Q: Two wires A and B are made of the same material. Wire A has a diameter of $$0.8\ \mathrm{cm}$$ and wire B has a diameter of $$1.6\ \mathrm{cm}. If $$the electron drift speed in wire A is $$3.2 \times 10$$^{-4}$$\ \mathrm{m/s}$$, what is the drift speed in wire B?

$$8.0 \times 10$$^{-5}$$\ \mathrm{m/s}$$

Q: A wire carrying a current of $$10\ \mathrm{A} is $$bent into a circular loop of radius $$0.1\ \mathrm{m}. $$What is the magnetic field at the center of the loop?

$$12.56 \times 10$$^{-5}$$\ \mathrm{T}$$

Question 1

Two point charges, $$q_1 = 2.0$$\ \mathrm{nC}$$ and q_2$ = 2.2\ \mathrm{nC}$$, are placed as shown in the diagram. Calculate the electric field at point $$p in $$component form. (Assume $$k = 9.0 	imes 10^9$\ \mathrm{N\cdot m^2$/C^2$$}$.)

Accepted Answer

$$(-20\ \mathbf{i} + 0.4\ \mathbf{j})\ \mathrm{kN/C}$$

Question 2

A pyramid-shaped object has four triangular faces of area $$A$$ and a base of area $$1.2A. $$The electric fields through the faces are as shown. Using Gauss' Law, does the pyramid contain a net positive, negative, or zero charge?

Accepted Answer

Positive charge

Question 3

What is the potential energy in electron volts (eV) of an electron-positron pair separated by $$0.051\ \mathrm{nm}$$?

Accepted Answer

$$-38.6\ \mathrm{eV}$$

Question 4

Why is lightning considered an example of dielectric breakdown?

Accepted Answer

Because air becomes conductive under high electric fields leading to a discharge.

Question 5

A defibrillator operates at $$1280\ \mathrm{V}$$ and delivers a current of $$18.8\ \mathrm{mA}$$ for $$50.0\ \mathrm{ms}. $$What are the power and energy delivered per pulse?

Accepted Answer

$$68.1\ \mathrm{W};\ 1.36\ \mathrm{J}$$

Question 6

Two wires A and B are made of the same material. Wire A has a diameter of $$0.8\ \mathrm{cm}$$ and wire B has a diameter of $$1.6\ \mathrm{cm}. If $$the electron drift speed in wire A is $$3.2 	imes 10$$^{-4}$$\ \mathrm{m/s}$$, what is the drift speed in wire B?

Accepted Answer

$$8.0 	imes 10$$^{-5}$$\ \mathrm{m/s}$$

Question 7

Which voltage vs. position graph correctly represents the circuit shown, traced clockwise from point A?

Accepted Answer

Graph 1 (first image after the circuit)

Question 8

A wire carrying a current of $$10\ \mathrm{A} is $$bent into a circular loop of radius $$0.1\ \mathrm{m}. $$What is the magnetic field at the center of the loop?

Accepted Answer

$$12.56 	imes 10$$^{-5}$$\ \mathrm{T}$$

Question 9

Which factor determines the direction of the magnetic force between two moving charges?

Accepted Answer

The sign of the charges and their direction of motion

Question 10

Two cylindrical coils are placed within one another. The outer coil has radius $$r_1$$ and $$n_1$$ turns per unit length; the inner coil has radius $$r_2$$ and $$n_2$$ turns per unit length. What is the mutual inductance per unit length?

Accepted Answer

$$n_2$$ \$$mu_0$$ $$n_1$$ \pi $$r_2^2$

Question 11

What is the phase difference between voltage and current in an ideal inductor?

Accepted Answer

Voltage leads current by 90 degrees.

Question 12

A resistor dissipates $$4.0\ \mathrm{W} of $$power when the rms voltage is $$15\ \mathrm{V}. $$What rms voltage is needed for the resistor to dissipate $$12\ \mathrm{W}$$?

Accepted Answer

$$34\ \mathrm{V}$$

Question 13

What is the characteristic behavior of voltage in a capacitor circuit?

Accepted Answer

Voltage lags current by 90 degrees.

Question 14

A space station uses a $$7\ \mathrm{W}$$ red laser (diameter $$4\ \mathrm{mm}) to $$deflect an $$80\ \mathrm{kg}$$ object $$20\ \mathrm{m}$$ away. The station has a mass of $$300,000\ \mathrm{kg}. (a) $$What is the radiation pressure force if the object perfectly reflects the light? (b) What is the gravitational pull? (c) Which force is greater?

Accepted Answer

Radiation pressure force: $$4.7 	imes 10$$^{-8}$$\ \mathrm{N}$$; Gravitational Pull = $$4 	imes 10$$^{-6}$$\ \mathrm{N}$$; $$F_G \u003e F_{RP} by a $$factor of 85

Question 15

Light from a monochromatic bulb illuminates a hole $$2.50\ \mathrm{m}$$ from the source. The bright patch on a wall $$1.50\ \mathrm{m}$$ behind the hole has a diameter of $$0.80\ \mathrm{mm}. $$What is the width of the hole?

Accepted Answer

$$2.00\ \mathrm{mm}$$

Question 16

A turtle in a spherical glass tank of radius $$25\ \mathrm{cm}$$ sees a dog's nose $$10\ \mathrm{cm}$$ from the outside edge. From the turtle's perspective, how far is the nose from the outside edge? (Ignore glass thickness.)

Accepted Answer

$$-18\ \mathrm{cm}$$

Question 17

A music practice room has two doorways, each $$1.1\ \mathrm{m}$$ wide and separated by $$3.1\ \mathrm{m}. A$$ $$512\ \mathrm{Hz}$$ tuning fork is struck outside. (i) Find all angles $$	heta at $$which no sound is heard inside. (ii) Will a student standing as shown have an explanation for not hearing the sound?

Accepted Answer

i. $$	heta = 10.2^\circ, 20.5^\circ, 30.8^\circ, 41.1^\circ, 51.4^\circ$$; ii. Yes, the student is standing at an angle of destructive interference.

Question 18

Ignoring the antineutrino, the decay of $$^{60}\mathrm{Co} at $$rest into $$^{60}\mathrm{Ni}$$ and an electron is $$^{60}\mathrm{Co} \$$rightarrow ^{60}$$\mathrm{Ni} + e. $$Which equation represents conservation of relativistic momentum for this decay?

Accepted Answer

$$\gamma_e m_e v_e = \gamma_N m_N v_N$$

Physics Final Exam Study Guidance – Electric Field, Gauss' Law, and More

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