# Homework Help: Voltage of electricity

1. Feb 21, 2015

### physicsnerd26

• User warned that template use is mandatory.
I'm doing a quality of measurements coursework at the moment, the coursework was to find the efficiency of a kettle, and to do this, one of the factors that I needed to find was the voltage.

So in 300ml of water, the voltage was 240.5V
400ml - 244V
500ml - 243V
600ml - 245V
700ml - 241.4V
800ml - 242.8V

I tried to graph it and it was completely scattered, I was just wondering if this would be normal, or does this mean that I did my experiment wrong?

2. Feb 21, 2015

### jbriggs444

It sounds like you are measuring the voltage provided by the wall outlet that powers the kettle. One would not expect the mains voltage to vary significantly based on the amount of water in your kettle.

3. Feb 21, 2015

### physicsnerd26

So I did it wrong? I used a plug-in power meter and recorded the voltage of each amount of water at the first minute everytime actually, not sure what went wrong there.

4. Feb 21, 2015

### jbriggs444

Voltage and power are not the same thing. Voltage is a measure of how much energy each unit of current will deliver. Unless you also measure how much current is being provided, it tells you nothing about how much power is being used.

5. Feb 21, 2015

### CWatters

That tells you there is no relationship between voltage and volume. If it had been a nice straight line that might have indicated they were related.

Your results are exactly what you would expect. The mains voltage is more or less constant but can vary moment to moment for a number of reasons unrelated to your experiment.

One option would be to calculate the average voltage and use that.

Last edited: Feb 21, 2015
6. Feb 21, 2015

### physicsnerd26

I also did the current. And the graph I made for it is just as scattered as the voltage.

7. Feb 21, 2015

### physicsnerd26

Should there be a relationship between them? I kinda get the feeling there should be, just not too sure since I'm not too familiar with the whole voltage and stuff.

8. Feb 21, 2015

### jbriggs444

Power and energy are not the same thing. Power is how much energy is delivered per unit time. Unless you also measured the time, it also tells you nothing.

Edit: I should have made it more explicit that if you multiply voltage by current you get "power".

9. Feb 21, 2015

### physicsnerd26

I measured that too, the time seemed about right, there was an increasing pattern there, my problem is the voltage and current being in a completely weird order.

10. Feb 21, 2015

### CWatters

No. In this experiment I would not expect a relationship between voltage and volume or current and volume.

11. Feb 21, 2015

### physicsnerd26

Okay, thank you.

12. Feb 21, 2015

### CWatters

It takes Energy to heat water. The more water you have to heat the more energy you need..

Energy = Power * Time
and
Power = Voltage * Current
so
Energy = Voltage * Current * Time

If Voltage and Current are constants then Time is a measure of the energy required. Try plotting Time vs Volume.

13. Feb 21, 2015

### jbriggs444

Of course if voltage and current vary slightly and randomly then a plot of energy versus volume should correlate even better.

14. Feb 21, 2015

### Staff: Mentor

I see that the course content is about quality of measurements. Presumably this means you'll be investigating things like measurement error, random errors, systematic errors, and accuracy versus precision in measurements.

Your voltage values may vary the way they do for any of several reasons. For example the device used to measure them may be flawed, exhibiting poor repeatability (precision). Or the voltage meter may only give readings accurate to 2% on the chosen scale. Or the meter may have been analog and different people made the measurements from different viewing angles of the meter. Or the voltage may actually be varying over time due to reasons entirely external to your experiment!

Whatever the reason you can try to figure out if the variations point to accuracy or precision issues in the experimental data.