AP Unit III

1
AP Unit III

• 2. Electric Field and Electric Potential

2

• a) Students should understand the concept of
  electric field, so they can
• (1) Define it in terms of the force on a test
  charge.
• (2) Describe and calculate the electric field of
  a single point charge.
• (3) Calculate the magnitude and direction of the
  electric field produced by two or more point
  charges.
• (4) Calculate the magnitude and direction of the
  force on a positive or negative charge placed in
  a specified field.
• (5) Interpret an electric field diagram
• (6) Analyze the motion of a particle of specified
  charge and mass in a uniform electric field.

3

• E F/Q V/d
• E kQ/r2 k 1/4pe0
• Calculate the electric field of an electron at a
  distance of 5 nanometers and 5 centimeters from
  the center.

5nm
x
5cm
x
4

• Calculate the electric field between 2 protons a
  distance of 2 cm apart at a point 5 mm from the
  first charge.

5

• Calculate the electric field in the center of a
  square and the center or each side of 4 charges
  of equal but opposite charge.
• If a positive charge was placed in the middle in
  which direction would it move?

1C
-1C
-1C
1 C
6
Millikans Oil Drop Experiment
V
FE QE
d
Fw mg
0V
7

• Two forces balance force due to gravity (mg)
  and electric force (QE)
• Electric Field Strength E V/d
• Therefore if the drop is stationary the two
  forces are in equilibrium (Fw FE)
• Thus mg QE QV/d

8

• If V is 1 000 V and d is 5 cm, and g is 10 m/s2
  and m is 1mg, calculate q.

9

• Millikan found that most of the charges recorded
  were multiples of 1.6 x 10-19 C
• If there was only one electron on our drop, what
  would the voltage have to be in order to
  stabilize it in mid air?