PHY 122 homework, continued. (Reading refers to College Physics by OpenStax.) Sec. 5 - Magnetic Forces and Fields.

Read: Ch. 22, sections 1 – 4, 7, first part of 9, 10.

A. 1. (2 pts) An electron is moving in the +x direction in a magnetic field which also points in the +x

direction. Does it feel a force? If so, in what direction?

2. (8) The horizontal conductor shown is 50.0 cm long, weighs .196 N,

and is in a 3.60 T magnetic field which points into the page. What

current must exist in the conductor for the tension in the supporting

wires to be zero? What is the direction of the current?

ans: .109 amp toward the right.

B. 1. (2 pts) Consider a stationary charged plastic ball.

a. Does it create an electric field? _____ Does it feel a force in an electric field? _____

b. Does it create a magnetic field? _____ Does it feel a force in a magnetic field? _____

2. (8) A long straight wire carries a current of 10 A. An

electron is fired at a speed of 1.0 x 106 m/s parallel to the

wire in the direction of the current, from a point 2.0 cm

from the wire. Find the force on the electron, including

its direction.

ans: 1.6 x 10-17

N in the –y direction.

C. 1. (2 pts) Consider a copper wire which has a current in it.

a. Does it create an electric field? _____ Does it feel a force in an electric field? _____

b. Does it create a magnetic field? _____ Does it feel a force in a magnetic field? _____

2. (2 pts) Consider a beam of electrons in a vacuum tube.

a. Does it create an electric field? _____ Does it feel a force in an electric field? _____

b. Does it create a magnetic field? _____ Does it feel a force in a magnetic field? _____

3. (6) The magnetic field over a certain region is B = 4.00 T in the positive x direction. An electron

moves in this field with a velocity 7.00 m/s in the negative z direction. Find the force on the electron,

including its direction.

ans: 4.48 x 10-18

N, in the +y direction

D. 1. (2 pts) What is the direction of the force on the electron shown?

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2. (8) Two parallel wires, 70.0 cm long, lie 7.50 cm

apart on a tabletop. They both carry 17.0 A in the

+x direction. Find:

a. The magnitude of the magnetic field, B, at the

position of one wire due to the other.

b. The magnitude of the force on one wire due to the other.

c. The direction of the force on the wire at the bottom of the picture.

ans: 45.3 T, 539 N, +y

E. 1. (2 pts) Explain why dropping a magnet on the floor can reduce its magnetization.

2. (8) Ionized atoms move in the +x direction through a region

where both an electric field (not shown) and a magnetic field (in

the –z direction) exist. The forces from these two fields have

opposite directions. For ions with a certain speed, the two

forces are the same size, so they cancel and allow the ions to go

straight. (Ions with other speeds are pushed to the side and miss

entering the final beam. This is how a beam of ions with a

single speed is created in devices such as a mass spectrometer.) The selected ions have a charge of +5e,

and are moving perpendicular to a .550 T magnetic field at 8.75 km/s.

a. What is the magnitude of the magnetic force?

b. What is the direction of the magnetic force?

c. What is the magnitude of the electric field vector, �⃑� ?

d. What is the direction of the electric field vector, �⃑� ?

ans: 3.85 fN, +y, 4.81 kV/m, -y

Sec. 6 - Induction.

Read: Ch. 22, the rest of section 9.

Ch. 23: sec. 1, 2, first page of 5, 7 & 9.

A. 1. (2 pts) A step-up transformer is designed to have an output voltage of 2200 V when the primary is

connected across a 110 V source. If there are 80 turns on the primary winding, how many turns are

required on the secondary? ans: 1600

2. (8) A single, plane wire loop of cross sectional area 8.00 cm2 lies in a plane which is perpendicular to a

magnetic field that increases uniformly from .500 T to 2.50 T in 1.00 s. What is the induced current if the

loop's resistance is 2.00 Ω?

ans: .800 mA

B. 1. (4 pts) A coil has an inductance of 3.00 mH, and the current through it changes from .200 A to 1.50

A in .200 s. Find the magnitude of the average induced emf in the coil during this time.

ans: 19.5 mV

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2. (6) The solenoid has a radius of 1.20 cm. When the current is turned

on, B in it changes from 0 to .45 T in .0750 s. Find the average emf

induced in the ring during this time.

ans: 2.71 mV

C. A stripe of gold paint is painted around the equator of a balloon. Its resistance is 2.0 x 10 –3

ohm. The

balloon's axis is parallel to the earth's magnetic field, which has a magnitude of 1.1 x 10 –4

Tesla here. The

balloon is deflated so that its radius decreases from r = 5.00 cm to r = 3.00 cm in .200 s. Find the average

current in the stripe during this time.

ans: 1.38 mA

D. 1. (3 pts) When the current in a certain coil is increasing at the rate of 1.20 A/s, it induces an emf of

96.0 mV in itself. What is the self-inductance of the coil?

ans: 80.0 mH

2. (7) A solenoid 2.50 cm in diameter and 30.0 cm long has 300 turns and carries

12.0 A. Calculate the magnetic flux through the surface of a disk of radius 5.00 cm

that is positioned perpendicular and centered on the axis of the solenoid, as shown.

ans: 7.40x10 –6

Wb

E. 1. (2 pts) Will a transformer operate if a continuously connected battery is used for the input voltage

across the primary? Explain.

