#13-Electric Power and Current 
  
Hello.  Welcome back.  


Today we're going to do a fairly simple and straightforward experiment using electricity.  


We want to look at some common 
household appliances and 


determine the power and the current that 
flows when the appliances are turned on.  


Now, power is important to us 
because it determines energy 


and consequently, the amount of money that you're going to spend when you turn on the appliance.  


We define power as the rate of energy transfer.  


So pretty obviously, a high power rating 
means energy is being transferred quickly 


which means that your bill is going to go up.  


Current, on the other hand, is defined 
as the rate of charged movement.  


Current is important to us for safety reasons. 
When too high a current is flowing, 


electrical wires heat up and that can lead to melted insulation, to sparks, and to fires in the home.  


And also remember that it's current which kills 
you, so whenever you're using electric devices 


you want to be very very careful.  


We'd ask you to inspect each appliance 
that you're using during the experiment.  


Check the cords.  Check the plugs.  Make sure they're not frayed at all and they look sound and substantial.  


If not, ask for help from the laboratory assistant.  We don't want you to get hurt during the course of the experiment.  


Let's begin then.  Your first step is to select 
four appliances that you're going to use.  


There are a number of them provided 
for you in the science center.  


We've got for example an electric 
toaster, we've got hot plates, 


we've got a number of different things 
for you to choose.  So pick four.  


If you look in your experiment sheet in your laboratory manual you'll find two tables.  


The first table, first column gives you 
a place to record the four appliances 


so write down the names of the 
appliances that you're going to use.  


Here's a little hint:  If you're going to use the toaster as one of your appliances put that one last.  


Because as you know the toaster doesn't stay 
on for a long time.  It will turn itself off.  


With the four appliances listed by name, you want 
to, in the next column, record the power rating 


as listed by the manufacturer.  Each 
appliance should have the power rating.  


You may have to search around.  You may have to look on the bottom, for example.  Find the power rating.  


Write that down next to the corresponding appliance.  


If you can't find a power rating, again, 
ask for help from the laboratory assistant.  


You want to, in the second column of your data 
table, record the rated power for each appliance, 


the one that the manufacturer gives to us.  
After that we're going to make a comparison.  


We're going to see what the actual 
power is for each of the appliances 


and to do that we use a device called a watt meter because power is measured as energy over time 


or joules per second which are called watts. 
we use a device called a watt meter.  


And the one that we've got for you is called 
"Watts Up?"  ...A little play on words.  


As soon as you plug it in you should 
get a reading on the dial right here 


but I want you to pay particular attention to the little mark listed over here in the column by the side.  


There are three positions, a dollar 
sign, KW for kilowatt, and HR for hour.  


You want that little indicater 
next to the KW position.  


So using the mode button, simply push the button enough times until the little black bar... 


that's not a minus sign, it's an indicater... 
is next to the KW position.  


There's no On/Off switch for the What's Up meter.  Simply plug it into an outlet and you're ready to go.  


What I'm going to do now is to plug in an appliance  and show you how to read the What's Up meter.  


So we take the power cord of an appliance, plug that in, and right away you see that we get a reading.  


Now the reading changes a little bit as the appliance heats up but it will stabilize after a bit.  


Notice here that the reading is in kilowatts.  


We want to move the decimal point three 
places to the right to change it to watts.  


So that there particular reading is not
.619 but rather 619... right now 618.  


We'll record that value down in 
the sheet next to the appliance.  


Do that for each of the four appliances 
that you're going to use.  


Another hint on safety:  Remember when 
you pull out a plug grab it by the plug.  


We don't want you grabbing ahold of the 
cord and yanking it out.  That's not safe.  


Once you've recorded the actual power 
readings for the four appliances 


you're done with the What's Up? 
meter and you can set it aside.  


The thing that we want you to see is how well actual power readings correspond with manufacturer power readings.  


That takes care of power.  Now we're 
interested in electric current.  


As you'll have seen in class and also explained in your laboratory manual we can determine the current 



flowing through an appliance if we know the power and the supply, potential difference or voltage.  


