Ohm's Law verification

Download ohm.ddw

Set up a resistor with a means of varying the Potential Difference from a smooth power source; connect current and PD sensors and Ohm's law is easily verified!

It is generally best to use a "spacebar" recording - you press the spacebar (or click the record button) every time you want to record a point on the graph. It is possible to set up a time-based recording and vary the PD smoothly but generally, this is more trouble to no advantage. With a spacebar recording you can go back, fill in gaps in the graph, and even reject points recorded too hastily before readings had settled.

Results like this are easily achievable but there are pitfalls and not everybody gets such nice results:

Note that above we said "a smooth power source". School general-purpose power supplies are not smooth! This seems to be a historical accident going back to the days when it didn't seem important and smoothing was expensive.

Unfortunately a sensor will record accurately the actual PD and current in an instant - they will not smear out the variations like bench meters used to. All too easily, what you get is a seemingly random set of results because you are actually playing a lottery on the instantaneous values. (For those of you who are really thinking about this it also reveals a common fault of A/D converters - they do not read each channel at exactly the same time!

Use a (rechargeable) battery supply or a smooth DC one.

Actually this use of non-smoothed supplies has other detrimental effects on school science investigations because the "averaged" values displayed by bench meters are not the true RMS (Root Mean Square - it means getting the true "average" for electrical measurements) ones so calculations of heating effects can easily be 20% or more wrong. (Calorimetry experimenters beware.)

Note that the Gradient cursor on the Graph menu will indicate the resistance value (slope of the graph). We have also used Fit function on the Data menu to draw the best straight line through the points.

See our similar experiment with a bulb to see that the rule is not as simple as it seems - we are surrounded these days by devices that depend for their working on their resistance changing with voltage etc. Tungsten filaments are so successful partly because the rising resistance has a self-limiting effect. The earlier Carbon filament was just the opposite and could easily "run away with itself".

Of course, Ohm's law is good and we wouldn't be where we are technologically without it but the full story is rarely simple.