## Units, you nit!

The S.I. System of Units is a thing of beauty: a lean, sinewy and utilitarian beauty that is the work of many committees, true; but in spite of that common saw about ‘a camel being a horse designed by a committee’, the S.I. System is truly a thing of rigorous beauty nonetheless.

Even the pedestrian Wikipedia entry on the 2019 Redefinition of the S.I. System reads like a lost episode from Homer’s Odyssey. As Odysseus tied himself to the mast of his ship to avoid the irresistible lure of the Sirens, so in 2019 the S.I, System tied itself to the values of a select number of universal physical constants to remove the last vestiges of merely human artifacts such as the now obsolete International Prototype Kilogram.

However, the austere beauty of the S.I. System is not always recognised by our students at GCSE or A-level. ‘Units, you nit!!!’ is a comment that physics teachers have scrawled on student work from time immemorial with varying degrees of disbelief, rage or despair at errors of omission (e.g. not including the unit with a final answer); errors of imprecision (e.g. writing ‘j’ instead of ‘J’ for ‘joule — unforgivable!); or errors of commission (e.g. changing kilograms into grams when the kilogram is the base unit, not the gram — barbarous!).

The saddest occasion for writing ‘Units, you nit!’ at least in my opinion, is when a student has incorrectly converted a prefix: for example, changing millijoules into joules by multiplying by one thousand rather than dividing by one thousand so that a student writes that 5.6 mJ = 5600 J.

This odd little issue can affect students from across the attainment range, so I have developed a procedure to deal with it which is loosely based on the Singapore Bar Model.

One millijoule is a teeny tiny amount of energy, so when we convert it joules it is only a small portion of one whole joule. So to convert mJ to J we divide by 1000.

One joule is a much larger quantity of energy than one millijoule, so when we convert joules to millijoules we multiply by one thousand because we need one thousand millijoules for each single joule.

In time, and if needed, you can move to a simplified version to remind students.

Strangely, one of the unit conversions that some students find most difficult in the context of calculations is time: for example, hours into seconds. A diagram similar to the one below can help students over this ‘hump’.

These diagrams may seem trivial, but we must beware of ‘the Curse of Knowledge’: just because we find these conversions easy (and, to be fair, so do many students) that does not mean that all students find them so.

The conversions that students may be asked to do from memory are listed below (in the context of amperes).

### 6 thoughts on “Units, you nit!”

1. Caseby's Casebook March 14, 2022 / 10:14 am

Forty years later I can still remember my physics teacher saying to us ‘150 what? Pink elephants?!!! Units!!!’!

2. Miss Ironfist March 14, 2022 / 2:30 pm

I love the pun and I love you. How come you never utter good puns in my presence? ILYF xxxx

3. Anthony Clowser March 19, 2022 / 9:29 pm

Important stuff, as is being precise with the multipliers. A couple of years ago at the 6th form ball the A level physicists and I were intrigued by the solvency of the (local) spring water at the table. All the concentrations of dissolved ions were quoted as Mg/l…. No one else on the table could see the problem!!

• e=mc2andallthat March 20, 2022 / 12:01 pm

OMG 1000 kg per cubic decimetre (!) Lower range of density of matter in a white dwarf is about 10 000 kg/dm^3 !