Answer Key
Electrostatics with Key
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University:
Purdue University -
Course:
PHYS 24100 | Electricity And Optics Academic year:
2024
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Views:
199
Pages:
8
Author:
Lilyana Hurley
Electrostatics Review Package
1. Which diagram shows the electric field near a negative point charge?
A. electric field lines pointing outwards from a point charge
B. electric field lines pointing inwards towards a point charge
C. electric field lines radiating outwards from a point charge
D. electric field lines converging towards a point charge
2. Which pair of values will cause the greatest deflection of an electron beam in a cathode ray tube?
ACCELERATING VOLTAGE DEFLECTION (PLATE) VOLTAGE
A. 400 V 20 V
B. 400 V 40 V
C. 800 V 20 V
D. 800 V 40 V1
3. The magnitude of the net electric field at P in the diagram below is 5.0 x 10^3 N/C.
E_net = E_1 + E_2
(5.0 x 10^3)^2 = (kQ_1/R_1^2)^2 + (kQ_2/R_2^2)^2
Q_1 = 4.0 x 10^-6 C
Q_2 = 3.0 x 10^-6 C
Find the magnitude of charge Q_2.
diagram with two point charges and a point P
4. a) Find the electric potential energy at point A and at point B. (Note: 1.0 µC is 1.0 x 10^-6 C)
Q = -15.0 µC
V_A = kQ/r_A
= (9 x 10^9)(-15 x 10^-6)/3
= -4.5 x 10^4 V
V_B = kQ/r_B
= (9 x 10^9)(-15 x 10^-6)/5
= -2.7 x 10^4 V
b) What is the potential difference between A and B?
ΔV = V_B - V_A = -2.7 x 10^4 V - (-4.5 x 10^4 V) = 1.8 x 10^4 V
c) 0.036 J of work must be done to move a charge from A to B. Find the magnitude and polarity of this charge.
ΔV = ΔE/Q
Q = ΔE/ΔV
= 0.036 J / 1.8 x 10^4 V
= 2.0 x 10^-6 C
5. The diagram shows the electric field lines near two point charges, L and R. Identify the polarity of these point charges.
electric field lines around two point charges
**Polarity of L Polarity of R**
A. Negative Negative
B. Negative Positive
C. Positive Negative
D. Positive Positive
6. An electron orbits the nucleus, which carries a charge of +9.6 x 10^-19 C. If the electron's orbital radius is 2.0 x 10^-10 m, what is its potential energy?
E_p = kQq/r
= (9 x 10^9)(-1.6 x 10^-19)(9.6 x 10^-19) / (2.0 x 10^-10)
= -6.9 x 10^-18 J
7. Two charges are positioned as shown in the diagram below.
Q1 = 8.0 µC
Q2 = -2.0 µC
A * 4.0 m * 2.0 m * B
a) Find the magnitude and direction of the electric field at A.
E1 = kQ1/r1^2 = 2000 N/C (to the right)
E2 = kQ2/r2^2 = 4500 N/C (to the left)
E_net = E1 - E2 = -2500 N/C (to the left)
b) A charge placed at A experiences a force of 4.0 x 10^3 N towards the right. What are the magnitude and polarity of this charge?
Q = F/E = 1.6 x 10^-6 C (positive)
8. In a cathode ray tube:
A. protons are accelerated from anode (positive) to cathode (negative)
B. protons are accelerated from cathode (negative) to anode (positive)
C. electrons are accelerated from anode (positive) to cathode (negative)
D. electrons are accelerated from cathode (negative) to anode (positive)
9. Charge Q2 is located 5.0 m from charge Q1 as shown:
Q1 = 2.0 x 10^-6 C
Q2 = 5.0 x 10^-6 C
+ 2.0 m + 3.0 m +
Q1 * * * * * Q2
How much work must be done to move charge Q2 2.0 m closer to charge Q1?
W = -ΔU = -kQ1Q2(1/r2 - 1/r1)
= 1.2 x 10^-2 J
10. An electron passing between parallel plates 0.025 m apart experiences an upward electrostatic force of 5.1 x 10^-16 N.
electron between two parallel plates
a) What is the magnitude of the electric field between the plates?
E = F/q = 3.2 x 10^3 N/C
b) What is the potential difference between the plates?
ΔV = Ed = 80 V
c) On the diagram below draw in the connections to the power supply necessary for the electron to experience this upward force. 1
diagram showing a parallel plate capacitor with an electron between the plates
11. Which of the following diagrams shows the electric field between two equal but opposite charges?
four diagrams showing electric field lines between two charges
12. A proton initially at rest is accelerated between parallel plates through a potential difference of 700 V.
ΔE = qΔV = 1/2 mv^2
v = √(2qΔV/m) = 3.7 x 10^5 m/s
What is the maximum speed reached by the proton?
diagram showing a proton being accelerated between two parallel plates
Electrostatics with Key
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