Electric Fields “forces at a distance”

Electric Charges: Electric charge is a fundamental quantity that is responsible for all electric phenomena. Charge is a property of all atomic particles. Charge can be positive, negative, or neutral.

Electrical Forces: Opposites attract. Likes repel. This is due to a force arising from the electric charge on particles. We will learn more about this force later...

Coulomb’s Law: k q q’ FE = ------------------- d2 k = 9 x 10 9 Nm2/C2 q = one charge q’ = another charge d = distance between the charges This is the law that quantitatively relates the attraction or repulsion of electric charges.

Charge Unit: 1 Coulomb, C, = 6.25 x 1018 electrons. Since electrons are much too small and numerous to be counted individually, they are counted in groups called Coulombs. 1 Coulomb, C, = 6.25 x 1018 electrons.

Imagine there is a cluster of – charge as shown. Electric Field: The region around a charged particle through which a force is exerted on another charged particle. Imagine there is a cluster of – charge as shown. If a small + charge was placed, what force would it feel? E Fields! -

This “map of force lines” shows what a positive test charge will do when exposed to any particular field. When lines are closer together, that means the force is stronger. Here are some pictures of various fields:

Here are some pictures of various fields:

Electric Field Strength/Intensity Electric potential describes how strong the field is per amount of charge. Electric Field Strength = Electrostatic Force / charge E = Fe / q Field Strength Gravity Field Strength = Gravity Force / mass g = Fg / m

Example = 6.3 x 1018 N/C Use E = Fe/q Fe=1.0N q =1.60 x 10-19 C What is the magnitude of the electric field strength at a point in a field where an electron experiences a 1.0-newton force? Use E = Fe/q Fe=1.0N q =1.60 x 10-19 C = 6.3 x 1018 N/C

Question: Q: Suppose you had a charged rod, and an oppositely charged hoop as pictured. Describe the electric field between them. Describe the electric field inside the hoop. + -

However, inside the hoop, there is surprisingly no field at all!!! A: Between, the field points directly from the (+) to the (-). That’s the direction a positive test charge would want to go. However, inside the hoop, there is surprisingly no field at all!!!

Shielding: This last question points out an odd, but important fact: The electric field inside a conducting surface is zero! Anywhere inside a conducting surface, the forces on you cancel out, giving no electric field. Even though you may be closer to one end ( less distance), on the other end, there is more charge pulling on you. These effects cancel out and give 0 field.

Lightning Strike? Q: If you are inside your car when it is struck by lightning, you will survive. Why?

A: The electrons repel themselves to the extreme outside of the car. Two electrons wouldn’t want to be near each other on the inside by you. The electricity flows around the outside of the car, not through you. This would happen even if there were no tires at all on the car!

Work and the Electric Field Work is done by the electric field in moving the positive charge. If the charge were moved counter to the field, work would be required.

Electric field strength is same at every point between plates. If d between two is relatively small compared to area, electric field is uniform!. Parallel to each other. Electric field strength is same at every point between plates.

Magnitude of electric force on an electron or a proton located at any point between two given oppositely charged parallel plates is the same. Particle speed increases as it approaches plates of opposite sign.

Electric potential = potential energy / charge How much work is done, depends on the amount of charge. Electric potential describes how much work is done, per amount of charge. Electric potential = potential energy / charge

Electric Potential Unit: A Joule per Coulomb is defined as a Volt. 1 V = 1 J / 1C A volt is the basic unit of electric potential. Named after Alessandro Volta, he invented the electric battery. DieHard

Mechanical and Electrical Energy Gravitational potential energy is derived from the earth’s gravitational field. Electrical potential energy, is derived from an electric field. Despite the name similarity, electric potential is NOT the exact same as potential energy. Electric potential describes how much work could be done per amount of charge.

Mechanical / Electrical Energy Two rocks are at the same height, the larger one has more PE. Two charges have the same electric potential, the larger charge has more PE.

V=W(Joules) / q(Coulombs) W = work done against field or energy acquired working with field (Joules or eV)  q - amount of charge moving through field (Coulombs) V - Potential Difference (volts) Ex) It takes 6 Joules of work to move 2 Coulombs of charge between 2 points in an electric field. What is the potential energy difference (voltage) between these 2 points?  

V = 3 Volts or J/C V =6 Joules / 2 coulombs Ex) It takes 6 Joules of work to move 2 coulombs of charge between 2 points in an electric field. What is the potential energy difference (voltage) between these 2 points?   V=W(Joules)  q(Coulombs) V =6 Joules / 2 coulombs V = 3 Volts or J/C

Both groups of charges on the side are at the same electric potential (voltage). However, it would take much more work to move the lower one closer since it has a larger charge...

Example If an elementary charge is moved against an electric field through a potential difference of one volt, the work done on the charge is: W=Vq =(1.00V)(1.60 x 10 -19 J) =1.60 x 10 -19 J (this gain in potential energy is called the electronvolt, eV!)

Example Moving a point charge of 3.2 x 10-19 coulomb between points A and B in an electric field requires 4.8 x 10 -18 joules of energy. What is the potential difference between these points? V=W/q 4.8x10 -18 J / 3.2x10 -19 C = 15V