Electric Fields Chapter 14.1

What do you already know about charged particles? Like charges repel. Opposite charges attract. Electric charges exert a FORCE on each other (similar to how masses exert a gravitational force on each other).

Inverse Square Law Recall that gravity follows the inverse square law with respect to the distance between objects (Universal Law of Gravitation) We see the same behaviour with charges:

Coulomb’s Law Is similar to Newton’s Universal Law of Gravitation Uses the following quantities: –k – Coulomb’s constant (Nm 2 /C 2 ) k = 8.99*10 9 Nm 2 /C 2 –q 1 & q 2 – charge in coulombs on each object (C) –r – distance between objects (centre to centre) (m) However, unlike the gravitation law, it is possible to have both positive and negative forces

Coulomb’s Law Calculates ELECTROSTATIC force Electric charges exert forces on each other

What exactly is a Coulomb? A measure of electric charge equal to the charge of 6.25 x electrons Therefore 1 electron (or proton) =1.60x C A Coulomb is known as elementary or fundamental charge. It is the smallest indivisible amount of charge. All charges are multiples of elementary charge.

Prefixes A Coulomb is a relatively large amount of charge. Often we want to express smaller amounts. You should know the following prefixes for this unit: 1.0 x C = 1 microCoulomb (μC) 1.0 x C = 1 nanoCoulomb (nC) 1.0 x C = 1 picoCoulomb (pC) 1.0 x C = 1 femtoCoulomb (fC)

Coulomb’s Law Positive force and repulsion will result when you have: –Two positively charged particles –Two negatively charged particles Negative force and attraction will result when you have: –One positively and negatively charged particle Electrostatic force is a non-contact force that gets smaller the farther two charges are away from each other.

Fg vs Fe Differences: Fg is only a positive force but Fe can also repel. Fg is much weaker than Fe

Example A small sphere carrying a charge of - 8.0μC exerts an attractive force of 0.50N on another sphere carrying a charge with a magnitude of 5.0 μC. a) What is the sign of the second charge? b) What is the distance between these two spheres? (distance of separation) a) positive b) 0.85 m

Practice Problems Page 638 –1-5

Example 2: 3 Charges What is the net electrostatic force on B?

Step 1: Free Body Diagram of B (NOTE: Signs indicate if the force is attractive or repulsive NOT DIRECTION on the x-y plane therefore the diagram is very important ). Step 2: Find the electrostatic forces on B from the other charges. Step 3: Vector Addition – find the force and direction (tan) Answer: 1.0 x 10 2 N [E 32.6’ S]

Example 3: 4 Charged Particles Find Fnet on A: Fab = 0.01N Fac = N Fad = N

Fab = 0.01N Fac = N Fad = N Answer: 9.49 x N [69.9’]