Physics Teacher Store

In Physics class today, we were learning about batteries and their function in complex electrical circuits.

Mr. Lyons: Until this point, we have worked with what are called "ideal batteries." They have no internal resistance. I can buy this type of battery at the Physics Teacher Store, so my batteries can have no internal resistance. You however, not being physics teachers, can only have other types of batteries, and these do have some internal resistance.

(Mr. Lyons continues lecturing; the following happens several minutes later)

Mr. Lyons: This battery (points to a drawing on the board) has an internal resistance of 0.5 ohms. If I short circuit it, what it the power used?

Student A: Wait, Mr. Lyons, is there really a Physics Teacher Store?

Mr. Lyons (smiling mischievously): Of course! It's where I get all my zero resistance batteries and wires! They also sell massless rope and massless, frictionless pulleys. And every winter, they have a sale on frictionless ice!