Download Physics 2145 Spring 2015 Test 1 (4 pages)

Physics 2145
Spring 2015
Test 1
(4 pages)
Name:_________________ February 20, 2015
Total Score:_________/120__
Constants: electron mass me = 9.11 x 10-31 kg
proton mass mp = 1.67 x 10-27 kg
e = 1.602 x 10-19 C
k = 9.0 x 109 N•m2/C2
𝐹=𝑘
∣𝑞1 𝑞2 ∣
𝑟2
𝑄
ε0 = 8.85 x 10-12 C2/N•m2
𝐸=𝑘
∣𝑞 ∣
𝑟2
𝐴
𝐹⃗ = 𝑞𝐸⃗⃗
𝑈=𝑘
𝑞1 𝑞2
𝑟
𝑞
𝑉 = 𝑘𝑟
𝑈 = 𝑞𝑉
𝑈 = ½𝑄𝛥𝑉 = ½𝐶(𝛥𝑉)2 = ½
𝐶 = 𝛥𝑉
𝐶 = 𝜅𝜀0 𝑑
𝛥𝑉 = 𝐸𝑑
𝐾 = ½𝑚𝑣 2
𝛥𝐾 = −𝑞𝛥𝑉 parallel: 𝐶𝑒𝑞 = ∑𝑖 𝐶𝑖
series:
1
𝐶𝑒𝑞
𝑄2
𝐶
1
= ∑𝑖 𝐶
𝑖
____1.(5) A proton enters the region of uniform electric field between two
charged plates with an initial velocity directed parallel to the plates, as
shown in the figure. It is deflected to the left. The direction of the electric
field is ____, and the electric potential al A is _______ than the
potential at B.
A) right, larger
B) right, smaller
C) left, larger
D) left, smaller
A
____2. (5) Which of the following statements is FALSE?
A) The electric field vector points from higher to lower potential.
B) The equipotential surfaces are perpendicular to the electric field vector.
C) In electrostatic equilibrium, different points on the surface of a conductor can be at different
values of electric potential.
D) In electrostatic equilibrium, the electric field inside a conductor is zero.
____3. (5) A dipole is placed in a uniform electric field. The dipole moment is the vector which
is pointing from the negative to the positive charge. Which of the following is true?
A) The dipole never experiences a net force.
B) The dipole never experiences a net torque.
C) The dipole only experiences a net torque if the dipole moment is parallel to the electric field.
D) The dipole experiences a net force if the dipole moment is perpendicular to the electric field.
____4. (5) If the potential difference across a capacitor in doubled, the capacitance
A) doubles
B) remains unchanged
C) halves
D) decreases by a factor of four
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Physics 2145 Test 1 Spring 2015
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B
5. (20) A parallel plate capacitor consists of two circular plates of radius 5 cm that are spaced
2mm apart with air between the plates. The electric field between the plates is 3.6×105 V/m.
a) (5) Calculate the capacitance.
b) (5) Calculate the potential difference between the plates.
c) (5) Calculate the amount of charge stored on each disk.
d)(5) Calculate the energy stored in the capacitor.
6.(20) The potential difference between two parallel plates is 800V. An electron is launched from
the negative plate. It strikes the positive plate with a speed of 2.0×107 m/s. What was the
electron’s speed as it left the negative plate?
f)(5) By how much, in eV, did the kinetic energy of the electron change during this process?
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7. (20) For the capacitor circuit in the figure,
C1 = 6 pF, C2 = 5 pF, C3 = 15 pF, and C4 = 20 pF.
a)(15) Find the equivalent capacitance.
A
5 pF
6 pF
20 pF
B
b) (5) If the applied voltage Vab = 20V, find the total charge on the system.
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15 pF
8. (40) Two charges, q1= +2.0nC and q2= - 8.0nC,
are located as shown in the figure. Charge q1 is
located on the positive y-axis at y=6.0cm, q2 is in
the first quadrant at x= 6cm, y=6cm. Point O is at
the origin.
y
q 1 = +2.0nC
q 2= 8.0nC
a)(5) Draw the electric field vectors created by each
of the charges at point O.
b)(10) Calculate the magnitudes of the electric fields
created by each of the charges at point O.
O
6.0cm
x
c)(10) Calculate the x- and y- components of the net electric field at point O.
d)(5) Calculate the magnitude of the net electric field at point O.
e)(5) Calculate the electric potential at point O
e)(5)A +2.0nC charge is placed at point O. What is the magnitude of the electric force this charge
experiences?
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