Coulomb’s Law Physics 102 Professor Lee Carkner Lecture 10

Direction of Forces   but,  opposites attract, like repel  On a coordinate system +F is to +x or +y and –F is to –x or -y

Units of Charge   For most electrostatic problems use coulombs, meters and newtons  1 e = 1.60 X C  Charge is represented by the variable q

Electric Force  The force between two charged objects is represented by Coulomb’s law: F = k q 1 q 2 /r 2  Where:   q 1 and q 2 are the two charges (in coulombs) 

Inverse Square Law  The formula for the gravitational force is:  Both are inverse square laws   Distance is more important than charge or mass  Decreasing the distance will greatly strengthen a force, increasing the distance will greatly weaken the force

Computing Electrical Force   Force is a vector quantity   Direction depends on charge   Repulsive if charges are same sign   equal and opposite

Resultant Force  To find the net force from several forces add the force from each vectorially:  Put an x-y axis on the system   F 1, F 2, F 3 ….   F 1x, F 1y …   F x = F 1x + F 2x …   F 2 = F x 2 + F y 2

X and Y  Remember vector addition rules:  Components along axis computed from:    Total F:   Angle to x axis:  q F F x = F cos   F y = F sin 

Today’s PAL  What is the magnitude and direction of the force on charge q 1 ? 3 m 2 m q 2 = 5 C q 3 = 4 C q 1 = -2 C

Spherical Charge Distribution  Coulomb’s law applies only to point charges   Non-point charges exert complex forces 

The Electric Field  Electrical and gravitational forces act at a distance   The area near the charge is said to be occupied by an electric field   The test charge is small enough so that its field does not affect the main one

Defining the Electric Field  E = F/q 0  E = k q/r 2  i.e., the electric field at any point a radial distance r away from a charge q  Field exists whether there is a charge near it or not

Calculating the Electric Field   Need to find the magnitude and direction of the field from each charge and add vectorially 

Next Time  Read  Homework Ch. 16: P 12, 14, 17, 26