Today was my first day teaching recitation for the summer session Physics 202, the algebra-based second half of intro physics. Physics 202 focuses mostly on electricity and magnetism, where the first course (Physics 201) was mostly classical mechanics.
The summer semester is split into halves for university-wide administrative reasons that are somewhat obscure. The gist of it for me is that I’m only the TA for the second half, replacing the previous guy. As it happens my first topic was electromagnetic inductance, and after going through inductance, mutual inductance, energy in an inductor, and inductors in circuits I gave them a quiz with two questions.
1) How much energy is converted to light and heat by a 100 W bulb in 1 hour?
2) If the amount of energy found in 1) is stored in an inductor carrying a current of 10 A, what is the inductance?
I didn’t make this quiz myself, it’s verbatim from the book homework with the numbers changed. First day, don’t want to be too obnoxious.
A surprising number of people missed part 1. This is either after-lunch lethargy or I have a lot of repair work to do on their prior physics education. For those of you who haven’t had physics, you find the answer like this: a watt is a unit of power, which is energy per time. In the SI system energy is measured in joules while time is measured in seconds. A watt is one joule per second. If power is energy per time, energy has to be power multiplied by time. So we have (100 J/s)(60 s / minute)(60 minute/hour) = 360,000 J.
Part 2 is just plug-and-chug since we went over the equations in class and in fact I printed it on their quiz:
Solve for L:
Plug in and you get 7,200 H, where H stands for henrys, the unit of inductance. This is a whopping huge amount of inductance; the quiz is designed to drive home the point of scales to develop intuition for these sorts of problems. It looks like we have a ways to go, but that’s why I rake in the big bucks.
(I don’t actually rake in the big bucks)