What is it that flows in an electric circuit? Why are there
9 volts in a small transistor radio battery and only 12 volts in a huge
car battery? What is the difference between volts and amps.
We will try to answer these and other questions as well as provide experiments
to illustrate our points.
If we were to disassemble a 9 volt battery we would find it is made from
6 cells each of which develops 1.5 volts. They all add up to
9 volts. With car batteries there are six cells each producing
about 2 volts to add to 12 volts. In almost all batteries the cells
are wired in series so the voltages add.
If we also disassembled a christmas tree light string we would find it
is made of 2 banks of 50 bulbs each. These 2 banks are connected in
parallel. Each bulb operates at 120 ( volts) / 50(bulbs)
or 2.4 volts. These bulbs will light at 1.5 volts.
Figure 1 Diagram of a 100 bulb 110 volt christmas tree light
But to get back to our main point. What is the difference between
volts and amps. The easiest way to understand this is to compare a
electrical circuit producing power to a water hose. We
don't usually use water hoses to produce power but we could if we wanted
to. All that would be required is a water wheel that could change
water power to mechanical power. The important point here is how much
water would be required to perform some task. Suppose we had
a water wheel connected to a generator which was connected to a light
bulb. Now we need water to run it. So we go
out and get a 5 gallon bucket of water. Will that work.
No. We need pressure. So we go out and get a water pistol.
It squirts a stream of water 10 feet up. A fair amount of pressure.
Will that work? No The reason it will not work is while
the pressure is ok the quantity of water or the flow is too small.
Once we understand that we need both pressure and a large flow of water
we connect a garden hose to our generator. This works.
The key point here is that the energy we are putting into the generator is
not only pressure like the water pistol had or only flow like we had in the
bucket. It is the product of pressure and flow. Indeed
if we want a lot of power a fire hose is the best thing for delivering
a high pressure as well as a large flow.
Now back to electrical circuits. If we connect the light
bulb to a very high voltage source perhaps a spark plug wire on a running
engine it will not light. The spark plug has lots of voltage
( a few thousand volts) but very little current .
if we connect it to a current supply like a soldering gun that produces
many amps but very small voltage it will also not light.
Just like the water powered generator that needed both pressure and flow we
need both volts and amps to operate our light bulb.
Voltage is like pressure. It pushes the electrons through the wire.
Amps or current is like water flowing. It is a measure of
quantity per second. You can tell circuits that carry a
lot of current by looking at the size of the wire just as you can look at
the size of a pipe. A car starter motor uses 12 volts at
a about 500 amps or so. This is a lot of current.
That is why the wires to a car battery are so big.
In the following experiments we will connect various batteries to various
light bulbs to demonstrated series and parallel circuits and measure amps
and volts. Note that at no time will we use the line voltage
in houses. All these experiments use batteries.
A series string of christmas tree lights with the plug and receptacle
One nine volt battery
A nine volt battery clip with two wires attached. ( don't let the wires
touch each other)
One size D battery ( properly called a cell) volts =
6 wire nuts to fit above wire
One roll of electrical tape
One wire stripper
The above parts are commonly available The multimeter varies from
3 to 100 dollars. I used one from Jameco which cost about $20.
It is aviable HERE costs about
$19 and is priduct number 645618.
1 ) Preparing the battery.
Cut 2 six inch lengths lengths of wire from the string.
Strip one end of each wire. place one end on top of the battery and
tape it in place. Place the other end of the wire against the bottom
of the battery and tape it in place. Add more tape until both
wires are secure. Now strip the remaining ends. Don't let
them touch each other as it will pull large currents from the battery and
2) Lets make a circuit.
With the wire stripper cut out a single bulb
with 2 long wires attached. ( there may be two single
bulbs with 3 wires attached. We will ignore these.
Now strip the ends of each wire and connect one wire to a wire from
the flashlight cell and the other to the remaining wire from the cell.
The light should light. This is an electrical circuit.
Notice that it is necessary to have a loop and electricity flows from the
battery through the bulb and back to the battery.
3) Lets measure the current.
Turn on the meter. The meter can be damaged if it is not used
correctly. Always double check the settings before connecting anything
to the meter. Set the range scale to 10 amps and plug the wires
into the 10A connector and the common connector.
Now disconnect one pair of wires and connect one meter lead to one wire
and one to the other. The meter should read about .07 amps.
Note that we are not using the 200 milliamps scale because the internal resistance
of the meter affects the readings.
4) Lets make a parallel circuit.
Cut and strip a second bulb and wire it in parallel
with the first
5) Now connect it to the battery with the meter in the circuit.
Note that it still lights but the current is now .15 amps. With
parallel circuits the total current is the sum if the individual circuits.
6) Lets add a third bulb.
Cut and strip a third bulb and connect it in series to the
existing two. note that the current is now .22 amps.
7) lets get more bulbs and connect 6 bulbs in parallel. when
we connect them up they all light and the meter reads .4 amps.
Parallel circuits are very important. Many of the lamps in
your house and car are wired in parallel.
8) Now lets make a series circuit. for this we
cut 6 bulbs all in series as shown in the picture.
9) If we try to light them they will not light.
1.5 volts is not enough voltage to light all of them. Lets find
out why this does not work. strip some of the insulation from between
the first and second bulb. Now set the meter to 2 volts
and move the leads to the proper terminals. Now connect
the meter across the first bulb
10. now we see why the bulbs do not light. There is only .2
volts across each bulb. We need about 1.5 volts to light them.
11) Now is the time to switch to the 9 volt battery.
Be sure to change the meter back to 10 amps by changing both the leads and
the range switch. What happens. With our new 9 volt source the
bulbs all light. and the current through the string is .06 amps.
This same .06 amps flows through all the bulbs.
12) Now lets remove the meter re-plug it and set the range switch for
2 volts, and measure the voltage across the first bulb.
It is up to 1.27 volts.
1) When we measure volts we set the meter to volts, light
the string and put the meter across a single bulb.
2) When we measure current we set the meter to Amps break the circuit
and put the meter in the loop.
3) There are 2 ways to light 6 bulbs. The first is with
one 1.5 volt battery and 6 bulbs in parallel. The second is with 6
bulbs in series and a 9 volt battery.
About that car battery. So why is a nine volt battery
so small and a car battery so big. The car battery is
big because it needs to provide a lot of power to start a car engine but
as the power is volts times amps it gets it's high power not from a lot
of volts but from a lot of amps it can deliver over 500 amps while a nine
volt battery is made to deliver a fraction of 1 amp.
OTHER COMMENTS ON LIGHT BULBS
The resistance of light bulbs is non linear. this means that if
1.5 volts causes a current of 70 ma then .15 volts will not cause a current
of 8 ma. It will be something less that that. this is because
the resistance changes as the filament heats up. The company
Hewlett Packard was started by selling an audio oscillator. At that
time it was difficult to make a audio oscillator with a constant amplitude
output at different frequencies. HP solved the problem
by including a light bulb in the circuit. The non linear resistance
of the bulb is what enabled their audio oscillator to maintain a constant
amplitude over a range of output frequencies.