Understanding the Induction of an EMF and its Effect on Vehicle Speed

[v_pino]

I did part a) of this question but got stuck when it comes to part b). It asks how the induction of an emf causes the vehicle to slow down.

When the aluminium disc rotates, this conductor cuts through a magnetic field so there is an emf induced. So this induced emf means that there is an induced current (the eddy currents). Is kinetic energy converted into the internal energy of the aluminium disc? So is this what slows down the vehicle?

However, there are areas in the mark scheme that I didn't understand:

1) (I^2)R losses - why is there power loss?

2) (direction of) force opposes motion & opposing magnetic fields - eddy currents are not the result of back emf, right?

 

[jbsteele]

There is no back emf in this case. 3) Why is there an opposing force? What is the cause of this? The answer to part b) of this question is that the eddy currents induced in the aluminum disc create a resistance to the motion of the disc. This resistance is caused by the interaction of the eddy currents with the magnetic field, and it results in energy losses in the form of heat due to I²R losses. As a result, the motion of the disc is slowed down, and this in turn slows down the vehicle. The direction of the force opposing the motion of the disc is opposite to the direction of the motion. This is because the eddy currents produce a magnetic field that opposes the magnetic field of the surrounding magnets, resulting in a repulsive force.

 

[Moetasim]

3) Energy converted to heat - why is there heat produced?

I would like to provide a response to the understanding of the induction of an EMF and its effect on vehicle speed. Firstly, let's review the concept of electromagnetic induction. This is the process by which a changing magnetic field can induce an electric current in a conductor. In the case of the aluminium disc in a vehicle, as it rotates, it cuts through the magnetic field produced by the permanent magnets in the motor. This creates a changing magnetic field, which in turn induces an EMF (electromotive force) in the disc.

Now, let's address the question of how this induction of an EMF causes the vehicle to slow down. As mentioned earlier, the induced EMF leads to the flow of eddy currents in the aluminium disc. These currents create their own magnetic field, which opposes the original magnetic field produced by the permanent magnets. This creates a force that acts against the motion of the disc, causing it to slow down. This is known as the Lenz's Law, which states that the direction of an induced current will always be such as to oppose the change that produced it.

Now, let's address the areas in the mark scheme that you mentioned. Firstly, the (I^2)R losses refer to the power loss due to the resistance of the conductor. As current flows through the aluminium disc, some of the energy is lost as heat due to the resistance of the material. This is a natural occurrence in any electrical system and is known as Joule heating.

Secondly, the force that opposes motion and the opposing magnetic fields are related to the eddy currents. As mentioned earlier, these currents create their own magnetic field, which opposes the original field. This opposing force acts against the motion of the disc, causing it to slow down.

Lastly, the conversion of energy into heat is also related to the Joule heating mentioned earlier. As the eddy currents flow through the conductor, they experience resistance, which leads to the production of heat. This heat is a form of energy that is lost from the system and contributes to the slowing down of the vehicle.

In conclusion, the induction of an EMF in the aluminium disc leads to the production of eddy currents, which in turn creates opposing magnetic fields and forces that act against the motion of the disc. This results in a loss of energy through Joule heating, ultimately leading to the slowing down of the