There's an equation that we'd like you to use saying that current is equal to power divided by voltage.  


                 I = P divided by V  



We're going to use the power ratings that we measured... the actual power consumptions... 


and divide those by the voltage or potential difference which is being supplied.  


Now, that varies across the United States.  
Generally it's between 110 and 120 volts 


and we'd like you to use 120 
volts for this experiment.  


So in the fourth column of your response 
sheet, divide the measured power by 120 


to get the theoretical current which 
flows through each appliance.  


Those values should be rounded to the nearest tenths or one decimal place.  So for example, 4.7 or 10.3


Don't carry out the divisions to 
many many significant digits.  


That tells you what current should 
be flowing through the appliance.  


Then we're going to measure the 
actual currents that flow.  


We're going to do this in terms of plugging 
more than one appliance in at a time.  


Our homes are connected in a parallel connection.  


What that means is that every time you plug in another appliance, more current is drawn from the source 


and the total current in the circuit increases.  


What we want to do first before we plug 
anything in, is total up our currents.  


You're going to take the values of theoretical current that you have in the fourth column of Data Table 1 


and transfer them down to Data 
Table 2.  But this time adding.  


Your first appliance current goes at the top 
of the table. Add to that the second current 


so that you get the first and the second 
together and record that in the second space.  


Then add the third theoretical current.  Get 
that total and record it in the third place.  


Finally, add all four theoretical currents and put the total in.  That total should remain less than 25 amps.  


25 amps is a substantial amount of 
current. We don't want to exceed that.  


In fact, you can do a quick check as 
you're writing down your power values.  


If you add up all the powers and you get more 
than 2,500 watts, that's probably too much.  


At this point now we've got our currents, at least the theoretical ones which are flowing through our system.  


We want to measure actual currents.  To 
do that we use a device called an ammeter.  


Current is measured in amps and the device that we 
use is an ammeter.  And I've got one here for you.  


Now, this ammeter does go up to 25 amps 


and if we get a look at the face there's 
a couple of things that I want to show you.  


First we want to check and make sure that the 
needle is zeroed when nothing is plugged in.  


That is to say, on the face right here, with 
no appliances plugged in, we have 0 current.  


There's a little adjustment screw on the face 
right here that can be used to align the needle 


but let me warn you that the meter is very 
fragile, so if you have any questions at all 


you again ought to ask for some help.  


If we look at reading the meter you can see the 
big numbers across... 5, 10, 15, and so on.  


In between 0 and 5 we've got five divisions.  
Each major division is one amp and then 


in-between, the small divisions, are a half an amp.  


So you ought to be able to record your 
values to the nearest half of an amp.  


Well, this device is plugged in because 
we want the currents flowing through it.  


So I've got that plugged into the mains.  


It's a substantial heavy-duty cord so 
that we can safely carry the currents.  


As we're going to plug in more than one appliance at a time we want to use a device that allows us to do that 


and we've got a power strip here.  


Now, the power strip is a safe way to 
plug in several appliances at once.  


We don't want to use the little multiple connecters that you find in the dimestore.  Those aren't safe.  


If we've got several appliances we 
want to get a power strip like this.  


That power strip needs to be plugged into the 
ammeter. And so we'll bring up the power cord.  


That plugs into the side of the box 
over here.  We get that plugged in 


and this particular device has an On/Off switch.  


We'll turn that on and that should give us an indicater light saying that the power strip is hot.  


What we're going to do now is simply 
plug in each of the appliances.  


Let's use a hot plate as our first appliance 
here.  We'll plug it into the power strip.  


And as we do, our ammeter comes on telling us that 
in fact the hot plate was left on by someone.  


That's something you don't want to do.  


I've turned it off now.  As we turn it 
on, we should see the current pick up.  


Go ahead and turn that all the way up to the high setting. That looks to me like it's just about 5 amps.  


Maybe we want to record that as 4.9 or 5.0 and that's the current flowing with one appliance connected.  


Let's go on to our second appliance now.  


