Electric Charge, Force and Field

Properties of Charge Unlike charges attract, like charges repel Force between charges varies as an inverse square (same as gravity) Charge is conserved Charge is quantized p+ is 1800× heavier than e- Benjamin Franklin

Electrical Properties Conductor Allows easy current Metals Insulator Prevents current flow Plastics, glass, & air Semiconductor Intermediate behavior Carbon, silicon, & germanium

Coulomb’s Law q1 q2 r F12 F21 e0 - Permittivity of free space

Superposition The net force from multiple charges is additive. Ex. A charge of +0.02 C is located at the origin and a charge of -0.05 C is located at (10 m, 0 m). What is the force exerted on a +0.03 C charge located at (5 m, 5 m)?

Electric Field The force per unit charge experience by a charge. For a point charge the electric field is Units (N/C)

Drawing Electric Fields Lines begin on positive charges and end on negative charges. # of lines leaving a charge is proportional to the magnitude of the charge. Therefore, the strength of the electric field is proportional to the density of the electric field lines. No two field lines can cross.

Field Lines How would you draw the field lines for the following sets of charges? Point Charge Dipole Like Charges See 2-D Electrostatics Applet at http://www.falstad.com/mathphysics.html Can also look at the 3-D Electrostatic Fields Applet

Continuous Charge Use superposition to add all the infinitesimal charges, dq. If charge is uniformly distributed, then Line Charge Surface Charge Volume Charge

Electric Field on a Wire Ex. A wire has a length of 2 m and a charge of +20 mC. What is the electric field at a point 4 cm away from the center of the wire? (Assume the charge is uniformly distributed.)

Charge in a Field Charge will accelerate in the direction of the electric field lines. Ex. What is the acceleration of an electron in a 25 N/C electric field? e-

Torque on a Dipole E Remember then q+ q- 2a F is a unit vector going from the negative to the positive charge