Back emf & Lenz’s Law

Analyze qualitatively and quantitatively, with reference to energy transfers and transformations, examples of Faraday’s Law and Lenz’s Law 𝜀 = −𝑁 𝛥𝛷 𝛥𝑡, including but not limited to:

  • The generation of an electromotive force (emf) and evidence for Lenz’s Law produced by the relative movement between a magnet, straight conductors, metal plates and solenoids
  • The generation of an emf produced by the relative movement or changes in current in one solenoid in the vicinity of another solenoid


Following on from Experiment: Lenz’s law, we will cover the theory behind it in greater depth as well as some examples seen in motors.

EMF is the electromotive force which is created to oppose the change in flux. In the lesson on Lenz’s law explored this.

The reason a DC motor is limited to a maximum torque is due to back emf. When the motor rotates, back emf increases in the motor to oppose the change in flux created when the rotor rotates. As a result the voltage supplied – the voltage from the back emf will eventually equal 0. Therefore, the motor has reached its maximum velocity.

When a motor is put under load, it is more likely to burn out. By slowing the rotation of a motor with a heavy load, the back emf is also reduced since there is less of a change in flux. Since the voltage supplied is not increased, therefore, the net voltage is greater then usual. This greater net voltage can result in the components burning up or melting.

Additionally, back emf is extremely useful for magnetic breaks. A magnetic break works by a large aluminium coil rotating in a fixed permanent magnetic field. As a result of the coil moving relative to the magnetic field, creating a change in flux, eddy currents are created in the disk. These eddy currents oppose the change in flux (Lenz’s Law)


Question 1. [4 marks]

Analyze the impact of back emf in motor design and how it is minimized.

Question 2. [4 marks]

Compare and contrast back emf in a motor and an electric break.

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