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 string.


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.  



Equipment needed:

A series string of christmas tree lights with the plug and receptacle  cut off.  

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  = 1.5

One multimeter  

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 drain it.     



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.  

SERIES CIRCUITS  

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.  




SUMMARY  
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.