2ObjectivesProperly explain/predict the behavior of objects, before and after being electrically charged/discharged.Use equations and constants to solve word problems involving electric field, forces and motions of charged particles.
3Definition Electric Field is defined as Force per charge Units are Newtons per Coulomb (N/C)So, what’s an “Electric Field?”You have probably heard or even spoken of a “FORCE FIELD”Esp in science fictionYou have been using equations to describe Gravity Fields already.Let’s see how such “Force Fields” compare.
4Gravity vs. Electricity Gravity Field Electric Field
5NoteLittle q, the “test” charge needs to be very small so that its own field will not significantly change the charge distribution causing the surrounding field.Charge on a small object can be treated as if it is concentrated at a point, we call this a “point charge.”
6Example 1A metal covered Styrofoam ball of mass kg and charged to 4.0 micro C is placed in an electric field and suspended motionless in midair. What is the strength of the electric field?
7Solution Start with F=ma! Free body diagram. F = qE – mg = 0; E=mg/q = x 9.8 / 4.0 x 10-6 =12000 N/CFe = qEFg=mg(12250 if you don’t like sig figs or units)
8Drawing! Forces are vectors, so too must be the Electric Field. There are some conventions for drawing lines to represent an electric field, that will help explain the behavior of charged objects.Sign - Arrows on field lines point away from positive charge. They show the direction of force by the field on a positively charged particle.Density – The number of lines leaving/ending at an object are drawn in proportion to their charge. They represent the relative strength of the field.
10Example 3 Electric dipole Equal number of lines originate from pos charge and terminate at neg charge.Note symmetry.
11Conductors and Insulators Conductors – charge (electrons) moves freely – (metals)Insulators, …they don’t! (rubber, plastic, glass)Field lines can help us determine where the charge on a conductor will reside.Four rules…
12Rules for electrostatic equilibrium Electric Field is zero everywhere INSIDE a conducting material.Excess charge on a conductor resides entirely on its surface.The electric field caused by a charged conductor is always perpendicular to its surface.Charge is more concentrated at the smaller radius of irregularly shaped objects.
13Why?If there were an electric field inside a conductor, what would happen to the charge there?It’s complicated, but has to do with inverse square nature Coulomb’s Law.If it weren’t perpendicular, it would shove the charges sideways, not equilibrium.Less sideways component of force.
14Van De GraaffA number of the properties we’ve discussed can be demonstrated.Like charges repelNet Charge migrates to the outsideField Lines perpendicular“Dipole”
16One more note on the “point charge”. So the charge on the dome of a Van de Graaff is on the dome surface.Through methods of calculus you could show that the field created can be determined if you consider all the charge to be located all at the center. This is due to symmetry and charge repulsion.You can also show that the field everywhere inside is zero.This will greatly simplify calculations you will need to do.
17Example 4A Van de Graaff dome of radius 0.20 meters is charged to 3.00 x 10-4 C. What is the strength and direction of the electric field at the following distances from the center?5.0 cm20.0 cm1.0 meter
18Picture 1.0 m 20.0 cm 5.0 cm Zero inside! E=keQ/r2 and radialy outward on surface and outside.E=kq/r23.00E-048.99E+09E+07 N/C1 8.08E+19 N/C
19Millikan Oil drop experiment Shows quantized nature of charge. (He got the Nobel Prize for this one.)+-
20Free Body Oil Drop -q mg falling -q qE mg rising After thousands of trials, Millikan observed discrete velocities (very small) and concluded charge came in discrete amounts
21Faraday Ice Pail Experiment Charge will migrate outward in conductors.Touch a charged conductor to another just like it, ½ the charge will move to it.But if you touch the inside of the neutral container, all of it will transfer!Let’s draw to explain.
22Note Coulomb’s Law is also written as: You can see that the values of e0 and ke. must be related.
23Wrap upProperly explain/predict the behavior of objects, before and after being electrically charged/discharged.Use equations and constants to solve word problems involving electric field, forces and motions of charged particles.Like charges repel, opposites attractNet charge resides on surfaceConcentrates at small radiusNo field insideField lines perpendicularPoint in direction a positive charge will be accelerated.Charge from one object will be shared with a neutral object if touched on the outsideCharge will be completed given away when charged object is touched inside anotherCharge is conserved, stripping one electron leaves a net charge of +e one beind and -e with the electron.Charge is quantized at 1.6e-19 C per elementary charge.F=kqq/r^2E=F/q=kq/r^2
24Summary Like charges repel, opposites attract Charge is conserved, stripping one electron leaves a net charge of +e one beind and -e with the electron.Net charge resides on surfaceConcentrates at small radiusNo field insideField lines perpendicularPoint in direction a positive charge will be accelerated.Charge is quantized at 1.6 x C per elementary charge.Charge from one object will be shared with a neutral object if touched on the outsideCharge will be completly given away when charged object is touched inside another