Hard Physics MC: How do I go about doing this question?

[envisagator]

A 100 g bar magnet is released from rest and falls vertically through a small coil connected to a light globe. the wires have neglible resistance.
After falling through a height of 1.00 m, the bar magnet is moving at 3.8m/s.
How much electrical energy was converted to other forms of energy by the light globe?

[jamonwindeyer]

A 100 g bar magnet is released from rest and falls vertically through a small coil connected to a light globe. the wires have neglible resistance.
After falling through a height of 1.00 m, the bar magnet is moving at 3.8m/s.
How much electrical energy was converted to other forms of energy by the light globe?

Welcome to the forums envisagator!

So let's ignore the coil for the moment. What speed would the magnet be falling at based purely on an acceleration of \(g=9.8\) metres per second per second downwards over 1 metre? We have a formula for that:



This gives it a kinetic energy of:



However, we weren't going that quickly, we were going at 3.8 metres per second. So we actually have a slightly smaller amount of kinetic energy. \(K_e'=\frac{1}{2}\times0.1\times3.8^2\approx0.72J\).

The difference between these is 0.26J, and that's the kinetic energy you lose due to the electromagnetic braking effect of the induced currents in the coil. It is converted to electrical energy (the induced currents). Since the wires have no resistance, all of this goes to the light bulb to be converted into other forms of energy (heat, light, etc).

So the answer is D