1. Coulomb’s Law

2. Electric Force  Two positively charged spheres will repel each other.  The force will cause a deflection until equilibrium is reached.  The apparatus can measure the properties of electric force.  r m,q 2 q1q1

3. Inverse Square  Closer spheres exhibit more electric force.  The electric force decreases as the square of the distance.  This is an inverse square force. Compare to the force of universal gravitation  r m,q 2 q1q1

4. Total Charge  Increasing the charge of either sphere increases the total force. Linear increase with either charge  The force is proportional to the product of the two charges.  r m,q 2 q1q1

5. Coulomb’s Law  The force also depends on the material separating the charges. Simplest material is a vacuumSimplest material is a vacuum Expressed as a constant k = 8.99 x 10 9 N m 2 / C 2Expressed as a constant k = 8.99 x 10 9 N m 2 / C 2 Permittivity of free space:  0 = 1/4  kPermittivity of free space:  0 = 1/4  k  The formula is known as Coulomb’s Law. The sign of the charges indicates attraction or repulsionThe sign of the charges indicates attraction or repulsion

6. Vector Force  The electric force is a vector.  The force acts in a line between two charges.  The force one charge is equal and opposite to the force acting on the other charge of a pair.  Multiple charges add their forces as vectors. q1q1 q1q1 q2q2 q2q2 q3q3

7. Electricity vs. Gravity  A hydrogen atom consists of an electron and proton at an average distance of r = 0.53 x 10 -10 m.  Find the relative forces due to electric charge and gravity between the particles.  Useful facts: q e = q p = 1.6 x 10 -19 Cq e = q p = 1.6 x 10 -19 C m e = 9.1 x 10 -31 kgm e = 9.1 x 10 -31 kg m p = 1.67 x 10 -26 kgm p = 1.67 x 10 -26 kg  From Coulomb’s law:  From Newton’s Gravitation:  The ratio is: next