How Should Error Bars Be Placed in an Ohm's Law Lab Graph?

[fomenkoa]

Hey everyone.

I am doing an Ohm's law lab where there is a variable source, a fixed resistor, a voltmeter across the resistor, and an ammeter in series with the resistor.

The fixed resistor is an unknown value but has a tolerance of 5%

You need to figure out its value by plotting Voltage VS Current and the slope of the graph is resistance.

My question is...I need to add error bars to the graph but I don't know where to put them...the x-axis is current and the y-axis is voltage, so would I put X-error bars of 5%, or Y-error bars of 5%, or both?

My guess is that since the resistor has tolerance of 5%, and resistance depends on Voltage and current, both x and y axes need to have 5% error bars

Thanks,
Anton.

 

[jtbell]

You measured the current and voltage using two meters, right? In that case, the error bars should reflect how well you can measure each of those quantities. If they're analog meters, I usually try to interpolate to 1/10 of the smallest scale division, and use that 1/10 for the uncertainty. If they're digital meters, I usually use 1/2 of the smallest unit on the display. In either case, if the reading fluctuates visibly, I try to eyeball a +/- range.

The tolerance on the resistor shouldn't figure into the graph itself at all. Measure the slope of the graph to estimate the resistance, and then compare that with the value marked on the resistor. If they're closer than the tolerance, you're OK.

[added: If you really don't know the "actual" value of the resistance, or at least aren't supposed to know, then I don't see how the tolerance could come into this at all.]

 

[thepatientmental]

Hello Anton,

You are correct in thinking that both the x and y axes should have 5% error bars. This is because the tolerance of the resistor affects both the voltage and current measurements, which in turn affect the calculated resistance. Adding error bars to both axes will give a more accurate representation of the potential range of values for the resistance.

When adding the error bars, make sure to include both the positive and negative values. For example, if the measured voltage is 5V with a 5% error, the error bar would extend from 4.75V to 5.25V. This will show the potential range of values for the voltage measurement.

I hope this helps and good luck with your lab!