Electrostatics

1
Electrostatics

• Part 3

2
Electric Field vs Gravitational Field

• The electric field can push AND pull charges
• Because there are two types of charges ( and -)
• The gravitational field can only pull
• Only positive masses

3
Cause of Electric Field

• Charges cause an electrical field.
• Each charge creates its own electric field in the
  surrounding space.
• Electric Field is a vector quantity
• Has magnitude and direction
• Use Electric Field lines(lines of force) to show
  the Electric Field

4
Drawing the Electric Field

• Electric field direction from charge that creates
  the field
• Away from positive charge
• Toward negative charges
• The lines begin on positive charges and end on
  negative charges.
• Draw lines perpendicular to charge
• Lines never cross

5
Calculating the Electric Field

• Electric field strength
• The closer to the charge, the stronger the field
• The closer the field lines, the stronger the
  field
• The farther away, the weaker the field

6
Calculating the Electric Field

• Electric field strength
• The number of lines are proportional to strength
• The more lines the stronger the field

7
Calculating the Electric Field

• Electric field strength (quantitative)
• E kq/d2
• Depends on Charge and distance
• E Electric field strength (Newton/Coulomb, N/C)
• k 9.0 x 109 Nm2/C
• q charge causing the field (Coulomb, C)
• d or r distance (meters, m)

8
Practice Problem 1

• A fly accumulates 3.0 x 10-10 C of positive
  charge as it flies through the air. What is the
  magnitude and direction of the electric field at
  a location 2.0 cm away from the fly?

q 3.0 x 10-10C d or r 2.0 cm _____ m E
? equation E kq/ r2 E (9.0 x
109)(3.0x10-10) (0.020)2 E 6750
N/C
0.020
9
Charges in Electric Field
Electric Field force exerted on positive
charge If q force in same direction (pull)
of the field If -q force in opposite
direction (push) of the field
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The electric field

• The strength of the electric field
• E F/q
• E strength of the field (Newton/Coulomb, N/C)
• F Force exerted on charge in field (Newton, N)
• q charge (Coulomb, C) that sits in the field
  experiences the force

11
Practice Problem 2

• A charge of -2.0 x10-6C is placed in a uniform
  electric field of strength 5000 N/C that points
  downward. What is the magnitude and direction of
  the force experienced by this charge?

12
Practice Problem 2
A charge of -2.0 x10-6C is placed in a uniform
electric field of strength 5000 N/C that points
downward. What is the magnitude and direction of
the force experienced by this charge?
E F / q
5000 N/C F / (-2.0 x 10-6C)
(-2x10-6 )(5000)F
-0.01NF magnitude of the force direction? - 2
charge, the force on the electron is opposite
the field!