Five Chapter 3

Electric Fields

• Electric and gravitational forces occur without contact. In fact, all forces occur without contact.
• These forces are explained by the presence of fields.
• Fields:
  • The region of space where an effect can be detected.
  • The value of an effect at a point in space.
  • The set of all values for an effect.
• Essentially the field transmits the effect (force)
• Mass produces a gravitational field (a curve in space).
• Electric charges produce electric fields.
• A mass inside a gravitational field experiences a gravitational force and an electric charge inside of an electric field experiences an electric force.
• We represent fields by drawing a series of force vectors. For a gravitational field, the vectors point towards the object producing the field. For an electric field, we need a rule for defining the direction, since we can have both positive and negative charges.
• Direction of an electric field – The same as the direction of the force on a positive test charge placed in the field.

• For A, at every point surrounding A, a positive test charge will be repelled. Therefore we draw the force vectors as shown.
• For B, the vectors are drawn as shown, because a positive test charge at any place in space will be attracted to B.
• The amount of space between the field lines indicates relative intensity of the field (The closer the lines, the stronger the field).
• Electric Field Intensity – The electric force per unit positive charge.

• Units of E are N/C

• The equation gives the magnitude of the electric field.
• positive values of E indicate the field points away from the charge
• negative values of E indicate the field points towards the charge
• Because electric fields are vector quantities, then when more than one charge is present, the electric field is the vector sum of the individual fields.
• Besides multiple charges distorting electric fields, large objects may also distort the shapes of the fields.
  • E.g. Two large parallel plates
• For oppositely charged plates, each plate can be considered to be made up of n point charges.
• The electric field between the plates is then the vector sum of the fields made from the 2n point charges. Calculus is required to find the sum of the 2n fields from the 2n charges.

• Generally
  • Field intensity outside the plates is about zero
    • The field is constant between the plates. The field lines are straight and equally spaced, and perpendicular to the plates.
    • The magnitude of the field depends only on the magnitude of the charge on each plate.
  • Uniform electric field – An electric field that is constant in both magnitude and direction.
• Parallel plates produce uniform electric fields
• Fields around like and oppositely charged spheres are also distorted.

• Here is an example of using the electric field equation.

December 6, 2013