click on the picture to play electrical hokey Electric Field Let's take a single electric charge, Q, and put it somewhere. The space around it is different from the space without charge. We have created a situation in which we could have an electric force. All we have to do is bring in a second charge, q, to feel the force. Without q, there is no force ....but we still have the condition that we could have a force. We say that the space around charge contains ELECTRIC FIELD. How to measure/find the strength (magnitude and direction) of electric field at particular location P due to charge Q? A test charge, q, placed at P will experience an electric force, F - either attractive or repulsive. r Q P F q r Q P F q Definition of electric field, E, at a point P distance r away from Q. The magnitude of the electric field is defined as the force per unit charge. F E= q As F contains q, E DOESN’T depend on q at all, only on Q. Electric field at any point P in space is always in the direction of the force on a positive test charge if it were placed at the point P. N E C The other way around: If you know electric field E at a point where you place a charge q, that charge will experience the force F: F=qE E E q F F q Electric field of a charged particle/point charge A charged particle Q creates an electric field. Q F E Field independent ◊ magnitude E q = k r2 of test charge the same value on the sphere of radius r around ◊ direction – radially outward or inward example: Q=1.6x10-19 C + q E r = 1x10-10 m -19 E = 9109 1.610 (10-10)2 = 2.91011 N/C q positive test charge (to the right) Question Say the electric field from an isolated point charge has a certain value at a distance of 1m. How will the electric field strength compare at a distance of 2 m from the charge? It will be ¼ as much – inverse square law for force between two charges carries over to the electric field from a point charge. We use “Electric Field Lines” to visualize el. field. Convention / agreement Direction indicates direction in which a positive test charge would be pushed – direction of the force!!!. 321011 N/C Electric Field of a Point Charge 2.91011 N/C + This is becoming a mess!!! E Electric Field Lines 1. Density gives strength # lines proportional to Q lines never cross! 2. Arrow gives direction Start on +, end on - negative charge So always point away from + charges, towards – charges… positive charge Denser lines - stronger field el. field decreases with distance more lines revels stronger field due to greater charge Electric field lines can never cross. If they crossed, that would mean that a charge placed at the intersection, would be accelerated in TWO directions at once! This is impossible! If two sources are creating electric fields in the same place, we have to add the two vectors and get a resultant vector representing the NET ELECTRIC FIELD. Question? What is the direction of the electric field at point C? 1) Left 2) Right 3) Zero Away from positive charge (right) Towards negative charge (right) y Net E field is to right. C x Question? What is the direction of the electric field at point A? 1) Up 2) Down 3) Left 4) Right 5) Zero A x Question? What is the direction of the electric field at point B? 1) Up 2) Down 3) Left 4) Right 5) Zero y B x Question? What is the direction of the electric field at point A, if the two positive charges have equal magnitude? 1) Up 2) Down 3) Left 4) Right 5) Zero A x Electric field of a capacitor Uniform electric field (the one that has constant magnitude and direction is generated between two oppositely charged parallel plates. Edge effect is minimazed when the length is long compared to their separation. FOR MORE CLICK Electric field of a charged conducting sphere in electrostatic equilibrium Electric field outside a charged sphere (evenly distributed charge q over surface) at distance r from its center is the same as if the charge q is concentrated at the center of the sphere: q r R E=k E=k q r2 q at the surface R2 What is electric field inside the sphere? OK. Let’s start. In general if you have many charges you have to find el. field of each of them and then add them up as vectors to get net el. field at certain point. So imagine a solid!!!!!!!!! Good luck. Different approach: Conductor is in electrostatic equilibrium when there is no net motion (flow ) of charge within a conductor or on its surface. • Conductor electrons free to move • charges in electric field feel the force F = Eq • only free electrons can move in the conductor so they will move until E = 0 inside a conductor if not then electrons would respond to its presence and be accelerated within the conductor. And that is not conductor in electrostatic equilibrium. Q.E.D If such conductor has excess charge, it resides entirely on the conductor’s outer surface running away from each other as far as possible. El. field is still zero everywhere inside the conductor. Therefore, electric field of charged conducting sphere in electrostatic equilibrium is: E=k r E=0 q R E=k q at the surface R2 q r2 El. field just outside a charged conductor is perpendicular to the conductor’s surface. if not Charge tends to accumulate where the curvature is the