Ohms Law and Power

[joey7]

Just wondering if someone could try explain this to me:

If you decrease voltage with a constant power I must increase right?
but according to Ohms law if the resistance is constant if V decreases so must I?

[Rish_007]

would the resistor then be non-ohmic?

[lzxnl]

Just wondering if someone could try explain this to me:

If you decrease voltage with a constant power I must increase right?
but according to Ohms law if the resistance is constant if V decreases so must I?

You can't decrease the voltage and keep the power constant for an ohmic device, precisely because of the dilemma you've proposed.
For an ohmic resistor, the current flow is directly proportional to the energy drop per charge across the resistor. If you change the energy drop per unit charge across the resistor, the current flow WILL decrease. So will the power.

Alternatively, P=V^2/R. If V changes, P MUST change. Unless it's non-ohmic. So yeah.

[joey7]

So does that mean that in questions with transmission lines and power drop across those lines, the power lines resistance is non ohmic?

[lzxnl]

Huh? There's nothing wrong with power transmission line questions...

[joey7]

Say you increase the voltage of transmission, the Current then decreases right?

[SocialRhubarb]

The voltage drop across the transmission lines actually decreases.

They may be carrying 10,000 V, but if at the end of the lines the voltage is still 9,999 V, the voltage drop across the lines is still only 1 V.

[joey7]

Oh yeah I understand, so in a question where you had to explain how increasing the voltage of transmission decreased power loss how would you word it? I wrote that with constant power if V increases I must decrease according to the equation P=VI, is that wrong?

[SocialRhubarb]

Probably easiest to justify using , and talk about how increasing voltage decreases current and hence power loss.

[lzxnl]

I think you're confusing two concepts.
The voltage of transmission is the voltage difference between the two wires, at the start and the end of the circuit. The voltage lost across the transmission wires is a different quantity.

Think of the entire transmission as a series circuit with a weird transformer in the middle and think of the power source as just a voltage source.
Then, the potential difference over the voltage source is constant. The voltage loss, however, is the voltage over the wires, which can be seen as a resistor in themselves. It's not the same voltage as the voltage across the transformer; that voltage is also something different. There are two voltages depending on which coil you're looking at, and those voltages are the potential differences between each half of the same coil. So really, when we get to a transformer, it's as if we have a new circuit with a new voltage source.
As you know, when the transformer voltage goes up, the current drops. The current is really what determines the voltage. Try not to think of electric transmission questions in terms of voltages, because otherwise you'll confuse the various voltages. Just think of it in terms of current like what SocialRhubarb said. There is nothing trippy about the currents, except that they increase as the voltages decrease (across the transformer and NOT the wires! Those voltage drops are still just IR). Increase voltage across transformer => drop the current => drop the voltage loss across the power lines.