Lab 7, sample answers

As you are working through lab #7, either in the lab or in Croc physics, you should pay attention to WHAT you are measuring and seeing.
Here are some sample measurements that you probably won't get, but if you look through here you should be able to understand the way that the voltage drops across the circuit and the current works its way through the circuit.
The brightness of a given bulb can essentially be measured in Watts (power usage), which you can find by multiplying the Voltage across a bulb and the current through that bulb.

Power and current and Voltage, oh my!

The block Wed/Thurs gives us some time to straighten out some of the different things we are learning about, as well as time to work on the labs.

Current activities:
* Labs up through #7,
* Energy Audit
* P=IV, V=IR homework
* Croc Physics Extra Credit


The quiz will be on Monday. The focus of the quiz will be the lab couple of electricity labs, and the Voltage homework (potential difference). For this quiz, you need to be able to reason through which light bulbs will be bright, and why. This involves understanding how resistance prevents the flow of electricity, how current can split to different pathways in a parallel branch, and how voltage "drops" across light bulbs as electricity goes around the circuit.


Voltage can be thought of as the "elevation" on Chutes and Ladders, batteries would be considered the ladders, and light bulbs would be the slides. Notice that some slides go from very high to very low, while others only make a small change. This would correspond to light bulbs that would be bright (large voltage change, thus a large current rushing through the bulb, providing tons of energy each second).

Energy and Power

Reminder that the Energy Audit, which I gave you two weeks ago, is due Friday 10/27.
This means that you need to keep track of all the things that you use in your house for a 24 hour period.
AND you need to examine an electrical bill, and your electric meter
Learn about reading the meter here:
http://www.dom.com/customer/vares_meter.jsp
(the image to the right has a reading of 01074 )

And here's a copy of my electric bill (an overestimation from a few months ago)
There are several factors to determining the total cost.
One is the actual cost divided by the total kWh used.
What dominion power prefers to do is take the "supply service" charge divided by the total kWh used. This way they can ignore the distribution service (which is also based on the amount used).

I used 2348 kWh of electricity in 32 days, and it costs me $174.96.
Thus, the cost was about 7.4 cents per kWh, and I used 73.4 kWh of electricity per day.
That is equivalent to having about 50 lightbulbs (60 Watts) turned on continuously.
OR, two space heaters running nonstop (1500 Watts)

There is also a homework assignment for resistance/power equation stuff coming up.

Static Electricity lab

Monday we did a short lab on static electricity.
We rubbed a plastic rod on a rabbit fur and examined the charge with an electroscope.
Here's a "virtual lab" demonstration of what we were seeing.
http://www.shep.net/resources/curricular/physics/P30/Unit2/electroscope.html

here's another, similar one, showing attraction via polarization:
http://physics.weber.edu/amiri/director/dcrfiles/electricity/pithBallS.dcr

We'll finish our discussion Tuesday about this phenomena:
1) polarization
2) charging by conduction
3) charging by induction
4) lightning rods


In short:
1) polarization -- the temporary rearrangement of charges in an object because of the influence of another charged object nearby.
2) charging by conduction -- direct transfer of charge by contact. (sparking, etc.) [A negatively charged rod, brought up and touching an electroscope will charge the scope negatively]
3) charging by induction -- indirect transfer of charge by first polarizing, and then grounding. (This results in the opposite charge).[a negatively charged rod brought near an electroscope will polarize it, meaning the negative charges already present in the electroscope will get pushed back. If we provide them a path out of the electroscope (a ground) they will leave. The net charge remaining on the electroscope will then be positive -- OPPOSITE the charge of the rod]
4) lightning rods - their primary job is to prevent lightning strikes, by helping to slowly trickle charges between sky/ground. A lightning cloud polarizes the ground, and buildings (etc.) thus appear to have the opposite charge of the cloud, thus making the building a very appealing target for a discharge of static charge. The lightning rod will slowly neutralize this imbalance, helping to prevent lightning strikes.

VOLTAGE

The past week was a bit discombobulated with all the activities...
But here's the summary:
Voltage.

That was the main focus of discussion for the week.
Voltage, measured in Volts, is a measure of the
ELECTRIC POTENTIAL.
The units are Joules/Coulombs.
It is a measure of how much ooomph! each charge carries with it.

