Experiencing Electricity 29Experiment 4: Varying the VoltageWe want to know R, the resistance. So, we use the version of Ohm’s Law that
puts R on the left side:
R= V/INow plug in the values:
R = 3.5/0.02Run this through your pocket calculator if you find decimals confusing. The
answer is:
R = 175ΩIt so happens that 175Ω isn’t a standard value. You may have to settle for 180
or 220Ω, but that’s close enough.
Evidently the 470Ω resistor that you used in Experiment 3 was a very conserva-
tive choice. I suggested it because I said originally that you could use any LED
at all. I figured that no matter which one you picked, it should be safe with
470Ω to protect it.
Cleanup and Recycling
The dead LED can be thrown away. Everything else is reusable.
FundAmentAls
Decimals
Legendary British politician Sir Winston Churchill is famous for complain-
ing about “those damned dots.” He was referring to decimal points. Because
Churchill was Chancellor of the Exchequer at the time, and thus in charge of all
government expenditures, his difficulty with decimals was a bit of a problem.
Still, he muddled through in time-honored British fashion, and so can you.
You can also use a pocket calculator—or follow two basic rules.Doing multiplication: combine the zeros
Suppose you need to multiply 0.04 by 0.005:- Count the total number of zeros following both of the decimal points. In
this case, three zeros. - Multiply the numbers which follow the zeros. In this case, 4 × 5 = 20.
- Write down the result as 0 followed by a decimal point, followed by the
number of zeros, followed by the multiplication result. Like this: 0.00020,
which is the same as 0.0002.
Doing division: cancel the zeros
Suppose you need to divide 0.006 by 0.0002: - Shift the decimal points to the right, in both the numbers, by the same
number of steps, until both the numbers are greater than 1. In this case,
shift the point four steps in each number, so you get 60 divided by 2. - Do the division. The result in this case is 30.