Intermediate Electromagnetism and Electromagnetic Induction

Electromagnetic brakes are very useful devices in industry. One interesting application I’ve seen for this technology is the mechanical load for an automotive dynamometer, where a car is driven onto a set of steel rollers, with one roller coupled to a large metal disk (with electromagnets on either side). By varying the amount of current sent to the electromagnets, the degree of mechanical drag may be varied.

Incidentally, this disk becomes very hot when in use, because the automobile’s power output cannot simply vanish - it must be converted into a different form of energy in the braking mechanism, and heat it is.

It might help students to visualize the polarity if they imagine a resistive load connected between the two output terminals, and then figured out which direction induced current would go through that load. Once that determination is made, voltage polarity (considering the coil as an energy source) should be easier to visualize. A mistake many beginning students make when doing this, though, is to fail to recognize the coil as the source of electrical energy and the resistor as the load, so be prepared to address this misunderstanding.

If this does not help, suggest they first identify the magnetic polarity of the coil’s induced field: determine which end of the coil is “trying” to be North and which is “trying” to be South. Of course, no induced field will form unless the coil has a complete circuit to sustain the induced current, but it is still helpful to imagine a load resistor or even a short completing the circuit so that induced current and thus induced magnetic polarity may be visualized.

This question is both a quantitative application of Faraday’s Law and an application of Lenz’s Law.