A Crash Course in Electric Fields

By Leah Chernoff

What are Electric Fields? What do they do?

• An E-field (E) exerts a force on charged particles in space. F=qE, when F is force and q is a charged particle.

• E-fields are created by voltage differentials between points in space. E= -∆V/R, when ∆V is the voltage difference over a certain distance R.

Why Does Voltage Affect the E-Field?

• For a point charge Q, V= kQ/R• For a point charge Q, E= kQ/R2

• So, E= -dV/dr (the negative accounts for the fact that electric fields are directed in the direction opposite to the direction of increasing voltage; that is, fields go from high voltage to low voltage.)

Field Lines• THEY CAN NEVER CROSS

(like in Ghostbusters)• Negative charges always feel

force opposite of the electric field

• Double the field lines means twice as strong

• Positive E-field --> lines go out; lines go in for negative E-field

• Confused? Check out this cool simulation! http://www.vias.org/simulations/simusoft_efield.html

Example Problems

There is a 10V voltage difference over 25 m. What is the magnitude of the electric field?

ANSWER: E = ∆V/R; 10/25 = .4 V/m

What is the force on a particle with charge -3 C in this field?

ANSWER: F = qE F = -3(.4) = 1.2 N in the opposite direction of the field

Useful Equations and Facts

• Units: N/C or V/m• E = -dV/dr = kQ/R2

• F = qE• Another cool

simulation website: http://www.its.caltech.edu/~phys1/java/phys1/EField/EField.html