Electric forces and electric fields

1
Electric forces and electric fields
2

• 1Proprieties of electric charges
• (Give an ex. of electrically charged)
• Electric charge can be or
• Like charges repel one another and unlike
  charges attract one another
• Electric charge is always conserved
• The object become charged because charge is
  transffered from one object to another
• An object may have charge of e, 2e, 3e
• e 1.60219x10-19C
• SI unit C (Coulomb)

3

• 2 Insulators and conductors
• In conductors, electric charges move freely in
  response to an electric force. All other
  materials are called insulators (give an ex. of
  each)
• Semiconductors are between conductors and
  insulators.
• An object connected to a conducting wire buried
  in the Earth is said to be grounded.
• Induction charging of a conductor

4

• Charging an object by induction requires no
  contact with the object inducing the charge.
• 3. Coulombs Law
• An electric force has the following properties
• It is directing along a line joining the two
  particles and is inversely proportional to the
  square of the separation distance r, between them

5

• It is proportional to the product of the
  magnitudes of the charges, q1and q2, of the 2
  particles
• It is attractive if the charges are of the
  opposite sign, and repulsive if the charges have
  the same sign
• The magnitude of the electric force
• Fke (q1q2/r2)
• ke Coulomb constant
• ke 8.9875x109N m2/C2

6

• 4. Electric Field
• The electric field E produced by a charge Q at
  the location of a small test charge qo is
  defined as the electric force F exerted by Q and
  qo divided by the charge qo .
• EF/qo Eke (q/r2)
• Si unit N/C

7

• Pb. Strategies
• 1. Draw a diagram of the charges
• 2. Identify the charge of interest
• 3. Convert all units in SI
• 4. Apply Coulombs Law
• 5. Sum all the x- components of the resulting
  electric force
• 6. Sum all the y-components of the resulting
  electric force
• 7. Use Pythagorean theorem to find the magnitude
  and the direction of the force

8

• 5. Electric field lines
• 1. The electric field E is tangent to the
  electric field lines at each point
• 2. The number of lines per unit area through a
  surface perpendicular to the lines is
  proportional to the strength of the electric
  field in a given region
• Rules for drawing electric field lines
• -The lines for a group of point charges must
  begin on charge and end on charge
• - The number of lines drawn leaving a charge or
  ending a charge is proportional to the
  magnitude of the charge
• - No two field lines can cross each other

9

• 6. Conductors in electrostatic equilibrium
• When no net motion of chartge pccurs within a
  conductor, the conductor is in electrostatic
  equilibrum
• 1. the electric field is zero inside of the
  material
• 2. any excess charge on an isolated conductor
  resides entirely on its surface
• 3. the electric field just outside a charge
  conductor is perpendicular to the conductors
  surface
• 4. On an irregularly shaped conductor , the
  charge accumulates at sharp points, where the
  radius of curvature of the surface is smallest

10

• 9. electric flux and Gausss Law
• The electric flux ( the number of the field
  lines) is proportional to the product of the
  electric field and surface of the area
• FE EA
• FE EA cos?
• For a close surface, the flux line passing into
  the interior of the volume are negative, and
  those passing out of the interior of the volume
  are positive

11

• E ke q/r2
• A 4pr2
• FE EA4p ke q
• Permittivity of free space
  eo1/4p ke 8.85x10-12C2/N m2
• Gausss Law
• FE q/ eo
• The electric flux through any closed surface is
  equal to the net charge inside the surface
  divided by the permittivity