The analogies we have used are elevation, and pressure.
Imagine cliffs of various heights. A stream of water going over a small cliff will release a certain amount of energy as it crashes to the bottom.
The same stream of water going over a much higher cliff will release much more energy.
The stream of water is the current. The height of the cliff is the voltage.
If you don't have an elevation change, you won't get any stream of water at all.
And the bigger the cliff, the more "oomph" you get out of each gallon of water.
Voltage is like that.
Stack more batteries up, and you get a brighter lightbulb. This means greater oomph per charge, and it also means more charge will be flowing.

Of course it is a little different than that. But that gets us pretty far in understanding it. We also talk about "pressure", and can model a battery as a "pump" that pulls in water and pushes it out with energy.
Remember, the charges are already in the wire. The battery just pushes them around. Putting energy into the system by doing that.

The stronger the pump, the more charge you can move around, AND the more energy per charge.

We had two worksheets this past week on Voltage.
The first one is was examining the basic definitions of voltage, the other one was focusing on diagrams showing batteries and light bulbs where we consider the importance of "potential difference".
"Potential difference" is the term that we use for the difference in voltage between one side of the light bulb, and the other.
As we talked about in class, nobody is afraid of Kansas. Even though they are at 4000 ft above sea level. Since there is almost no variation in elevation, you don't have to worry about falling, or being caught in rapids. Same with electricity. No difference in voltage levels, means no current to flow.
This is why birds can perch on a high voltage power line without being electrocuted. But if they straddle two wires, at two different voltage levels, then they will provide a path for current to go from high to low, and they will fry. You can see this with squirrels trying to step from wire to pole, or kites connecting wire to ground with a piece of string. (Wet string, or metal string is best for conducting).

Ground is an important concept that we will keep developing. This is the "universal zero voltage level". The earth is a good "dumping ground" for charge.

key terms

As we get deeper into electricity, we need to learn the important terminology to be able to discuss electrical stuff.
The BAD thing is that everyone has heard the terms, and almost NOBODY (including you, probably) knows what they mean.
Voltage, current, resistance, power.
All are technical terms that you must use correctly.
Current (I) we've talked about. Measured in Amps, it is the rate of flow of charge through a circuit.
Resistance (R) is the amount of resistance to flow. It can limit the amount of current, and also can cause the heating in things like filaments. Measured in ohms.
Voltage (V) is electric pressure. The greater the voltage, the greater the current.
The relationship between these variables is: I=V/R or V=IR

The key ingredient in voltage is the idea of DIFFERENCE. Nobody is afraid of Kansas. Even though it has an elevation (in the western side) as high as 4000 feet above sea level. The reason nobody is afraid, is that there isn't much elevation variation. Similarly with electricity. Only a DIFFERENCE in voltages really makes a difference. That's why we connect to the positive AND the negative. Just like (take a moment to let this sink in...) a waterfall needs to have a top AND a bottom. It won't work with just all the water at the top (hanging out in Kansas). It needs to have a bottom as well (a path to flow to).

Tomorrow, we'll start looking more into the concept of voltage, and learn some static electricity stuff.

Important info:

Quiz Friday. It will cover basic circuits, light bulbs, electric current, and circuit diagrams.

Current homework due Friday.

Energy Audit due Oct 27.

• You need to look at your household energy bill
• you need to look at your electric meter
• you need to record your energy usage for a 24 hour period.
• you may need to borrow one of the watt meters to measure your usage.
• I only have three of these, and there are 50 students. So start early, share nicely.
• No whining about the meter not being available on the last day!!!
  

Electric current:
So, wires are made of metal. Metal wire has bazillions of electrons in it already. If you try to put in just a few more on one end, then some pop out the other end. The voltage source (battery) is the pusher. Since the wires are already "full" of electrons, the "pulse", or "domino effect" is what is important. Almost at the speed of light, the signal travels across the wire, so the light turns on instantly. The electrons that are moving through the wire, actually travel slowly.

Since there are bazillions of electrons (like 10^18 or 1000000000000000000 electrons) moving through the wire each second in a standard lightbulb circuit. That's a lot.

So, Mr. Coulomb said: let's just take 10^18-ish electrons and call that "one Coulomb". Then we can talk about coulombs per second of electric current traveling through the wires.

Mr. Ampere said: "OK, we can call 1 Coulomb/sec an AMP". :-)

And thus, electric current is measured in Amps.

Circuit diagrams:

This is not a *Drawing* but rather a diagram. You've been looking at them for more than a week now in our lab write-ups. I have been going increasingly away from "pictoral" representations and towards schematic diagrams. The menu in croc physics that you used for your first Vlab is pictoral. Next to that menu is the schematic menu.

Here's a helpful link to become familiar with diagrams of electric circuits.

http://www.kpsec.freeuk.com/cdiags.htm