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 (Nm2/C2) k = 8.99*109 Nm2/C2 q1 & q2 – 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 1018 electrons Therefore 1 electron (or proton) =1.60x10-19C 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 10 -6 C = 1 microCoulomb (μC) 1.0 x 10-9 C = 1 nanoCoulomb (nC) 1.0 x 10-12 C = 1 picoCoulomb (pC) 1.0 x 10-15 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 an attractive 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

Question 5 page 638 Two identical objects have charges of +6.0µC and -2.0µC, respectively. When placed a distance d apart, their force of attraction is 2.0N. If the objects are touched together, then moved to a distance of separation 2d, what will be the new force between them? HINT: When the charges touch they leave with an identical force as the charges transfer. Alt 230

Example 2: 3 Charges Three charges (Charge A: +3.0µC, B: +5.0µC, C: -3.0µC) are arranged in a right triangle. The distance between A and B is 0.050m, dBC = 0.040m. What is the net electrostatic force on B?

To Solve when 3 Charges Step 1: Free Body Diagram of charge you want (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 charge you want from the other charges. Step 3: Vector Addition – find the force and direction (tan) Answer: 1.0 x 102 N [E 32.6’ S] http://hrsbstaff.ednet.ns.ca/pboudrea/physics12.html

Example 3: 4 Charged Particles Find Fnet on A: Fab = 0.01N Fac = 0.00144N Fad = 0.00212 N

Fab = 0.01N Fac = 0.00144N Fad = 0.00212 N Answer: 9.5 x 10 -3 N [70’]

Practice Model Problem Page 639 Page 640-641 Questions 7, 8, 9, 10