Amongst the myriad inconveniences and troubles of a Physics teacher’s life, the choice of the symbols commonly used to represent voltage, current and resistance, must surely rank in the top ten.

**V is for voltage in volts, V**

Well, OK, that’s sensible enough. On a good day, I may even remember to call it “potential difference”. The sage advice of “Never use two words when one will do” is widely accepted. However, as a profession Physics teachers have decided to go it alone and completely ignore it. One can only hope that everyone got the memo.

**R is for resistance in ohms, Ω**

R for resistance? That’s fairly sensible too.

“But what’s that weird squiggly thing, Sir?”

“Ah, you mean the Greek letter *omega*? Because Physics is soooo enormous that the measly 26 letters of the Latin alphabet ain’t big enough for it…”

**I is for current in amps, A**

“WTφ? Are you taking the πΣΣ, Sir?”

“I know, I know! Look, if it helps, think of it as short for *intensité du courant *. . . Wait, don’t leave! Stop, I have many more fun Physics facts to teach you! Look, here’s a picture of Richard Feynman playing his bongo drums — nooooooooo!”

**Ohm’s Law: or is it more a sort of guideline?**

Let’s start with a brief statement of Ohm’s Law:

*V = I R*

Except, that’s ** not** Ohm’s Law; it’s actually the definition of resistance:

*R = V / I*

There is not a single instance where it is not true *by definition*. The value of resistance will always be equal to the ratio of the potential difference and the current.

Think of it like this. At room temperature, 1 V of potential difference can push (say) 0.5 A of current through the wire in a filament bulb. (I just love that retro 1890s tech, don’t you?)

This means it has a resistance of 1/0.5 = 2 ohms. However, bump up the potential difference to 6 V and the current is (say) 0.75 A. This means that is has a resistance of 6/0.75 = 8 ohms. Its resistance has *changed* because it has become hotter. In other words, its resistance is not constant.

Ohm’s Law is perhaps most simply stated as:

The potential difference is

directly proportionalto the current over a range of physical conditions (including temperature).

Using standard symbols:

*V *α* I*

or, taking *R’ *as a constant of proportionality:

*V = I R’*

You do see the difference, don’t you?

In the first example, R is not a constant value for a given range of physical conditions: for example it can get higher as the temperature increases.

In the second, R’ is *constant* over a range of temperatures and other physical conditions.

And so there we have it: V=IR can be a perfectly valid statement of Ohm’s Law, provided it is specified that R is constant. If one does not do that, then all bets are off…

In the meantime, here’s another picture of Richard Feynman playing the bongo drums. Enjoy!