Electric Fields

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Electric Fields

• Chapter 14.1

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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).

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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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Coulombs Law

• Is similar to Newtons Universal Law of
  Gravitation
• Uses the following quantities
• k Coulombs constant (Nm2/C2)
• k 8.99109 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

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Coulombs Law

• Calculates ELECTROSTATIC force
• Electric charges exert forces on each other

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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.

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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)

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Coulombs 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.

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Fg vs Fe

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

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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

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Practice Problems

• Page 638
• 1-5

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Example 2 3 Charges

• What is the net electrostatic force on B?

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• 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

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Example 3 4 Charged Particles

• Find Fnet on A
• Fab 0.01N
• Fac 0.00144N
• Fad 0.00212 N

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• Fab 0.01N
• Fac 0.00144N
• Fad 0.00212 N
• Answer 9.49 x 10 -3 N 69.9