greatest (sharp points). The fact that pointed objects create strong electric fields if charged is the reason for the shape of lightning rod. You’ll see. Summary: Conductors in electrostatic equilibrium no net motion (flow) of charge within a conductor . Then: El. field is zero everywhere inside the conductor If such conductor has excess charge, it resides entirely on the conductor’s outer surface. El. field is still zero everywhere inside the conductor. El. field just outside a charged conductor is perpendicular to the conductor’s surface. Charge tends to accumulate where the radius of curvature is smallest (sharp points). Electrical Shielding Electric field is zero inside conductor, regardles how strong the electric field is outside. Even more: electric field inside metal cavities is zero, regardles what’s going outside - so put electrical equipment in metal boxes. Outside may be very strong fields and high charges, but the charges on the metal surface rearrange to give 0 inside. This is why if lightning hits a car and you’re inside, you’re safe, at least if it is not too strong ot make everything melt. Electrons from the lightening bolt mutually repel and spread over the outer surface. Although the electric field they set up maybe great outside the car, the overall electric field inside the car practically cancels to zero. Radio signals cannot penetrate through a metal enclosure (mobile in an elevator) The metal bars (rebar) that reinforce the concrete in walls can interfere also Electrostatic shielding • The effect of the high voltage on the van de Graff generator stops on the outside of the metal cage Homer is SAFE! Lightning – great weapon of the Gods • causes 80 million dollars in damage each year in the US • kills ~ 85 people a year in the US • is all over in a thousandth of a second • carries up to 200,000 Amperes (currant) • temperature of a lightning bolt can reach 30,000°C – 5x hotter than the surface of the sun • causes the thunder! development of a lightning bolt 1. step: Positive and negative charges separate. A cloud becomes positively charged at the top and negatively charged at the bottom. Reasons for this charge distribution are complex and only partially understood. 2. step: As the negative charges collect at the bottom of the cloud it forces the negative charges in the ground to be forced away from the surface. This leaves the ground positive. If attractive force is large enough electrons will leap from negative region downward from the cloud to the ground. +++ +++ +++ +++ +++++++ +++ http://regentsprep.org/Regents/physics/ phys03/alightnin/default.htm 3. step: As the electrons approach the earth, el. field just above the surface becomes so intense that it ionizes atoms and molecules in the air. An intense flash of light is produced. 4. step: When first breakdown occurs, negative charge in the column farther above the earth can rush down through the region of ionized air. Now that flow of electrons originated farther above the earth causes a flash of light. That way flash appears to move up. The rapidly moving electrons excite the air along the path so much that it emits light. It also heats the air so intensely that it rapidly expands creating thunder. One thing to notice is that the positive charges that make up both the cloud and the ground do not move. Even the positive streamer launched by the ground (in the animation) is really only made up of positively charged air particles because the electrons left the particle. Electrons in atoms orbit the nucleus well defined orbits called stationary states. By absorbing energy atom can be raised to higher energy level called excited state. When excited electron jumps back to a lower level it emits photon. At ordinary temperatures and when not in strong electric filed most atom are in the ground state. But if the temperature is high and/or electric field very strong the air will turn into plasma. The light is generated when the stripped electrons in the plasma recombine with the ionized ions. The probability of recombination process is pretty high, so the time for the recombination to occur is short. The streaks of light from the lightening fade away almost immediately. When a meteor shower enters the very high regions of the atmosphere, streaks of light last for as long as 2 – 3 seconds before fading. Because the probability of freed electrons finding an ionized atom with which to recombine is very low (pressure and density are low - atoms are relatively far apart). Lightening Rods + + + +++ Place rod above a building, and connect it to ground. Then the point of the rod picks up electrons from the air (“leakage”), so prevents large build up of + on the building, hence decreasing chance of a lightning strike. + + + ++ + + ++ + + + + But even if there is a lightning strike (if leakage not enough), the electricity goes through rod to ground, rather than through building. While enjoying the Sequoia National Park from a lookout platform, this woman found her hair rising from her head. Amused, her brother took her photograph. Why should she run for cover? Five minutes after they left, lightning struck the platform, killing one person and injuring seven.