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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:
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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 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’]