VIEWS: 5 PAGES: 23 POSTED ON: 12/19/2009
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 1.602 * 10-19 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 * 109 N m2 / C2 • 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 / r2 • 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 / r2 • So, F = -q1 * q2 * k / r2 • (k is the same constant we had before) • Notice this is almost the same as the gravitational force – where: • F = -G * M1 * M2 / r2 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 / r2 • Or gravity: F = -G * M1 * M2 / r2 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.