# Teaching voltage

1. Apr 18, 2006

### Miranda2005

Hi, I am a teacher (new) and have never taught Physics before... GULP.

I have a learning outcome I must teach and am unsure of how to best do it. It is in the grade 10 (S2) Gen Science...

I must define voltage (electrical potential difference) as the energy per unit charge between 2 points along a conductor and solve related problems. Include: V = E/Q

Basically, I need to put this into understandable notes my students can use without the help of a textbook.

I also want to make about 5 practice problems for the students to work through...and an answer key.

Does anyone have any ideas?
Help!
Thanks,
Matt

2. Apr 18, 2006

### Hootenanny

Staff Emeritus
Wikipedia explains it quite well, http://en.wikipedia.org/wiki/Voltage although you may have to paraphrase somewhat. When I was at high school I would intially tought about voltage using the hydraulic pressure analogy, but I was never a fan.

Regards,
~H

3. Apr 18, 2006

### rbj

for secondary school, you can use a hydrological analog. a quantity of water that weighs one Newton is a colomb (the mass would be 1 Nt/g where g is 9.8 m/s2). current would be measured as how much water (measured in "coulombs")) passes past a point in a pipe per unit time. an ampere would be 1 "coulomb" per second.

then Voltage is just like water pressure. the pressure of water (at ground level) is directly proportional to the height of the level of water in the tank of a water tower. and that height represents exactly how much energy (in Joules) it takes to lift one "coulomb" of water (that is 1 Nt/g or about 102 grams). to lift that "coulomb" of water (that weighs 1 Newton) up 1 meter, requires 1 Joule of energy, so a pressure resulting from one meter of height would be 1 volt. that would be about 9810 Nt/m2 of pressure or a little less than 1 Nt/cm2.

does that help?

you can construct hydrological analogs for a resistor, a capacitor, and an inductor. even a crude hydrological analog for a transistor (an FET).

4. Apr 18, 2006

### ZapperZ

Staff Emeritus
Let's try this. Not sure if it will work with your students, but I'll run it by you anyway.

What if we want to know what is the electrostatic potential energy difference between two points in space? (remember that potential energy is really a measure of the difference in PE with respect to a reference value.) What we could do is use a test charge Q, and move it from Point 1 to Point 2, and find the work done. This work done is equal to the change in potential energy.

However, if we tell other people that value, we also have to tell them it was measured using a test charge Q! If we use a different test charge, our change in PE will be different! This is cumbersome to other people who would like to use other values of charge. So what do we do? We "normalize" or divide out the value of the test charge that we use. So we take the change in PE U and divided out the test charge that we used, so obtain a ratio U/Q. We define this new quantity as potential, V. This is the change in energy per unit charge.

So next time, someone wants to know what is the amount of work, or change in potential energy that occurs in going from Point 1 to Point 2 by some charge, let's call it q, all that person needs to do is look at the value of potential we have found, and just multiply it with whatever charge that is being used. So knowing V is helpful in the sense that it is not dependent on a particular value of test charge (even though, if someone wants to be picking, he/she can argue that it is for a 1 unit of charge).

Zz.

5. Apr 18, 2006

### Vosh

I won't be able to help until I personally truly understand the concepts, then I will be able to explain it in understandable terms to the lay person or to an 8 year old.

6. Apr 18, 2006

### Miranda2005

thanks!

Hi, this was my first time posting to this forum. Thank you for your swift and helpful replies. You can expect to see me here more often after all this help!
I appreciate this. Now I hope I can make it work!
Matt