Physics question about power and voltage
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Physics question about power and voltage

[From: ] [author: ] [Date: 11-05-07] [Hit: ]
953125 MWThe maximum power loss in the cable = 1.953125 MWI say maximum, because the amount of current flowing through the cable is determined by the customers! As people turn on their air conditioners, energy is required to operate the air conditioner. The electrons in the current carry the energy from the power station to the air conditioner.......

The amount of heat energy transferred, each second, from the wire to the air = Power loss

Below is the math!

P = V * I
V = I * R
P = (I * R) * I

P = I^2 * R

This equation helps you determine the magnitude of the power loss.

The power loss = 625^2 * 5 = 1,953,125 watts = 1.953125 * 10^6 watts = 1.953125 MW

The maximum power loss in the cable = 1.953125 MW


I say maximum, because the amount of current flowing through the cable is determined by the customers! As people turn on their air conditioners, energy is required to operate the air conditioner. The electrons in the current carry the energy from the power station to the air conditioner. So, more energy means more current flowing through the cable. More current produces more heat energy in the aluminum wire in the cable.


Heat energy produced = Power loss * time
Power loss = 1.953125 * 10^6 watts
1 watt = 1 joule per second.

Heat energy produced = 1.953125 * 10^6 J/s * time

In 1 second, the heat energy produced = 1.953125 * 10^6 J/s * 1s = 1.953125 * 10^6 Joules of heat energy

So, each second, 1.953125 * 10^6 Joules of electric energy is converted into 1.953125 * 10^6 Joules of heat energy


The total power from the power station = 250 MW
The total power after the power loss = 250 – 1.953125 = 248.046875 MW

This is the power that is available for the customers to use.

What is the current in the power lines? 625 amps
What is the power loss in the cable? 1.953125 MW


P = I^2 * R
When people have less appliances turned on, less current is flowing through the long distance cables. If the current = 400 amps:


Power loss = 400^2 * 5 = 8 * 10^5 watts
This means the 8 * 10^5 Joules of heat energy was produced in the wire each second.

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This is Ohms law: P = E ^ I and E = I * R and P = I2 * R.

One way to remember the 3 forms of Ohms law - Simple algebra:

_P_ and _E_ and _P_
E * I ...... I * R ..... I2 * R (Dots are for spacing only)

I use a simple method of determining this. Where
P = Power in Watts
E = EMF (Electromotive Force) in Volts
I = Current in Amperes
R = Resistance in Ohms.

Multiply going up, divide going down. So:
250 MW / 400 KV = 625 Amperes

P = 250,000,000 Watts
E = 400,000 Volts
I = 625 Amperes
R =

I*R drop in cable: E = I * R then P = E * I So:

P = 1,953,125 Watts lost.
E = 3125 Volts drop
I = 625 Amps
R = 5 Ohms
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