I = V/R
Plug in the numbers:
I = 9/50,000 = 0.00018 amps
Move the decimal point three places to convert to milliamps:
I = 0.18 mA
That’s a tiny current that will not produce much heat at 9 volts.
What about when you shorted out the battery? How much current made the wires get hot? Well, suppose the wires had a resistance of 0.1 ohms (probably it’s less, but I’ll start with 0.1 as a guess). Write down what we know:
V = 1.5
R = 0.1
Once again we’re trying to find I, the current, so we use:
I = V/R
Plug in the numbers:
I = 1.5/0.1 = 15 amps
That’s 100,000 times the current that may have passed through your tongue, which would have generated much more heat, even though the voltage was lower.
Could that tiny little battery really pump out 15 amps? Remember that the battery got hot, as well as the wire. This tells us that the electrons may have met some resistance inside the battery, as well as in the wire. (Otherwise, where else did the heat come from?) Normally we can forget about the internal resistance of a battery, because it’s so low. But at high currents, it becomes a factor.
I was reluctant to short-circuit the battery through a meter, to try to measure the current. My meter will fry if the current is greater than 10A. However I did try putting other fuses into the circuit, to see whether they would blow. When I tried a 10A fuse, it did not melt. Therefore, for the brand of battery I used, I’m fairly sure that the current in the short circuit was under 10A, but I know it was over 3A, because the 3A fuse blew right away.
The internal resistance of the 1.5-volt battery prevented the current in the short circuit from getting too high. This is why I cautioned against using a larger battery (especially a car battery). Larger batteries have a much lower internal resistance, allowing dangerously high currents which generate explosive amounts of heat. A car battery is designed to deliver literally hundreds of amps when it turns a starter motor. That’s quite enough current to melt wires and cause nasty burns. In fact, you can weld metal using a car battery.
Lithium batteries also have low internal resistance, making them very dangerous when they’re shorted out. High current can be just as dangerous as high voltage.
Fundamentals
Watt basics
So far I haven’t mentioned a unit that everyone is familiar with: watts.
A watt is a unit of work. Engineers have their own definition of work—they say that work is done when a person, an animal, or a machine pushes something to overcome mechanical resistance. Examples would be a steam engine pulling a train on a level track (overcoming friction and air resistance) or a person walking upstairs (overcoming the force of gravity).
When electrons push their way through a circuit, they are overcoming a kind of resistance, and so they are doing work, which can be measured in watts. The definition is easy:
watts = volts × amps
Or, using the symbols customarily assigned, these three formulas all mean the same thing:
W = V × I
V = W/I
I = W/V
Watts can be preceded with an “m,” for “milli,” just like volts:
Number of watts
Usually expressed as
Abbreviated as
0.001 watts
1 milliwatt
1mW
0.01 watts
10 milliwatts
10 mW
0.1 watts
100 milliwatts
100 mW
1 watt
1,000 milliwatts
1W
Because power stations, solar installations, and wind farms deal with much larger numbers, you may also see references to kilowatts (using letter K) and megawatts (with a capital M, not to be confused with the lowercase m used to define milliwatts):
Number of watts
Usually expressed as
Abbreviated as
1,000 watts
1 kilowatt
1 KW
1,000,000 watts
1 megawatt
1 MW
Lightbulbs are calibrated in watts. So are stereo systems. The watt is named after James Watt, inventor of the steam engine. Incidentally, watts can be converted to horsepower, and vice versa.
Theory
Power assessments
I mentioned earlier that resistors are commonly rated as being capable of dealing with 1/4 watt, 1/2 watt, 1 watt, and so on. I suggested that you should buy resistors of 1/4 watt or higher. How did I know this?
Go back to the LED circuit. Remember we wanted the resistor to drop the voltage by 3.5 volts, at a current of 20 mA. How many watts of power would this impose on the resistor?
Write down what you know:
V = 3.5 (the voltage drop
imposed by the resistor)
I = 20mA = 0.02 amps
(the current flowing through the resistor)
We want to know W, so we use this version of the formula:
W = V × I
Plug in the values:
W = 3.5 × 0.02 = 0.07 watts (the power being dissipated by the resistor)
Because 1/4 watt is 0.25 watts, obviously a 1/4 watt resistor will have about four times the necessary capacity. In fact you could have used a 1/8 watt resistor, but in future experiments we may need resistors that can handle 1/4 watt, and there’s no penalty for using a resistor that is rated for more watts than will actually pass through it.
Experiment 5: Let’s Make a Battery
Long ago, before web surfing, file sharing, or cell phones, kids were so horribly deprived that they tried to amuse themselves with kitchen-table experiments such as making a primitive battery by pushing a nail and a penny into a lemon. Hard to believe, perhaps, but true!
This is seriously old-school—but I want you to try it anyway, because anyone who wants to get a feel for electricity should see how easy it is to extract it from everyday objects around us. Plus, if you use enough lemons, you just might generate enough voltage to power an LED.
The basic components of a battery are two metal electrodes immersed in an electrolyte. I won’t define these terms here (they’re explained in the following section “Theory: The nature of electricity”). Right now all you need to know is that lemon juice will be your electrolyte, and copper and zinc will be your electrodes. A penny provides the necessary copper, as long as
