Transcript Chapter 20 Lecture Notes 2011

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.
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
Coulomb’s 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.
Coulomb’s Law
In Coulomb’s 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.
Samples
1.
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.
Sample
2. Which is the stronger force: electrical force or gravitational
force?
Why?
Electrical v. Gravitational
1.
2.
3.
4.
Electrical
attract and repel
objects are + or stronger
overcome gravity
easily
Gravitational
attractive only
mass always +
weaker
no match for
electrical force
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.
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?
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?
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?
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?
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.
Vector Sum of Electrical Forces
Steps:
1. Find the force exerted between individual bodies using
Coulomb’s 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
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:
a.
-1.5 mC
b. +3.0 mC
Linear Vector Forces
1.
2.
Forces in the same direction, add magnitudes
Forces in the opposite directions, subtract magnitudes
Note: Focus on the point charge in question
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?
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.
Sample
6. Will a point of equilibrium be reached if:
a.
+ point charge between + and +
b. + point charge between + and –
c. + point charge between – and –
d. - point charge between – and –
Sample
7. Where will equilibrium be reached for:
a.
Two +6 charge at x = 0 and x = 10 when a + point
charge is set between.
b. Two charges of +4 and -2 at x = 0 and x = 10 when a +
point charge is set between.
c. Two charges of -2 and -4 at x = 2 and x = 10 when a +
point charge is placed between.
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?
Electric Field
Area of influence around a charged body.
E = Felectric / test charge (qo)
NOTE:
1. Felectric = Kc (q1 q2 / r2) Substitute
2. Strength of E measured against the effect on qo
3. Unit is N/C
4. E is a vector
Electric Field
5. q is (+) radiates outward
6. q is (-) radiates inward toward
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?
Electric Field Lines
Lines of influence radiating from a point charge
1. (+) outward
2. (-) inward
3. The number of line define the strength
4. Like field line will NOT cross.
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 (-)