Electromagnetism in my physics class

1. Jan 9, 2005

sp00ky

Well I was learning bits about Electromagnetism in my physics class and we were learning about transformers and thought it was kind of neat. I was just wondering why transformers use alterneting current and why power must be delivered at such high voltages across transmission lines? Also, how is transformers used in delivering poweer to homes from the hydro-electic power plant?

2. Jan 9, 2005

pervect

Staff Emeritus
Transformers will not work with DC, only with AC. If you connect a battery across the terminals of a transformer, the current in the primary will rise linearly with time, due to the inductance of the primary, until eventually the core saturates at which point the current will rise even more quickly, until it becomes limited only by the resistance of the wire. This happens even with no load on the secondary.

When you hook up an alternating voltage to a transformer, it works as intended. Only a small amount of current "circulates" in the circuit due to the inducatance of the primary. Thus if there is no load on the secondary, very little current will flow in the primary either.

Transmission lines work at high voltages to keep the losses due to resistance down. The power loss due to the resistance of the lines is I^2*R. The power carried by a line is V*I. Therefore by making V high and I small, a minimum of power is lost due to resistance for a given amount of power transmitted.

3. Jan 9, 2005

vincentchan

The CHANGE of current generate magnetic flux, and the CHANGE of megnetic flux generate voltage.....Steady don't generate magnetic flux therefore we need alterneting current....
remember $$power=I^2R$$ the resistance in the transmission line generate heat, which is a waste of energy, in order to minimize the power loss, we wanna keep the current $$I$$ low.. the only way to do that is make the voltages high $$power=IV$$, if the power unchange low I implies high V.

4. Jan 10, 2005

sp00ky

Oh okay, makes sense. But how do transformers actually operate?

5. Jan 10, 2005

vincentchan

6. Jan 10, 2005

sp00ky

oh thanks for that link. Is this right: The primary coil receives alternating current and the secondary coil basically stimulates the potential difference? Dunno if stimulates if the best word for it.

7. Jan 10, 2005

vincentchan

yes, basically, you r rite