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Transcript
AP Physics B
Electrostatics Review
name___________________
1. Describe the similarities and differences between the laws of gravity (Newton's
Universal Gravitation) and Coulomb's law of electrostatic force.(write equations)
2. State FOUR ways an object can be charged and explain each.
3. When a comb attracts a tiny piece of paper sometimes one piece jumps away from
the comb. Explain why.
4. Draw the electric field lines surrounding two NEGATIVE charges a distance d
apart.
5. Find the magnitude and direction of the force between two charges of 1.5
nanocoulombs (1 nc = 1 x 10-9 C) one millimeter apart.
5a. What would happen to this force if the charges were tripled and the distance
bwtween then were reduced to two thirds its original value?
6. What is the net force on an UNCHARGED insulating sphere in a UNIFORM
electric field?
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7. What is the net force on an UNCHARGED conducting sphere in a UNIFORM
electric field?
8. Two parallel conducting plates are connected to a voltage source. What happens
to the magnitude of the electric field if the voltage is doubled and the distance
between the plates is reduced by half?
9. A small conducting sphere holds charge +Q. Another sphere holding charge -5Q
is R centimeters away from the first sphere. The force between the charges is 25
Newtons. The charges are briefly brought into contact and then returned to their
original positions. What is the force between the charges now? Has its direction
changed?
10. What is true about the electric field inside a conductor? Why?
11. What is true about the direction of an electric field just outside a conductor?
12. A charge -Q is placed at the center of a spherical conducting shell. Draw the
pattern of electric field lines.
13. Two very large parallel metal plates 5 cm apart are oppositely charged with a
potential difference of 10,000 volts between them. As an electron is moved from
the negative to the positive plate
(a) what happens to the electric potential energy of the electron
(b) what happens to the electrostatic force on the electron
(c) what happens to the electric potential at the location of the electron
(d) calculate the electric field strength midway between the plates
(e) if the electron were released at the negative plate, what would be its speed
when it reached the positive plate?
2
14. Charges +Q and -Q are located a distance L meters apart as shown below:
L
+Q
-Q
P
(a) Find an expression for the electric field strength(and its direction) at a point
midway between the charges on the line connecting them.
(b) Find an expression for the electric field strength at point P shown above. P is
equidistant from the charges but lies a distance L/2 below them. What is the
direction of the field?
(c) Find an expression for the electric potential at a point midway between the
charges (same as part (a))
(d) Repeat part (c) with both charges equal to +Q.
15. Two particles of charge Q are located at the opposite corners of a square of side
d(draw it)
(a) what is the magnitude and direction of the electric field at each of the
other corners?
(b) What is the potential energy of a particle of charge +q held at these other
corners?
3
16. A 1 keV electron (mass 9.11 x 10-31 Kg) enters the region between two parallel
conducting plates as shown below(note 1 eV = 1.6 x 10-19 Joules) In this region
the electric field strength is 10,000 N/C. The plates are 5.0 cm apart:
+
+
e-
Find
(a) the potential difference across the plates
(b) which plate is at higher potential
(c) the force on the electron
(d) the acceleration of the electron toward the positive plate
(e) does the electron gain or lose energy as it moves between the plates? Explain
(f) the speed of the electron as it enters the region between the plates
(g) the vertical component of the electron's momentum vector after traveling 20
cm to the right between the plates.
(h) the angle the electron is traveling above the horizontal at this time
4
17. A charged pith ball of mass m = 1.2 grams is held by a string (to form a
pendulum) in a region of uniform electric field E = 5000N/C horizontal and to the
right. The charge on the pith ball is 1.5 x 10-7C. Find
(a) the electrostatic force on the pith ball
(b) DRAW the situation. Be careful about which way the string hangs.
(c) make a free body diagram on which you identify all the forces
(d) find the angle from the vertical at which this "pendulum" hangs.
(e) The string is cut. Describe the motion of the pith ball.
5
Capacitance Questions
+
-
Capacitors store electric charge on parallel conducting (metal) plates closely spaced
together. Usually they are rolled up with wax paper in between metal foil layers. The
amount of charge that can be stored is
Q = CV
where the constant C (for that capacitor) is called capacitance(unit "farad"). For a
parallel plate capacitor which has plates of area A separated by distance d the capacitance
is
C = o A/d
o is called the permittivity of free space. o = 8.85 x 10-12 C2/N m2
The energy stored on a capacitor is U = 1/2 QV = 1/2CV2 = 1/2 Q2/C
18. A certain capacitor has a capacitance of 50pf (picofarads 1pf = 1.0 x 10-12 farad)
(a) If the plates are separated by 1.0 x 10-4 meters, what is their area
(b) When connected to 5.0 volts, what charge can this capacitor hold?
(c) What energy can this capacitor store?
(d) What charge could this capacitor hold when connected to 10 volts?
(e) what energy could it hold when connected to10 volts
(f) what would happen if the capacitor was connected to much higher voltage,
say 1000V?
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