents produce magnetic fields that oppose the motion.
The magnet may therefore experience a noticeable drag force.
This principle is used in some real braking systems.
Swinging Magnet Demonstration
A magnet can also be attached to a pendulum.
Allow it to swing freely.
If it swings through air, it gradually slows because of ordinary energy losses.
Now allow it to swing above a conducting metal plate.
The changing magnetic field produces eddy currents in the plate.
These create an opposing magnetic effect.
The magnet slows much more rapidly.
This is another clear demonstration of Lenz's Law.
Coil and Galvanometer Demonstration
A galvanometer is particularly useful because it shows both:
- Whether current is flowing
- The direction of current
Suppose the needle moves right when a magnet enters the coil.
When the magnet leaves, the needle moves left.
This tells us that the current has:
reversed direction
The direction of current depends on the direction of the change in magnetic flux.
Reversing the Magnet
Instead of using the north pole, turn the magnet around and move the south pole toward the coil.
The change in magnetic flux is now in the opposite direction.
Therefore:
The induced EMF reverses direction.
The galvanometer needle deflects in the opposite direction compared with the approaching north pole.
This is another useful experimental test of Lenz's Law.
Four Useful Cases
Suppose the same end of the coil is being observed.
North pole approaching → one current direction
North pole moving away → opposite direction
South pole approaching → opposite to north approaching
South pole moving away → opposite to south approaching
The exact clockwise or anticlockwise direction depends on which side of the coil is being viewed.
The important principle is:
Reversing the change in magnetic flux reverses the induced EMF.
Rotating a Coil in a Magnetic Field
Another important demonstration involves rotating a coil between magnetic poles.
As the coil rotates, the angle between the coil and magnetic field changes.
Therefore, the magnetic flux through the coil continuously changes.
This produces an induced EMF.
After half a rotation, the direction of the flux change reverses.
Therefore, the direction of the induced EMF also reverses.
This produces:
alternating current (AC)
From Demonstration to Generator
The rotating-coil experiment is the basic principle behind an electrical generator.
A generator converts:
Mechanical energy → Electrical energy
The coil or magnetic field is rotated mechanically.
This continuously changes the magnetic flux linkage.
An EMF is induced.
If the circuit is complete, current flows.
Large power-station generators operate using the same basic physical principle.
Bicycle Dynamo
A bicycle dynamo provides a familiar application of electromagnetic induction.
As the bicycle wheel turns, it causes part of the generator to rotate.
This produces changing magnetic flux through a coil.
An EMF is induced.
The electrical energy can then power the bicycle lights.
When the bicycle moves faster, the magnetic flux changes more rapidly.
This can increase the induced EMF.
Transformers
A transformer is another important application of electromagnetic induction.
Unlike the magnet-and-coil demonstration, transformers do not require a magnet to physically move back and forth.
Instead, an alternating current in one coil produces a continuously changing magnetic field.
This changing magnetic field passes through a second coil.
The changing magnetic flux induces an EMF in the second coil.
Transformers are essential for changing voltages in electrical power systems.
Wireless Charging
Wireless charging also uses electromagnetic induction.
An alternating current passes through a coil in the charging pad.
This produces a changing magnetic field.
A second coil inside the device experiences changing magnetic flux.
An EMF is induced in the device's coil.
The induced electrical energy can then be used to charge the battery.
This is essentially a sophisticated version of the same induction principles demonstrated with classroom coils.
Induction Cooktops
Induction cooking also relies on electromagnetic induction.
An alternating current flows through a coil beneath the cooking surface.
The changing magnetic field induces currents in suitable metal cookware.
These currents produce heating in the pan.
Rather than heating a burner first, electrical energy is transferred through electromagnetic effects that cause the cookware itself to heat.
Electromagnetic Braking
Eddy-current braking uses Lenz's Law.
A conductor moves through a magnetic field.
The changing magnetic flux induces eddy currents.
The eddy currents produce magnetic fields that oppose the motion.
The result is a braking force.
Applications can include:
- Trains
- Exercise equipment
- Roller coasters
- Industrial machinery
- Measuring instruments
Because the braking effect can occur without physical contact, there can be less mechanical wear than with friction-based braking alone.
Microphones
Some microphones also use electromagnetic induction.
Sound waves cause a diaphragm and attached coil to move.
The coil moves relative to a magnetic field.
This changing magnetic flux induces a small electrical signal.
The electrical signal represents the original sound wave and can be amplified, recorded, or transmitted.
Interpreting Experimental Observations
When analyzing an induction demonstration, ask:
- Is the magnetic flux changing?
- What is causing the change?
- Is the flux increasing or decreasing?
- How quickly is it changing?
- Is an EMF being induced?
- Is the circuit complete so current can flow?
- What does the meter reading show?
- Does the current direction agree with Lenz's Law?
- Where is the energy coming from?
These questions help connect observations with the underlying physics.
Worked Example — Magnet and Coil
A student pushes a north pole rapidly into a coil.
The galvanometer needle moves strongly to the right.
The student then holds the magnet still.
The needle returns to zero.
Finally, the student pulls the magnet out rapidly.
The needle moves strongly to the left.
Interpretation
While entering:
Flux changes rapidly → Large induced EMF
While stationary:
Flux does not change → No induced EMF
While leaving:
Flux changes rapidly in the opposite direction → Large EMF in the opposite direction
This behaviour is consistent with both Faraday's and Lenz's Laws.
