Electric Fields and Potential

1
Electric Fields and Potential

• Chapter 33

2
Electric Field Lines

• Electric fields have both magnitude and direction
  they are vectors
• The direction of the field lines is in the
  direction of force on a small positive test
  charge.

3
Electric Fields

• What are the fields around the following objects?

-

-














_
_
_
_
_
_
_
4
Electric Field Strength

• The electric field strength vector E at any point
  in an electric field is defined as the force per
  unit positive (test) charge placed at that point
  in the field
• EF/q
• SI Units NC-1

5
Electric Shielding

• Jeep Cherokee that was struck by lightning.

6
Electric Shielding

• If there is an electric field inside a conductor
  then the free electrons would distribute
  themselves to cancel the field
• There is no electric field in a conductor the
  charge is distributed around the outside

7
Electric Potential Energy

• To pull an electron away from an atom requires
  energy. It has electric potential energy.
• SI Units - Joules

-
-
8
Electric Potential

• A measure of how much energy each charge has.
• Electric potential electric potential
  energy charge
• SI Unit Joules/Coulomb volts (V)

Allesandro Volta (1745-1827)
9
Electric Potential

• How much work is required to move an electron
  across an electric potential of 100 volts?
• Notice how small a number this is. When dealing
  with sub-atomic particles we normally use a much
  smaller unit of energy.
• One electron volt (1 eV) is the work done when an
  electron moves through a potential difference of
  one volt.
• 1 eV 1.6x10-19 J

10
Electric Potential Questions

• Explain why passengers in a metallic aeroplane
  are not affected by the electric charge that
  builds up on its outside due to friction between
  air and the metal
• Does the Coulombic force exerted by one charge on
  another change if other charges are brought into
  their vicinity? Explain your answer.
• A proton is placed in the uniform field between
  two parallel oppositely charged plates. It is
  allowed to move and so accelerates towards the
  negative plate.
• Is the force on it constant during its movement?
• Does the attraction force on the proton increase
  as it nears the negative plate?
• Describe the energy changes that occur during the
  movement of the proton.
• What experiences the equal and opposite force as
  required by Newtons third law?
• Calculate the force acting on an electron in a
  hair as it comes within 1mm of a comb carry a
  static charge of 5.0x10-5C.
• How far apart in a vacuum must two charges of 1C
  be placed in order to produce a force of 1000N
  between them?
• Three spheres, each carrying a charge of
  1.00x10-11C are placed at the corners of a
  right-angled isosceles triangle, with equal sides
  of 10.0cm, as in the diagram. Find the resultant
  force acting on B.

10cm
10cm