2. (8) We are looking into the end of a solenoid which lies along the z axis. It is

19.0 cm long, has 600 turns and carries 7.00 A. An electron at its center is

moving at 3.50 x 105 m/s in the +y direction. Find the magnitude and direction of

the force on this electron.

ans: 1.56 x 10 –15

N, –x direction

Sec. 7 - Self Induction. RL, RC, & LC Circuits.

Read: Ch. 21 sec. 6. Ch. 23 sec. 10

(The text does not cover LC circuits. I can refer you to other books if you would like.)

A. 1. (2 pts.) a. A capacitor is discharged through a resistor. If you graph current as a function of time, is

there any time where this function actually reaches a current of zero?

b. A capacitor is discharged through an inductor. If you graph current as a function of time, is there any

time where this function actually reaches a current of zero?

2. (8) When the switch is closed at t = 0, the charge on the capacitor is 17.0 μC. The

charge decreases from there until at t = 100 μs it is 6.16 μC. Find (a) the angular

frequency in radians per second, and (b) the inductance of the coil.

ans: 12 000 rad/s, .278 mH

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B. In the circuit shown, E = 20.0 V, C = 6.00 F, and R = 2.00 x 106 The

capacitor starts out uncharged. 7.00 s after the switch is closed, what are (a) the

charge on the capacitor, (b) the voltage across the capacitor, (c) the voltage across

the resistor, and (d) the current in the circuit?

ans: 53.0 C, 8.83 V, 11.2 V, 5.60 μA

C. 1. (2 pts) As the voltage increases across a capacitor being charged in an RC circuit, does the current

flowing into it increase, decrease, or stay the same?

2. (8) A 2.50 F capacitor initially holds 40.0 C of charge. At t = 0, it is connected to a 3.00 k resistor,

and starts discharging through it. At t = 10 ms, what is

a. the charge on the capacitor?

b. the voltage across the capacitor?

c. the current in the circuit?

ans: 10.5 C, 4.22 V, 1.41 mA

D. 1. (1 pt) When the circuit in problem 2, below, reaches its final, steady state, what is the inductance of

the coil?

2. (9) If E = 6.00 V, L = 8.00 mH, and R = 4.00 Ω,

a. What is the time constant of this circuit?

b. Calculate the current 250 μs after the switch is closed.

c. What is the value of the final, steady-state current?

ans: 2.00 ms, .176 A, 1.50 A

E. 1. (2) a. A capacitor, initially with 15 V across it, is discharged through a resistor. What is the voltage

across the capacitor after one time constant?

b. An uncharged capacitor is connected, through a resistor, to a 15 V battery. What is the voltage across

the capacitor after one time constant?

ans: 5.52 V, 9.48 V

2. (8) A 2.50 F capacitor initially holds 40.0 C of charge. At t = 0, it is connected to a 3.00 mH coil,

and starts discharging through it. At t = 1.30 ms, what is

a. the charge on the capacitor?

b. the voltage across the capacitor?

ans: -30.7 C, -12.3 V

Sec. 8 - A.C. Circuits

Read: Ch. 20 sec. 5. Ch. 23 sec 11 & 12.

A. Consider a series RLC circuit for which R = 200 Ω, L = 600 mH, and C = 26.5 μF. The applied

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voltage has an amplitude of 50.0 V, and a frequency of 60.0 Hz. Find

(a) the amplitude of the current,

(b) the phase angle between the current and the applied voltage,

(c) the amplitude of voltage across the inductor,

(d) the phase angle between the current and the voltage across the inductor.

ans: 211 mA, v leads by 32.2, 47.7 V, v leads by 90.0

B. 1. (1.5 points) An RLC series circuit is connected to a 120 V AC source. The circuit has a resistance of

10 Ω and an impedance of 20 Ω. What is the current?

