Module 11 The Electric Field

1
Module 11 The Electric Field

• Serway Faughn Chapters 15 16

2
Module Study Objectives

• Electric charge
• Electric force
• Electric field lines
• Electric potential
• Capacitance

3
The Electric Force

• A field force acting between charges
• Explains many natural phenomena
• Basis for electronic technology

4
Electric Charges

• Unlike charges attract, like charges repel
• Electric charge is conserved
• Charge is quantised
• Electric force varies with inverse square of
  distance

5
Conductors

• Materials in which charges move freely
• Eg. Conducting paths of metal in a circuit board

6
Insulators

• Materials which do not readily transport charge.
• Eg. Plastic, rubber.

7
Coulombs Law

• The electric force depends on charges q1 and q2
  and separation r.

8
Example

• Compare the electric and gravitational forces
  between the proton and electron of a hydrogen
  atom (separation 5.3x10-11 metre)

9
Magnitude of The Electric Field

• Magnitude defined by force and charge.
• Direction defined by direction of electric force
  on a positive charge.

10
Electric Field of Point Charge

• Magnitude depends of value of point charge q and
  distance from point r.

11
Electric Field Lines

• For describing electric field in any region.
• Electric field vector is tangent to the lines.
• Number of field lines per unit area is
  proportional to the field strength.

12
Conductor in Electrostatic Equilibrium

• Field is zero inside the conductor.
• Excess charge resides on surface.
• Field just outside conductor is perpendicular to
  surface.
• On an irregularly shaped conductor charge
  accumulates at sharpest points.

13
Electric Potential

• The Coulomb force is conservative and an electric
  potential energy can be defined.
• Electric potential is the potential energy per
  unit charge.

14
Difference in Electric Potential

• Defined by the difference in potential energy
  experienced by a charge.

15
Electric Potential and Field

• Electric potential difference between two points
  depends on electric field strength and distance
  between points.

16
Electric Potential Due to a Point Charge

• Depends on charge and distance from it.
• Electric circuits usually take Earth as zero
  potential.

17
Electric Potential Energy of a Pair of Point
Charges

• Depends on product of charges and separation.
• Units of Joules of electron volts

18
Charged Conductors

• Every point on the surface of a charged conductor
  in electrostatic equilibrium is at the same
  potential.
• Furthermore, the potential is constant everywhere
  inside the conductor and equals its value on the
  surface.

19
Capacitors

• Capacitors store and release charge.
• Uses in TVs, radio, camera flash units, heart
  defibrillators, computer keyboards, etc.

20
The Parallel-plate Capacitor

• The parallel-plate capacitor has equal and
  opposite charges.
• A potential difference occurs between the plates.

Q
?V
-Q
21
Capacitance Formula

• Capacitance is defined in terms of charge and
  potential difference

22
Parallel Plate Capacitor Formula

• The capacitance of two parallel metal plates
  depends on area and separation.

23
Capacitors in Parallel

• The equivalent capacitance of a parallel
  combination is given by the addition of the
  individual capacitances.

24
Capacitors in Series

• Capacitors in a series circuit are added
  according to their inverses.

25
Capacitor Energy

• The energy stored in a charged capacitor is a
  function of the charge, potential difference and
  capacitance.

26
Dielectrics

• A nonconducting dielectric multiplies capacitance
  by the dielectric constant, a property of the
  dielectric material.

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Summing up

• A discussion of the electric force leads to
  expressions for electric field, potential energy
  and potential.
• A discussion of capacitors, which are devices
  that store charge, leads to expressions for the
  capacitance of the common parallel-plate
  capacitor and for capacitor combinations in
  circuits.

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End of Module

• Any questions?