Worked Example — Slow and Fast Motion
A magnet is pushed into a coil slowly.
The galvanometer shows a small deflection.
The experiment is repeated with the same magnet moving much faster.
The galvanometer shows a larger deflection.
Why?
Faraday's Law tells us:
Faster motion → Faster change in magnetic flux → Greater induced EMF
The total flux change may be similar, but it occurs in less time.
Worked Example — Magnet Dropped Through Copper
A student drops a strong magnet through a copper tube.
It falls much more slowly than expected.
The student concludes that copper must be strongly attracted to magnets.
This conclusion is incorrect.
The better explanation is:
Moving magnet → Changing magnetic flux in copper → Eddy currents → Opposing magnetic field → Upward magnetic force
This is an example of Lenz's Law.
Faraday's Law and Lenz's Law Together
The two laws answer different questions.
Faraday's Law
helps determine:
How large is the induced EMF?
Larger rate of flux change produces larger EMF.
Lenz's Law
helps determine:
Which direction is the induced EMF or current?
The induced effect opposes the change that produced it.
Together, the laws provide a powerful description of electromagnetic induction.
Common Mistakes
Mistake: Thinking a Magnet Must Touch the Coil
Physical contact is not necessary.
A changing magnetic field through the coil is enough to induce an EMF.
Mistake: Thinking a Stationary Magnet Produces Continuous Current
If the magnet and coil are stationary relative to one another:
Flux is constant → No continuous induced EMF
Mistake: Thinking the Coil Always Repels the Magnet
When the magnet approaches, the induced effect resists the approach.
When the magnet moves away, the induced effect resists the separation.
The coil therefore does not simply "always repel."
Mistake: Saying Lenz's Law Opposes the Magnetic Field
More accurately:
Lenz's Law opposes the change in magnetic flux that produced the induced current.
Mistake: Confusing EMF and Current
EMF is an induced voltage.
Current flows only when there is a complete conducting path.
Mistake: Thinking Faster Motion Creates More Magnetic Flux
Faster motion primarily increases the rate at which the flux changes.
That greater rate of change produces a larger induced EMF.
Key Terms
Electromagnetic induction – production of an EMF by changing magnetic flux linkage.
Induced EMF – voltage produced by electromagnetic induction.
Induced current – current produced by an induced EMF in a complete circuit.
Magnetic flux – measure of magnetic field passing through an area.
Faraday's Law – induced EMF depends on the rate of change of magnetic flux linkage.
Lenz's Law – the induced effect opposes the change in magnetic flux that caused it.
Galvanometer – sensitive instrument used to detect small electric currents.
Eddy currents – circulating currents induced inside conducting materials.
Generator – device that converts mechanical energy into electrical energy using induction.
Transformer – device that uses electromagnetic induction to transfer electrical energy between coils.
Check Your Understanding
1. What happens to a galvanometer when a magnet is pushed into a connected coil?
2. Why does the galvanometer return to zero when the magnet is held still?
3. What happens when the magnet is pulled out of the coil?
4. Why does moving the magnet faster produce a larger meter deflection?
5. Can induction occur if the magnet remains stationary but the coil moves? Explain.
6. Explain how the magnet-and-coil demonstration provides evidence for Faraday's Law.
7. Explain how the reversal of the galvanometer needle provides evidence for Lenz's Law.
8. What would happen if the magnet were replaced with a stronger magnet and moved in the same way?
9. Why does increasing the number of coil turns increase the induced EMF?
10. Explain why a magnet falls slowly through a copper tube.
11. What are eddy currents?
12. Explain how Lenz's Law is connected to conservation of energy.
13. Describe how a rotating coil can produce an alternating EMF.
14. Explain how a bicycle dynamo uses electromagnetic induction.
15. Choose one of the following and explain how electromagnetic induction is involved:
- Transformer
- Wireless charger
- Induction cooktop
- Dynamic microphone
- Eddy-current brake
Key Takeaways
- Electromagnetic induction can be demonstrated using a magnet, coil, and galvanometer.
- Moving a magnet into a coil produces an induced EMF.
- Moving the magnet out reverses the direction of the induced EMF.
- A stationary magnet and stationary coil do not produce a continuous induced EMF.
- Relative motion matters—the coil can move instead of the magnet.
- Faster motion produces a greater induced EMF because magnetic flux changes more rapidly.
- Stronger magnetic fields and more coil turns can increase induced EMF.
- Faraday's Law relates induced EMF to the rate of change of magnetic flux linkage.
- Lenz's Law determines the direction of the induced effect.
- The induced effect opposes the change in magnetic flux that caused it.
- Eddy currents provide a striking demonstration of Lenz's Law.
- Generators convert mechanical energy into electrical energy using electromagnetic induction.
- Transformers use changing magnetic fields to induce voltages in other coils.
- Wireless charging, induction cooking, dynamic microphones, and electromagnetic braking all use induction.
- Practical demonstrations connect observable effects directly to the laws of electromagnetic induction.
Did You Know?
One of the most memorable induction demonstrations requires no battery and no powered electromagnet: simply drop a strong magnet through a copper tube. The magnet can appear to fall almost in slow motion. The changing magnetic field induces eddy currents in the copper, and those currents produce magnetic fields that oppose the magnet's motion. It is a remarkably direct demonstration of Faraday's Law, Lenz's Law, and conservation of energy working together.