Coulomb’s Law

Coulomb’s Law…

= the relationship among electrical forces, charges, and distance.

It is like Newton’s law of gravity. But, unlike gravity, electric forces can be attractive or repulsive.

Discovered by French Scientist, Charles Coulomb in the 18th Century.

Coulomb’s Law• F = K qAqB

d2

F = Force (N) Kc = (Coulomb Constant)9.0 X 109 N·m2/C2

q = charge (C for Coulomb) d = distance (m)

Important Information…

• The SI Unit of measure for electrical charge is a Coulomb (C)

• Charge of 1 proton is +1.6 x 10-19 C

• Charge of 1 electron is -1.6x 10-19 C

Remember…

• The charge (q) of a particle is NOT a vector quantity.

• Therefore, when described as “POSITIVE” or “NEGATIVE” that tells whether it is a proton or electron, but is NOT part of the magnitude of the charge.

• DO NOT PUT THE NEGATIVES IN THE FORMULA.

Sample Problem #1

• Two charges are separated by 0.030m. Object A has a charge of +6.0 C while object B has a charge of +3.0 C. What is the force on object A?

Sample Problem #1 ANSWER

F = 1.8 x 1014 N

Sample Problem #2

• A negative charge of -

6x10-6 C exerts an attractive force of 65 N on a second charge 0.050 m away. What is the magnitude of the 2nd charge?

Sample Problem #2 ANSWER

qb = +3.009 x 10-6 C

• Electric Field – a vector quantity that relates the force exerted on a charge to the size of the charge

• E=F/q or E=kc(q/d2)

• The magnitude of the electric field is measured in N/C (Newtons/Coulomb)

Electric Fields

Electric Field Lines

• Direction of arrows indicate the charge of the field

• Away from a positively charged object

• Toward a negatively charged object

• Strength of the field is denoted by the spacing between the lines (close = strong)

Electric Field Lines

Example

• Your hair acquired an equal amount of positive charge when you rubbed the balloon on your head. What is the strength of the electric field created by your head at the location of your feet, 1.5 meters below if the charge is 2e-9C?