Electric Charges

1
Electric Charges

• Conduction Transfer of a charge easily.
• Induction Influence transfer of a charge.
• (polarization of a charge)
• Insulator Does not transfer a charge easily.
• Semiconductor Inhibited transfer of a charge.

2
Electric Charges

• Sources of electric charges is
• The Atom
• Protons () constant
• Electrons (-) variable
• Loss electrons result in a net () charge
• Gain electrons result in a net (-) charge
• The SI unit for a charge is coulomb, C.
• One coulomb 6.24 x 1018 electrons or protons

3
Coulombs Law

• Defines the force acting between two charged
  bodies.
• (explains gravitational, magnetic and electric
  forces)
• Felectric Kc(q1q2) Kc Coulomb constant
• r2 9.0 x 109 Nm2/C2
• Electric force that acts between two bodies is
  equal the produce of the charges on the bodies
  and inverse the square of the distance between
  the bodies.

4
Coulombs Law

• In Coulombs Law the force is
• a vector quantity
• vector act along a line between the bodies .
• force applies to a point charge.
• distance between the bodies in from center of
  mass to center of mass.

5
Samples

• Consider the force F1 F and F2 -F acting on
  two charged particles separated by a distance, d.
  Explain the following
• a. the distance between the particles doubles.
• b. the charge on one particle doubles
• c. the charges on each particle doubles.
• d. the charge on each and the distance between
  doubles.

6
Sample

• 2. Which is the stronger force electrical force
  or gravitational force?
• Why?

7
Electrical v. Gravitational

• Electrical Gravitational
• attract and repel attractive only
• objects are or - mass always
• stronger weaker
• overcome gravity no match for
• easily electrical force

8
Sampler Coulombs Law

• 3. The electron and the proton of a hydrogen atom
  are separated, on average, by a distance of 5.3 x
  10-11 m. What is the electrical force acting
  between the two charged subatomic particles. The
  charge on each is respectively -/ 1.60 x 10-19
  C.

9

• 4. A negative charge of -2.0 x 10-4 C and a
  positive charge of 8.0 x 10-4 are separated by
  0.3 m. What is the force acting between the two
  charged bodies?

10

• 5. A balloon rubbed against wool gains a charge
  of -8.0 µC. What is the force between the balloon
  and the wool when the two are separated by a
  distance 5 cm?

11

• 6. A negative charge of -6 x 10-6 C exerts a
  force of 95 N on a second object when each are
  0.05 m apart. What is the magnitude of the second
  object?

12

• 7. Two electrostatic point charges of 60 µC and
  50 µC exert a repulsive force of 175 N. What is
  the distance between the two charges?

13
Vector Sum of Electrical Force

• Review vector addition from Section 3-2
• The resultant force on a single charge equals the
  vector sum of the individual forces exerted on
  that charge by all the other individual charges
  that are present.

14
Vector Sum of Electrical Forces

• Steps
• 1. Find the force exerted between individual
  bodies using Coulombs Law.
• 2. Use the charge to define the direction.
• 3. Find the x and y components.
• 4. Add up the total x and y force component
• 5. Use the Pythagoream Theorum to find the
  resultant.
• 6. Use tan to find direction Tan q
  Fytotal/Fxtotal

15
Sample

• 3. Four equal charges of 1.5 mC set at each
  corner of a square 5 cm on each side. Find the
  net force on a 5th charge placed in the center of
  the square if that 5th charge is
• -1.5 mC
• 3.0 mC

16
Linear Vector Forces

• Forces in the same direction, add magnitudes
• Forces in the opposite directions, subtract
  magnitudes
• Note Focus on the point charge in question

17
Sample

• 4. Three point charges of 5 mC at x 0m, 2 mC
  at x 3m, and -4 mC at x 5m. What is the net
  force acting on the point charge at x 3m?

18
Equilibrium

• Forces acting on a point charge will balance or
  reach a point where the net charge is equal to
  zero.
• Depends on the point charge and the magnitude of
  that defines the force.

19
Sample

• 6. Will a point of equilibrium be reached if
• point charge between and
• point charge between and
• point charge between and
• - point charge between and

20
Sample

• 7. Where will equilibrium be reached for
• Two 6 charge at x 0 and x 10 when a point
  charge is set between.
• Two charges of 4 and -2 at x 0 and x 10 when
  a point charge is set between.
• Two charges of -2 and -4 at x 2 and x 10 when
  a point charge is placed between.

21
Sampler

• Three charged bodies. q1 6µc at 0 m q2 - 2
  µc at 4m and q3 -1 µc at 6m. With respect to
  q2, what is the force total?

22
Electric Field

• Area of influence around a charged body.
• E Felectric / test charge (qo)
• NOTE
• Felectric Kc (q1 q2 / r2) Substitute
• Strength of E measured against the effect on qo
• Unit is N/C
• E is a vector

23
Electric Field

• 5. q is () radiates outward
• 6. q is (-) radiates inward toward

24
Sample

• 8. A proton has a charge of 1.6 x 10-19 C. A
  proton is moved into an electric field of 600
  N/C. What force does the proton experience?
• 9. The point within a electric field is to be
  4.5 x 10-6 c. The test charge is measured to be
  0.18 N at an angle of 20o. What is the magnitude
  and direction of the electric field strength?

25
Electric Field Lines

• Lines of influence radiating from a point charge
• 1. () outward
• (-) inward
• The number of line define the strength
• Like field line will NOT cross.

26
Sample

• 10. What conclusion can be reached about (a) and
  (b) on page 648.
• 11. Two point charges set some distance apart
• a. Draw the field lines with the () point twice
  that of the (-) point
• b. both charge are (-)
• c. both charges are equal () and (-)