Goal: To understand Electrostatics Objectives: 1)Understanding what charges are. 2)Knowing how to produce a charge. 3)How to calculate an electric field 4)Electric Force 5)Electric Potential 6)Other random stuff

What is charge? For the most part, charge is a measure of how many protons or electrons you have somewhere. Charge is measured in units of Coulombs (C). An elementary charge from a proton or electron has magnitude of * C. Like charges repel. Opposite attract. Charges can move.

How do you get charge? 1) Rubbing (static electricity) 2) Induction (charge obtained from a changing magnetic field) 3) Conduction (moving charge along a wire)

Electric Field Suppose you wanted to know where the water would flow when it rains. How would you do that?

Fields Fields are just a listing of possible potential at any given point. For rain you look at the Gravitational Field – which is just a fancy way of saying the topography. Water will want to flow downwards. We can do the same with electric fields.

Electric “Field” The Electric Field is just a measure of the electric topography. Since protons repel each other you can think of the protons as hills. The electrons would be pits or valleys. The elevation of some point near some charges would depend on the distribution of charges (much like your elevation depends on where you are compared to the hills and valleys). Units are in N / C.

Calculating the Electric Field First lets do it for just one charge. For one charge the equation is pretty straightforward: E = -k * charge / (distance * distance) k is a constant, and is 9 * 10 9 N m 2 / C 2 There is another equation for the electric field we will use later.

Force translation the Electric field is a topography of electric charges around you. At any point the electric field is just a sum of the topography from each charge. For each charge E = -qk / r 2 How would this translate to a force?

Ball downhill If you have a gravitational topography a ball will want to roll downhill. That is it will roll from a high elevation to a low one or a high field to a low one. The same is true of electric fields. A positive charge will want to move to a lower electric field. A negative charge will do the opposite and will want to move up to a higher valued electric field (moving uphill).

Now for the math The force on a charge is: F = q * E Where q is the charge the force is being applied to and E is the electric field that charge q is located at. Much like for gravity that F = m * g on the surface of the earth.

If we add in E If we have 2 charges called q1 and q2 then the force is: F = q1 * E, but E = -q2 k / r 2 So, F = -q1 * q2 * k / r 2 (k is the same constant we had before) Notice this is almost the same as the gravitational force – where: F = -G * M1 * M2 / r 2

Which force is stronger? During the next 5 min break think about which of the following is a stronger force? Electric force: F = -q1 * q2 * k / r 2 Or gravity: F = -G * M1 * M2 / r 2

Electric Potential Energy Electric Potential energy is similar to gravitational potential. It is the potential energy between any two charges. Energy is a force times a distance, so if you multiply the Electric Force times a distance (I am oversimplifying a little here) you get the Electric Potential. U = k q1 q2 / r (and is in units of Joules) Note how similar it looks to the equation for the force.

Direction? Other than calculating distances between charges will the directions matter? Why or why not? NO! Potential energies are SCALER values!

Insulators Insulators (like insulation for the house) is a material that won’t let charges move through it. Wood would be an insulator… What is a real world use for an insulator?

Conductor Materials that help the flow of charges are conductors. Ideally, you want to make a wire out of a good conductive material.

What about in between There are materials that can act as both conductors and inductors. These are called semi-conductors. These form vital electronic parts such as transistors (which revolutionized the electronics industry)

Super strength conductors The only draw back to conductors is that they heat up (and that means you are loosing power). If you send power across a long power line, you loose energy, and that looses you MONEY! But what if you could build a conductor that didn’t give any resistance to the flow of charge? You would have a superconductor

Superconductors Typically superconductors are made of materials that when cooled to VERY low temperatures (-300 to – 400 F) they allow charges to flow freely. There is a lot of uses for this, but it is not very practical unless you can build them to operate at temperatures that are much closer to room temperature.

Saving charges Collecting charges requires a device called a capacitor. This is usually a pair of sheets separated by a small distance on which you store charge. The negative electrons collect on one sheet. One the opposing sheet the electrons are repelled which can then flow through the rest of the circuit – leaving only a positive charge.

Flood! However, there is a limit. If you try to collect too much charge, you get a discharge. The electrons flow over like water flowing over a filled to capacity dam.

Dielectrics As you read in the book (hopefully) – you can shield charges. One way is by filling the center of the capacitor with a substance. This substance is a dielectric. The type of material will determine what fraction of the charge the other side actually sees. Water, for example, allows you to collect 80 times more charge than air.

Conclusion 1) We learned how to find the Electric Field and electric force 2) We have found how to do the electric potential. We have seen applications for electrostatics such as conductors and inductors. We have seen how to store charge.