2. (4 points) An RLC circuit is used in a radio to tune in a station broadcasting at 99.7 MHz. The

resistance in the circuit is 12.0 Ω, and the inductance is 1.40 μH. What capacitance should be used?

ans: 1.82 pF

3. (4.5 points) The rms output voltage of an AC generator is 200 V, and the operating frequency is 100

Hz. Assuming a phase angle of zero, write the equation giving the output voltage as a function of time.

ans: v = (283 V)cos(628t)

C. A generator drives a circuit as shown. Calculate the

a. inductive reactance,

b. capacitive reactance,

c. impedance, and

d. phase angle between the current and

generator voltage.

ans: 78.5 Ω, 1.59 kΩ, 1.52 kΩ, -84.3

D. 1. (2 point). An AC ammeter reads an effective current of 1.00 amps. What

is the value of A on this graph?

2. (8) An RMS current of 4.5 A flows through a 3.0 mH coil when it is connected to a 50 V (RMS), 400

Hz AC source. What is the coil's internal resistance?

ans: 8.16 Ω

E. 1. (2 pts) The first graph shows how the voltage across an

inductor varies with time. Starting at the same time the voltage

graph does, sketch a graph of the inductor’s current as a

function of time.

2. (2) The frequency in an RLC series circuit is varied from zero to

something quite high. Fill in a sketch on this graph showing how the

current varies as a function of frequency.

3. (6) An incandescent light bulb is connected in series with an inductor across a household outlet (120 V

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RMS, 60 Hz). If the bulb’s resistance is 107 Ω, and the RMS current is .748 A, find (a) the RMS voltage

across the bulb, (b) the RMS voltage across the inductor.

ans: 80.0 V, 89.4 V

Sec. 9 - Vibration & Waves.

Read: Ch. 16 sec. 1 – 3 & 9.

A. 1. (2 points) What exactly happens to the frequency of a harmonic oscillator if you

a. double its mass?

b. double its amplitude?

2. (8) Two side views of someone surfing are shown. For

this wave, what are the following? (a) period, (b)

frequency, (c) angular frequency, (d) wavelength, (e) speed

ans: 12 s, .0833 Hz, .524 rad/s, 30 m, 2.5 m/s

B. 1. (2 points) A wave's frequency is 5 Hz. What is its period?

2. (8) Suppose that the speed of the wave in the figure is 5.00 m/s.

a. What is its frequency?

b. Its amplitude?

c. If the frequency is now doubled, what will be

the wavelength of the new wave?

ans: 125 Hz, .10 mm, 2.0 cm

C. 1. (2 points) How do transverse waves differ from longitudinal waves?

2. (8) When this rope is shaken with a period of .111 s the resulting wave

has a wavelength of 2.66 m. The rope is 2.00 m long and has a mass of

.500 kg. What is the value of the suspended weight, which is providing

the tension in the rope?

ans: 144 N

D.1. (1 point) If a mass-spring system hanging vertically is oscillating, why does the motion eventually

stop?

2. (3) Light from a helium-neon laser has a wavelength of 632.8 nm. What is its frequency?

ans: 4.74 x 1014

Hz

3. (6) A simple harmonic oscillator has a spring constant of 100 N/m. Determine the change in its period

if its mass is changed from 6.00 kg to 6.50 kg.

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ans: increases by .0628 s

E. 1. (1 pt) If a fire siren went off one mile from your house, does the sound reach you a little sooner in the

summer, a little sooner in the winter, or is there no difference?

2. (1 pt) A heavy rope and a light rope, both under the same tension, are strung between two masts on a

ship. Sailors on one mast start shaking both ropes at the same time. Do the waves reach the other mast

sooner on the heavy rope, sooner on the light rope, or is there no difference?

3. (8) A spring is hung from the ceiling, and a 200 g object is attached to its lower end. The 1.96 N weight

of this object stretches the spring 22.0 cm. If this system is set vibrating, what is its frequency?

ans: 1.06 Hz

Sec. 10 - Sound. Standing waves.

Read: Ch 16 sec. 10. Ch. 17 sec 3 & 5.

(The text does not cover the inverse square law for intensity. I can refer you to other books.)

A. 1. (2 points) A piano plays a note, then a horn plays the same note at the same loudness. What is the

difference between these sound waves? (That is, what is the physical difference between sounds of

different quality?)

2. (8) (a) Calculate the length of a pipe that has a fundamental frequency of 240 Hz if the pipe is closed at

one end and open at the other. (b) What is its next resonant frequency above the fundamental?

ans: .357 m, 720 Hz

B. 1. (5 points) A standing wave is established in a 120 cm long string fixed at both ends. The string

vibrates in four node to node segments when driven at 120 Hz. (a) Determine the wavelength. (b) What

is the fundamental frequency of the string?

ans: 60.0 cm, 30.0 Hz

2. (5) Solar radiation has an intensity of 1340 W/m2 at Earth’s location, 1.50 x 10

11 m from the sun. Find

the intensity of solar radiation at the planet Mercury, 5.80 x 1010

m from the sun.

ans: 8960 W/m2

C. 1. (3 pts) The fundamental frequency of a certain string is 1300 Hz. What is the highest resonance

frequency of the string that can be heard by a person able to hear frequencies up to 20 000 Hz?

