Physics 2054C

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Physics 2054C Spring 2002

• Its Good To Be Back!

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Housekeeping

• Course Web Page is online.
• My Office Hours
• Monday from 1000 to 1100.
• Tuesday from 100 to 200.
• Lab Sections.
• Let me know if you are in a lab section and need
  to change.
• Provide a 1st and 2nd choice.
• CAPA.
• Need names of those without CAPA logins.
• Other Questions?

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Getting Started CAPA
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Charge Conservation

• Total Charge remains the same.
• Charges are not created or destroyed
• Only rearranged.
• Is the universe electrically neutral?
• Equal numbers of protons electrons.
• Charge is quantized in units of the charge on the
  electron (proton)
• 1.602 x 10-19 C.

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Electric Forces (Coulombs Law)

• Like Charges Repel.
• Opposite Charges Attract.
• Responsible for Chemical Interactions.

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Coulombs Law Newtons Law

• Both apply
• Electrical forces cause accelerations.
• It is a vector.

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Electric Fields

• Related to the Electric Force.
• Removes the dependence on one of the charges.
• E F/Q

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Rules for Drawing Electric Field Lines

• The lines must begin on a positive charges and
  terminate on a negative charges (or at infinity).
• The number of lines drawn leaving a positive
  charge or approaching a negative charge is
  proportional to the magnitude of the charge.
• No two field lines can cross.

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Superposition

• Add the effects of multiple charges.
• Fnet F1 F2 F3 .
• Calculate each of the forces as if the other
  charges were not present.
• Enet E1 E2 E3 .
• Calculate each of the electric fields as if the
  other charges were not present.

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Sample Problem

• Two charges are sitting on a table separated by a
  distance of 0.80 m. The one on the right (which
  we'll call charge "A") has a positive charge,
  while the one on the left (which we'll call
  charge "B") has a negative charge. Charge A has
  a magnitude of 1.5 x 10-3 C, while the magnitude
  of charge B is 2.0 x 10-3 C.
• To determine the electric field caused by charges
  A and B, you must first calculate the individual
  contributions to the electric field by each of
  the charges.
• What is the direction of the electric field
  contribution caused by charge A at any point
  between charges A and B?

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Sample Problem (continued)

• What is the direction of the electric field
  contribution caused by charge A at any point
  between charges A and B?
• What is the direction of the electric field
  contribution caused by charge B at any point
  between charges A and B?

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Sample Problem (continued)

• What is the direction of the electric field
  contribution caused by charge A at any point
  between charges A and B?
• What is the direction of the electric field
  contribution caused by charge B at any point
  between charges A and B?
• What is the direction of the electric field
  (including the effects of both charges) at any
  point between charges A and B?

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Sample Problem (continued)

• What is the magnitude of the electric field
  contribution caused by charge A at a point that
  is 0.30 m from charge A and is on the line
  between charges A and B?

EA kQA/R2 9.0x109 Nm2/C2 x 1.5 x
10-3 C / (0.3 m)2 150 x 106 N/C
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Sample Problem (continued)

• What is the magnitude of the electric field
  contribution caused by charge B at this point?

EB kQB/R2 9.0x109 Nm2/C2 x 2.0 x
10-3 C / (0.5 m)2 72 x 106 N/C
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Sample Problem (continued)

• What is the magnitude of the electric field at
  the specified point?

Enet EA EB -(150 x 106 N/C 72 x 106
N/C) Enet -222 x 106 N/C In this case the
negative sign means the electric field is to the
left.
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Sample Problem (continued)

• If a negative charge (we'll call it charge "C")
  is placed between charges A and B, what is the
  direction of the electrostatic force on this
  charge?

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Sample Problem (continued)

• What is the magnitude of the force on charge C if
  it is placed at the point specified if C is 4.0 x
  10-3 C?

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Insulators Conductors

• Charges can move in conductors.
• Charges cannot move within insulators.

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CAPA 9

1. An electron (mass m 9.1110-31 kg) is
   accelerated in the uniform field E (E 1.33104
   N/C) between two parallel charged plates. The
   separation of the plates is 1.25 cm. The electron
   is accelerated from rest near the negative plate
   and passes through a tiny hole in the positive
   plate, as seen in the figure. With what speed
   does it leave the hole?

F qE ma a qE/m
v2 vo2 2a(x-xo) v2 2ax 2(qE/m)x
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CAPA 9 (continued)

1. An electron (mass m 9.1110-31 kg) is
   accelerated in the uniform field E (E 1.33104
   N/C) between two parallel charged plates. The
   separation of the plates is 1.25 cm. The electron
   is accelerated from rest near the negative plate
   and passes through a tiny hole in the positive
   plate, as seen in the figure. With what speed
   does it leave the hole?

v2 2ax 2(qE/m)x v2 2 ( 1.602 x 10-19 C
1.33x104 N/c / 9.11 x10-31 kg )
0.0125 m 58.7 x 1012 (m/s)2 ? v 7.6 x
106 m/s
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Next Time

• Brief Review for Quiz
• Forces from Electric Charges.
• Motion of electric charges.
• Chapter 17 - Electrical Energy