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:

221

rqkqFe

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 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 102 N [E 32.6’ S]

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: 9.49 x 10 -3 N [69.9’]