# Loss of energy and energy loss prevention in circuits

## Homework Statement

why is there a loss of energy in DC circuits and power loss is reduced in AC when transmitted

## The Attempt at a Solution

Loss of energy in a DC circuit is due to the resistance of the wire cauing the wire to heat up therefore energy is being lost by heat. Energy loss can be prevented in AC circuits when transmitted by using transformers to "boost" the voltage. The voltage can also be boosted in DC however it is more difficult.

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berkeman
Mentor

## Homework Statement

why is there a loss of energy in DC circuits and power loss is reduced in AC when transmitted

## The Attempt at a Solution

Loss of energy in a DC circuit is due to the resistance of the wire cauing the wire to heat up therefore energy is being lost by heat. Energy loss can be prevented in AC circuits when transmitted by using transformers to "boost" the voltage. The voltage can also be boosted in DC however it is more difficult.
That's pretty close to correct. The problem statement wasn't very explicit, but it sounds like you are talking about AC Mains power transmission over long-distance transmission lines. Can you say a bit more about why using high voltage for that energy transmission minimizes the energy losses?

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Daniel2244
CWatters
Homework Helper
Gold Member

## The Attempt at a Solution

Loss of energy in a DC circuit is due to the resistance of the wire cauing the wire to heat up therefore energy is being lost by heat.
Same happens with AC.

Energy loss can be prevented in AC circuits when transmitted by using transformers to "boost" the voltage. The voltage can also be boosted in DC however it is more difficult.
You should explain why boosting the voltage reduces the power lost.

Daniel2244
Can you say a bit more about why using high voltage for that energy transmission minimizes the energy losses?
Is it becasue by using a higher voltage there is a lower current lower current therefore less impedence?

Last edited:
berkeman
Mentor
Is it becasue by using a higher voltage there is a lower current lower current therefore less impedence?
Close! Using the higher voltage for transmission means less current flows through the resistance of the wires, so there is less voltage drop in the wires and therefore less energy loss in the wire.

Daniel2244
Close! Using the higher voltage for transmission means less current flows through the resistance of the wires, so there is less voltage drop in the wires and therefore less energy loss in the wire.
As you said earlier
it sounds like you are talking about AC Mains power transmission over long-distance transmission lines
would this also apply to circuits because I have to "Use diagrams and clear desciptions to show the differnces and similarities between DC and AC in simple electrical circuits. The Work should include loss of energy in DC circuits and power loss reduction of AC when transmitted". This baffels me as I can't think of a reason why there would be a loss of energy in DC circuits becasue the current doesn't have to travel long distances therefore the energy loss would be equal between AC and DC as AC wouldn't need stepping up. (Unless I'm overthinking) and the power lines are a circuit anyways thanks for the help :)

CWatters
Homework Helper
Gold Member
What Berkeman said.

Power loss = Current * Voltage drop
Voltage drop = Current * Resistance
so
Power loss = Current2 * Resistance

So reducing the current is very important. For the same power transmitted, if you double the voltage the current halves and the power loss goes down by factor of four.

berkeman and Daniel2244
CWatters
Homework Helper
Gold Member
would this also apply to circuits
A power line is a "circuit", just a very big one.

because I have to "Use diagrams and clear desciptions to show the differnces and similarities between DC and AC in simple electrical circuits. The Work should include loss of energy in DC circuits and power loss reduction of AC when transmitted". This baffels me as I can't think of a reason why there would be a loss of energy in DC circuits becasue the current doesn't have to travel long distances therefore the energy loss would be equal between AC and DC as AC wouldn't need stepping up. (Unless I'm overthinking) and the power lines are a circuit anyways thanks for the help :)
High Voltage DC _is_ used for some long transmission lines although it's not as common as AC.

I think they are asking you to do several things to show your overall understanding. I would answer as follows...

1) Draw two versions of a simple circuit comprising a voltage source and resistor load or light bulb. One with a DC voltage source and one with an AC voltage source. Add graphs showing the voltage and current vs time to both drawings. Describe the circuits and graphs.

2) Draw another similar circuit to represent a voltage source/power station, long wires with resistance and a load/town. Explain how power loss in the wire is calculated.

3) Explain how increasing the voltage reduces the power loss for a given power transmitted. Perhaps with the aid of another drawing with transformers both ends.

Daniel2244
CWatters
Homework Helper
Gold Member
I can't think of a reason why there would be a loss of energy in DC circuits becasue the current doesn't have to travel long distances
Some DC circuits draw a lot of current. So much that even short wires have significant losses. Consider the starter motor in a car or truck. They can draw 100's of Amps. To reduce losses in the cables they make them fat. This is possible because the cables are reasonably short. It would be expensive to make power lines very much fatter because they are a lot longer. Truck starter motors draw more current than car starters so they also use 24V batteries in trucks rather than 12V batteries to keep things under control.

Daniel2244
Some DC circuits draw a lot of current. So much that even short wires have significant losses. Consider the starter motor in a car or truck. They can draw 100's of Amps. To reduce losses in the cables they make them fat. This is possible because the cables are reasonably short. It would be expensive to make power lines very much fatter because they are a lot longer. Truck starter motors draw more current than car starters so they also use 24V batteries in trucks rather than 12V batteries to keep things under control.
Thanks for the help much appreciated.

berkeman