Question 1

A system consists of three point charges arranged in a triangle. The total potential energy of the system is given by $$U_{Tot} = -\frac{kq^2$}{2d}. $$What does the negative sign in this expression physically represent?

Accepted Answer

The configuration is bound; work must be done to separate the charges to infinity.

Question 2

An electron is accelerated from rest through a potential difference of 1500 V. What is its final speed? (Given $$e = 1.60 	imes 10$$^{-19}$$ C, m_e = 9.11$$ 	imes $$10^{-31} kg)$$

Accepted Answer

$$v = 2.30 	imes 10^7$ m/s

Question 3

A proton is placed at the origin in the presence of two point charges as shown. The potential energy of the proton is calculated using $$PE = q_p \varphi_{Tot}. If$$ $$\varphi_{Tot} = 6.08476 	imes 10$$^{-10}$$ V and q_p = 1.6$$ 	imes $$10^{-19} C$$, what is the potential energy?

Accepted Answer

$$9.74 	imes 10$$^{-29}$$ J

Question 4

A copper wire of length 178 m and resistivity $$1.72 	imes 10$$^{-8}$$ \, \Omega \cdot m$$ carries a current of 2.00 A when connected to a 1.20 V battery. What is the resistance of the wire?

Accepted Answer

$$0.6 \, \Omega$$

Question 5

Refer to the circuit with a 20 V battery, a 10 $$\Omega$$ resistor in series with a parallel combination of 5 $$\Omega$$ and 15 $$\Omega$$ resistors. What is the equivalent resistance of the circuit?

Accepted Answer

$$13.75 \, \Omega$$

Question 6

In the same circuit, what is the total current supplied by the battery?

Accepted Answer

$$1.45 A$$

Question 7

For the circuit above, what is the current through the 5 $$\Omega$$ resistor?

Accepted Answer

$$1.09 A$$

Question 8

A 15.0-$$\muF $$capacitor is charged to 50.0 V and then disconnected from the battery. A dielectric with $$k=5.00 is $$inserted. What is the new capacitance?

Accepted Answer

$$75.0 \, \mu F$$

Question 9

What is the potential difference across the capacitor after the dielectric is inserted (with the battery disconnected)?

Accepted Answer

$$10 V$$

Question 10

A capacitor ($$C = 20.0 \, \mu F) is $$charged in an RC circuit ($$R = 0.100 \, M\Omega$$, $$V = 60.0 V). $$What is the charge on the capacitor 8.00 s after closing the switch?

Accepted Answer

$$1.20 	imes 10$$^{-3}$$ C

Question 11

In an RC charging circuit, the voltage across the capacitor as a function of time is given by $$\Delta V_C = V(1 - e$$^{-t/	au}$$). If$$ $$	au = 2.00 s$$, what is $$\Delta V_C at$$ $$t = 8.00 s $$for $$V = 60.0 V$$?

Accepted Answer

$$58.9 V$$

Question 12

Which equation correctly relates the work done to assemble a system of point charges to the system's potential energy?

Accepted Answer

$$W = U_{Tot}$$

Question 13

A 15.0-$$\muF $$capacitor is charged to 50.0 V. What is the energy stored in the capacitor before inserting a dielectric?

Accepted Answer

$$0.0188 J$$

Question 14

In the calculation of the electric field at the origin due to two point charges, why is the $$y-$$component of one field negative?

Accepted Answer

Because the field points in the negative $$y-$$direction due to the geometry of the charge placement.

Question 15

A wire has a resistance $$R$$, length $$L$$, and cross-sectional area $$A. $$Which equation gives the resistivity $$\rho$$?

Accepted Answer

$$\rho = \frac{RA}{L}$$

Question 16

If the total electric field at the origin due to two charges is $$5.46 	imes 10$$^{-10}$$ N/C at 47.3$$^\circ$$ below the x-axis, which of the following best describes the vector addition process used?

Accepted Answer

Sum the $$x$$ and $$y$$ components of each field, then use the Pythagorean theorem and arctangent to find magnitude and direction.

Practice Exam 2 – Step-by-Step Physics Guidance

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