Forum Home
PC World Chat
Thread ID: 57777	2005-05-12 09:41:00	Physics question, non-school related	george12 (7)	PC World Chat

Post ID	Timestamp	Content	User
354471	2005-05-12 09:41:00	OK, I can't think why this is. Say I have a 2,000W electric motor. Imagine it is trying to drive a car up a hill, but the hill happens to be the exact steepness that it is in "Stall". The electric motor's peak torque is when it is stalled - this is also when it draws the most current. Say it's running off 100V, and therefore drawing 20A. It is using 2kW of power. But it's doing no "work" according to the laws of science. It's certainly putting effort into keeping the car stationary, and it's certainly providing force, in fact quite a lot of it. But all in all, it's drawing 2kW, and outputting 0W. Why? If it's just making 2kW of heat, wheres the force coming from? But if it's making power, how does that work with W=FD. It's exerting a lot of force, but over no distance. Therefore work over any amount of time is 0 joules, therefore no power is being produced. It confuses me greatly. Anyone care to explain the concept I am missing? Cheers	george12 (7)
354472	2005-05-12 10:00:00	You have really answered your own questions George. There is no mechanical work being done when the car is stationary. The motor is producing the 'force' to hold the car still. Going back to fundamentals, Flemings Left Hand Rule for the direction of force on a conductor carrying a current in a magnetic field, ie an electric motor. The 2KW will appear as heat via ohmic and copper losses, and the motor will heat up until it either fails or an overload trip cuts in, whereupon the car will roll back down the hill :) I can't see your difficulty in seeing this :confused:	Terry Porritt (14)
354473	2005-05-12 11:49:00	The only thing I had difficulty seeing was that if all 2kW were being used as heat, and that heat wasn't the thing holding the car still, then where was the energy for it coming from. But I get it now :). Thanks.	george12 (7)
354474	2005-05-12 23:59:00	It reminds me of a teacher way back, who told us that if we were walking on the level, at a steady pace, no work was being done. There would only be work done if we started going uphill. :) (Holds up hand and asks...."So how come teacher I get tired if I walk for miles and miles along the flat?") That is why jogging along the flat only hammers and damages the leg joints, and it would be better for these types to climb hills instead thus performing work and getting cardiovascular exercise :D	Terry Porritt (14)
354475	2005-05-13 03:16:00	Of course, we're missing a few more "motor" things here. A 2KW motor won't pull 20A at 100V when stalled. Its windings won't be 5 ohms. They will be a very small fraction of an ohm. With no back-emf (which depends on the rotation speed), I'd expect lots of smoke very quickly.	Graham L (2)
354476	2005-05-13 05:24:00	Good point. I have a couple of "100W" motors which draw 480W when stalled. Anyway, not really particularly relevant here. So if I'm holding a 6KG weight out at arms length (not exactly a small strain), all the energy it takes is being used as heat? So then if I move it up a bit, I am doing work. Does that mean that less power is being used as heat in my arm? Or the same amount of heat and some to the movement?	george12 (7)
354477	2005-05-13 05:40:00	It's not quite the same process. You are still supporting the mass of the "weight" (and your arm) against gravity. If you raise the total mass, that much "work" will be added to the energy input.	Graham L (2)
1