Electric Fields and Forces

Electrostatics “Electricity at rest”

Electric Charges Fundamental positive charge – proton Fundamental negative charge – electron If an object gains electrons – becomes negative If an object loses electrons – becomes positive

Measuring the Charge Unit of charge is coulomb (C) Elementary charge (e) of electron and proton – 1.6x10-19 C. 1 C – 6.24x1018 electrons Braun Electroscope – device for detecting electric charge

Conservation of Charge Charge cannot be created or destroyed, only transferred from one object to another.

Transferring of Charges (Electroscopes, Metal Spheres)

Coulomb’s Law Force two charges have on each other Related to Newton’s Law of Universal Gravitation q1 and q2 are charges r – distance between centers k – electrostatic constant (9.0x109 Nm2 /C2) Fe = magnitude of force (N)

Force vs. Distance Force Distance

Example Calculate the magnitude of the force between two positive charges, q1 = 3.0 x10-6 and q2 = 6.0 x10-5 C, separated by a distance of 9.0 m. (Positive value of F – Repulsion) (Negative value of F – Attraction)

Electric Field A field exists when an electric force is exerted on a charged particle Vector Quantity Idea developed by Michael Faraday Electric Field Lines – Models to represent an electric field Lines ALWAYS go AWAY from positive charges

Electric Field Lines

Electric Field Lines Two Equal and Opposite Charges Two like charges (+ and -) Parallel Plates Hollow Sphere Directionality of a point charge.

Electric Field Strength F – Force (N) E – Electric Field Strength (N/C) q – charge (C)

Example A test charge of +2.0x10-6 coulomb experiences a force of 2.4x10-3 N when placed in an electric field. Determine the magnitude of the field.

Potential Difference The work required to bring a charge from infinity to that point. When work is done, energy is used Scalar Quantity

Potential Difference V – Potential Difference (Volts, V) W – Work (Joules, J) q – charge (Coulombs, C)

Problem When a charge of -4x10-3 C is moved between 2 points in an electric field, 0.8 J of work is done on the charge. Calculate the potential difference between the points

Electron Volt Electron Volt (eV) is the energy required to move one elementary charge (1.6x10-19 C) 1 eV = 1.6x10-19 C