Electromagnetic Braking
HSC Physics Syllabus
- relate Lenz's Law to the law of conservation of energy and apply the law of conservation of energy to:
Electromagnetic Braking Explained (Video)
This video applies the law of conservation of energy and Lenz's Law to magnetic braking.
What Is Electromagnetic Braking?
Electromagnetic braking is a primary application of electromagnetic induction that brings a moving system to rest through the use of magnets.
Unlike friction brakes (which use brake pads pressing against a wheel), electromagnetic brakes use the interaction between a magnetic field and a moving conductor.
The mechanism relies on three sequential physics concepts:
-
Faraday’s Law of Induction: A changing magnetic flux induces an electromotive force (EMF).
-
Eddy Currents: This EMF drives circulating currents within a bulk conductor.
-
Lenz’s Law: The magnetic field created by these currents opposes the change that caused them, resulting in a braking force.
Faraday's Law
When a conductive wheel rotates through a magnetic field, it experiences changes in flux. By Faraday's law, EMF is induced in the wheel. This EMF in turn produces eddy currents as shown below.
Lenz's Law
The eddy currents are induced in a direction such that the associated magnetic fields will act to oppose the initial change in flux.
In the area of the wheel moving towards the magnetic field, the eddy currents will be anti-clockwise as viewed from above. This creates a magnetic field with North pole facing up, and South pole facing down. This magnetic field repels the external magnetic field produced by the magnet, generating a counter-torque to slow down the wheel's rotation.
Simultaneously, the part of the wheel moving away from the magnet experiences a decrease in magnetic flux. The eddy current induced will be clockwise as viewed from above, producing a magnetic field (South pole facing up, North pole facing down) that attracts the magnet. This also generates a counter-torque to slow down the wheel's rotation.
This principle of magnetic braking is used to bring cars to rest. When the car brake is off, the electromagnets near the wheels are turned off. This means that there will be no external magnetic field to magnetically brake the wheels.
Advantages of Electromagnetic Braking
There are many advantages to magnetic braking:
Minimal Wear and Tear
Since the braking force is generated by magnetic fields interacting with electrons inside the metal, there is no physical contact between the braking mechanism and the moving wheel or rail.
Unlike friction brakes (like brake pads on a car), there are no parts rubbing together to wear out. This significantly reduces maintenance costs and the need to replace parts, which is ideal for high-use systems like trains.
Smooth Braking Profile
The magnitude of the induced EMF (and thus the braking force) is directly proportional to the rate of change of flux. This means the braking force is proportional to the velocity of the vehicle.
As the vehicle slows down, the braking force naturally and instantly decreases. This prevents the "jerky" halt often experienced with mechanical brakes and naturally prevents wheels from locking up or skidding.
At high speeds, the rate of change of flux is very high, producing a massive opposing force. Electromagnetic brakes are exceptionally effective when a vehicle is moving fast (e.g., a Shinkansen bullet train), providing powerful stopping power exactly when it is needed most.
Fail-Safe Capability
Some systems e.g. roller coasters use strong permanent magnets rather than electromagnets. This creates a passive safety system. Even if there is a total power outage, the laws of physics (Lenz's Law) will still apply, and the roller coaster car will be slowed down by the magnets at the end of the track. No electricity is required for the brakes to engage.
Electromagnetic Braking and Law of Conservation of Energy
Electromagnetic braking can be analysed in terms of the law of conservation of energy. Any moving body has kinetic energy, and this is converted into electrical energy (the current produced).
The electrical energy is then lost as heat through resistive heating.
Testing Your Understanding
Practice Question 1
(a) All transformers which operate using the principle of Faraday’s electromagnetic induction have less than 100% efficiency. Explain why this is the case. (4 marks)
(b) Explain one strategy that is used to overcome the problem(s) you outlined in (a). (2 marks)
Practice Question 2
The primary coil of a transformer has 4800 turns and is supplied by 240 V AC. The secondary circuit operates a small electric motor of resistance 192 ohms which requires 0.5 A.
(a) Calculate how many turns the secondary coil should have.
(b) The current flowing in the primary circuit is 0.21 A. Calculate the efficiency of the transformer.
Previous section: Operation of AC Induction Motors