The Grainger College of Engineering
Physics Van

Voltage, Current, and Power Lines

Most recent answer: 10/22/2007

Q:
WHAT IS load,voltage,current,resistor. My next question is that why such a high voltage is used for transmitting the electricity in the far villages.Why we use live and neutral conductors.Kindly make the relevent useful diagrams.
- Iqra Shaheen
RAWALPINDI, Pakistan
A:

Iqra,

I will do my best to summarize these questions, but you may wish to look at a physics book for more detailed answers.

Current is related to the flow of electrons through something. The more electrons are flowing, the bigger the current is. The only catch here is that current flows in the opposite direction of electrons. For example, if electrons are moving right, current is moving left. This is because the direction of a current in a circuit is defined as the direction positive charges would move, and electrons are negative.

Voltage has to do with potential energy. The important thing to know about voltage is that it is only meaningful when you’re looking at more than one point. For example, if a bird lands on a high voltage power line, it doesn’t matter because it’s not connected to anything with a lower voltage. If, however, you stand on the ground and touch a high voltage (compared to ground) power line, you will have current flowing through you since current flows from places with higher voltages to places with lower voltages.

A resistor is an example of a load. Resistors restrict current flow through a circuit. (The electrons don’t easily get from one side to the other.)

Now onto your other question....

High voltages are used in power lines because you are trying to send power from one place to another. Power is roughly equal to the current times the voltage. That means that if you want to send a lot of power you can use:
a) high current
b) high voltage
High voltage is actually needed for to send power over long distances because if you use high current, a lot of the power is lost to the resistance in the wires. This doesn’t happen as much with high voltage since the currents are smaller!

~Ann


(published on 10/22/2007)

Follow-Up #1: power transmission

Q:
But this is in total contrast to the ohm’s law. With the resistance remaining the same, high voltage will induce high currents. How is it then the power lines transmit high voltages with low current????????
- Kalind Pathak (age 30)
Ahmedabad, Gujarat, India
A:
Excellent question, but there is a transformer involved! Although for a given set of wires, their resistance is fixed, the resistance that they get hooked up to (in series) is very different for high and low voltage transmission lines. High voltage lines have to include transformers to step the voltage down to lower values. The primary coil of the transformer (the coil which the high-voltage is connected to) has very high impedance. So for a given power delivery, a circuit with higher voltage between the wires has lower current flowing. That mean that the voltage drop along the wire is lower in the high voltage circuit.

Mike W.

Lee H

(published on 10/22/2007)

Follow-Up #2: high voltage power lines

Q:
why are high cables transporting electricity from a power station is preferred to be under high voltage?
- luci (age 18)
south africa
A:

Since power is voltage times current, high voltage lets you use lower current. That means you can use smaller wires.

Mike W.


(published on 08/16/2013)

Follow-Up #3: more on transformers and power lines

Q:
Mike your answer needs some additional explanations. Although it's true that with a smaller current you can use smaller wires, that is not the main reason why we use high voltages for transmission lines. To keep it simple, this has to do with the Joule Effect, especially via the Joule Heating. Since there is no such thing as an electrical wire with 0 resistance, when charged particles (i.e. an electric current) move inside a wire, they encounter a tiny bit of resistance. For short distances that resistance is negligeable, however over miles of distance that resistance can be significant. According to the Joule Heating effect, the formula P = I^2 X R (where P = the loss of power due to the Joule Heating, I^2 = the electrical current intensity squared and V = voltage) says that as we use higher and higher voltages, I gets smaller and smaller. So for a fixed R and a smaller I, we reduced P ; the loss of power due to the Joule Heating. It's a bit like having a better "miles per gallons" but for power lines!
- Anonymous
A:

You're quite right that the reason you need larger wires when the current is larger is to keep the Joule heating from heating up the wires too much.

Mike W.


(published on 07/07/2014)

Follow-Up #4: power limit of voltage stabilizer

Q:
1.When we use voltage stabilizer we have to take care of total watt used in our house.But why we have to calculate them.Because as per my knowledge Voltage Stabilizer is only for voltage conversion why it is related with power ? 2.In our home from pole, how much power & current is coming ?
- Neeraj Sahu (age 34)
Noida,UP,India
A:

The voltage stabilizer has to handle the current that goes out to the house. That current is the power divided by the voltage. So when you use a lot of power a lot of current is drawn from the stabilizer. The components in it heat up when a lot of current flows through them. When too much current flows, they will be damaged. For example a wire might melt and even cause a fire. A fuse or circuit breaker should be used so that the current will shut off before that happens.

Mike W.

posted without vetting until Lee returns from the Serengeti 


(published on 10/30/2014)

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