Physics 2113 Jonathan Dowling Physics 2113 Lecture 05

Physics 2113 Jonathan Dowling Physics 2113 Lecture 05 Electric Fields I Charles-Augustin de Coulomb (1736 -1806)

What Are We Going to Learn? A Road Map • Electric charge - Electric force on other electric charges - Electric field, and electric potential • Moving electric charges : current • Electronic circuit components: batteries, resistors, capacitors • Electric currents - Magnetic field - Magnetic force on moving charges • Time-varying magnetic field & Electric Field • More circuit components: inductors. • Electromagnetic waves - light waves • Geometrical Optics (light rays). • Physical optics (light waves)

Coulomb’s Law Charles-Augustin de Coulomb (1736 -1806) For Charges in a Vacuum k = Often, we write k as:

Sir Michael Faraday’s Electric Lines of Force Electric Force Field Faraday (1791– 1867)

E-Field is E-Force Divided by E-Charge Definition of Electric Field: E-Force on Charge +q 1 –q 2 P 1 P 2 E-Field at Point –q 2 P 1 P 2 Units: F = [N] = [Newton] ; E = [N/C] = [Newton/Coulomb]

Electric Fields • Electric field E at some point in space is defined as the force experienced by an imaginary point charge of +1 C, divided by 1 C. • • • Note that E is a VECTOR. Electric Field of a Point Charge Since E is the force per unit charge, it –q E +1 C is measured in units of N/C. R We measure the electric field using very small positive “test charges”, and dividing the measured force by the magnitude of the charge.

Compare to Gravitational to Electric Fields Gravitational Force: (Units: Newtons = N) Gravitational Field: (Units: N/kg) Electric Force: (Units: Newtons = N) Electric Field: (Units: N/C) Given the Field, Find the Force: (Vector Form)

Compare to Electric Field to Gravitational Field Lines m Note: Field Exists in Empty Space Whether Test Mass m is There or Not! Electric Field Lines +q Note: Field Exists in Empty Space Whether Test Charge +q is There or Not!

Electric Field Lines • Field lines: useful way to visualize electric field E • Field lines start at a positive charge, end at negative charge • E at any point in space is tangential to field line • Field lines are closer where E is stronger Example: a negative point charge — note spherical symmetry

Direction of Electric Field Lines E-Field Vectors Point Away from Positive Charge — Field Source! E-Field Vectors Point Towards Negative Charge — Field Sink!

+qs _ +q. R + ICPP

Superposition of F and E • Question: How do we figure out the force or field due to several point charges? • Answer: consider one charge at a time, calculate the field (a vector!) produced by each charge, and then add all the vectors! (“superposition”) • Useful to look out for SYMMETRY to simplify calculations! • If you never learned to add vectors in 2110 you’ll be in serious trouble in 21132! See online review http: //phys. lsu. edu/~jdowling/ PHYS 21024 SP 10/Vectors. pdf

ICPP Total electric field -2 q +q • 4 charges are placed at the corners of a square as shown. • What is the direction of the electric field at the center of the square? (a) Field is ZERO! (b) Along +y (c) Along +x -q +q is the test charge y +2 q x

ICPP: Which Way is Enet?