We've got another kind of heater.  This is quite an old fashioned one.  Looks like it's on its last legs.  


This has no On/Off switch so I'd 
expect that when I plug it in 


I would get an increase in the current 
just as soon as it's plugged in.  


And sure enough...



We've got two appliances and as I mentioned 
before, when we're connected in parallel 


each time we add an appliance 
the total current goes up.  


Another appliance that I've brought 
along today is a heat lamp. 


We've chosen mostly heat producing devices because heat producing device generally have quite high currents.  


If we plug this one in nothing happens of course because our heat lamp is not turned on.  



If we do turn it on... and I'll keep this away from the camera because it puts out quite a bit of light.  


As soon as we turn it on we see 
another increase in the current. 


We're going to record that value in our 
third position under the actual currents.  


Finally our fourth appliance.  I'll go ahead and 
use the toaster here.  And we'll plug that in.  


Nothing happens of course when we first 
plug it in because it's not turned on.  


But as soon as we press down the plunger 
on the toaster there goes the current. 


So we're going to record that value as well. 
You notice that it flicked off right there.  


That's probably because we exceeded the current limitations so you want to be careful as you do this.  


Once you've recorded the four currents 
with each of the appliances connected, 


you're done with the measurements for this part. We want to go ahead and disconnect each of the appliances.  

Notice again that I'm using the plugs to do that.  Pull these out.  Don't yank them out by the cords. 


We want to remember to turn off each of the appliances, and we're done with the major part of the experiment.  


We've totalled up the theoretical currents and 
then we've measured the actual currents.  


And at this point you want to make a comparison.  
How well do your totals compare.  


If they compare well we want you to say so.  
If there are differences, record those.  


Remember that small differences are 
expected whenever we make measurements.  


And so your task is to use your 
judgment and decide whether 


the measured currents and the theoretical currents compare closely or if there are major differences.  


And then we'd like you to speculate on 
why the differences might have occurred 


so write that down on your response sheet as well.  


Let me get rid of some of these appliances, and then we can look at the last part of the experiment.  


In the last part of the experiments we 
ask you to look at a three way lamp.  


A three way lamp normally is 
rated with three power ratings.  


Generally 50 watts, 100 watts, and 150 watts.  


I'd like you to look at the lightbulb 
you have and write those down.  


Then we're going to plug in our three way lightbulb.  And we're going to test the currents that flow.  


First of all, determine the theoretical 
currents. You know how to do that already.  


Divide the power rating by the voltage.  So take our 50 watts and the divide it by 120 volts and write that down.  


We'd expect that as the power rating 
goes up, the currents should go up.  


So write down the three theoretical currents 
that you'd expect to get from the lightbulb.  


So go ahead and turn on the lightbulb... 
I won't do that for you now... 


and record the actual current flowing 
at 50 watts, the lowest setting.  


Do it again at the middle setting.  
And do it again at the highest setting.  


Again we're looking at how well theoretical 
currents compare with actual measured currents.  


The differences that you find might be attributable 
to our device right here, our measuring device, the ammeter.  


If the ammeter is not perfectly accurate 
of course our results will not be accurate.  


So think about things that could cause what we consider to be experimental error in the experiment.  


Once you've completed that you're all but done.  


There is one more thing though that we 
ask you to think about and that's safety.  


Again I've mentioned looking at the power cords and the plugs to make sure that they're substantial.  


And certainly you know not to stick 
your finger into electrical outlets.  


But when I talk about safety here we're 
talking about fuses and circuit breakers.  



Why do we have fuses and circuit 
breakers in our houses?  


Well, these are devices which are current-limiting devices and if you want to know more about them... 


and of course you do... attend your class, your instructors is going to talk about them there.  


Well, that's all we have today.  
This is a simple experiment.  


You should be in and out of the 
lab in about thirty minutes.  


Again, please be careful.  We don't want 
you to get hurt while you're in the lab.  


Show your response sheets to the lab assistants and always ask for help if you have any questions.  


Until next time, that's all for today... Bye for now.