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Electric Fields Chapter 14.1. What do you already know about charged particles? Like charges repel. Opposite charges attract. Electric charges exert a.

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Presentation on theme: "Electric Fields Chapter 14.1. What do you already know about charged particles? Like charges repel. Opposite charges attract. Electric charges exert a."— Presentation transcript:

1 Electric Fields Chapter 14.1

2 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).

3 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:

4 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

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

6 What exactly is a Coulomb? A measure of electric charge equal to the charge of 6.25 x 10 18 electrons Therefore 1 electron (or proton) =1.60x10 -19 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.

7 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)

8 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.

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

10 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

11 Practice Problems Page 638 –1-5

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

13 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]

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

15 Fab = 0.01N Fac = 0.00144N Fad = 0.00212 N Answer: 9.49 x 10 -3 N [69.9’]


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