ans: 19 500 Hz

2. (7) A loudspeaker sends out 8.00 watts of power in the form of sound waves. The speaker is on top of a

tall telephone pole, so the waves spread out uniformly in all directions. Find

a. the sound intensity 5.00 m from the speaker.

b. the sound level there, in decibels.

ans: .0255 W/m2, 104 dB

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D. 1. (1 point) Water drips into a bathtub from both the faucet and the showerhead. At one particular

moment, drips from only the faucet would make the wave shown on the left. Just drips from the shower

would make the wave on the right. With both waves present at the same time, what is the displacement of

the water’s surface at the point indicated? Each vertical division is 1 mm.

2. (1 pt) Either of two loudspeakers produces 50 dB, corresponding to an intensity of 1 x 10–7

W/m2, by

itself. Multiple choice, pick one: If both are sounded at the same time, there will be

(a) 100 dB and 2 x 10–7

W/m2 (b) 100 dB but not 2 x 10

–7 W/m

2

(c) 2 x 10–7

W/m2 but not 100 dB (d) neither 100 dB or 2 x 10

–7 W/m

2

3. (1) If you double your distance from a light source, what happens to the intensity? (Include how much

it changes, not just a vague “increases” or “decreases.”)

4. (7) A string in a piano is 0.400 m long, has a mass of 3.00 g, and is under 800 N of tension. What is its

fundamental frequency?

ans: 408 Hz

E. 1. (1 point) The picture shows a string with a standing wave on it, in

several of the positions the string goes through. When in the center

position (flat), is the velocity of point b in the same direction as a’s, in

the opposite direction, or zero?

2. (9) In the afternoon, as 400 cars per minute pass on the highway outside the window, the sound level in

a motel room is 80.0 dB. What is the average sound level in the room later, when only 65 cars per minute

are going by? Assume that the cars are the only significant source of sound.

ans: 72.1 dB

F. From now on, each assignment will include review questions. You are just as likely to be given the

review quiz as any of the others.

The ammeter indicates .100 A flows through it in the direction shown. Find the

unknown emf.

ans: 8.00 V

Sec. 11 – Electromagnetic Waves. Reflection and Refraction.

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Read: Ch. 24 First page of section 3.

Ch. 25 sec. 2 – 4, 6 & 7.

A. 1. (2 pts) Explain the difference between a real and a virtual image. (That is, what makes a real image

real and what makes a virtual image virtual.)

2. (8) In a certain video projector, the LCD which acts as the object is 12.0 cm from the lens. The image

on the screen is 4.00 m away. (There are actually three LCD panels, one each for red, green and blue, but

let’s not complicate things.)

a. What is the focal length of the lens?

b. If the LCD is 2.50 cm by 3.50 cm, what will the dimensions of the image on the screen be?

ans: 11.7 cm, 83.3 cm by 117 cm

B. 1. (1 point) Do radio waves travel at the speed of sound, or at the speed of light?

2. (1) Which color has the longest visible wavelength?

3. (2) There are two ways to make an electric field: 1) An E⃑⃑ field is found around a charge and 2) An E⃑⃑

field is induced by a changing B⃑⃑ field. Make a similar list of the way(s) to make a magnetic field.

4. (6) A 2.0 cm high object is placed 20.0 cm in front of a concave mirror whose radius of curvature is

60.0 cm. Calculate the image’s position, size and character (real or virtual? upright or inverted?).

ans: -60 cm, 6.0 cm, virtual & upright.

C. A 2.0 cm high object is placed 20.0 cm in front of a concave mirror whose radius of curvature is 60.0

cm. Use a ray diagram (not equations) to find the position, size and character of the image.

ans: -60 cm, 6.0 cm, virtual & upright.

D. 1. (2.5 points) State the law of reflection and illustrate its meaning with a diagram.

2. (3.5) The index of refraction of a certain material is 1.58. What is the speed of light in this material?

ans: 1.90 x 108 m/s

3. (4) A beam of light travels from benzene into water. If the angle of incidence in the benzene is 48,

what will be the angle of refraction in the water?

ans: 57

E. 1. (2 pts) An electromagnetic wave can travel through empty space, far from

any charges or currents. Where do the fields which vibrate as the wave goes by

come from?

2. (8) A light ray is incident at an angle θ on the top surface of a block of

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polystyrene as shown. Find the maximum angle θ for which the refracted ray undergoes total internal

reflection at the left vertical face of the block if it is immersed in water.

ans: 30.3

F. 1. (1 point) A small light bulb is located at the focal point of a lens, seen here

from the side. Three typical rays leave the bulb and hit the lens. Finish the

picture, showing what the rays do after that.

2. (9 pts, Review) A loop of wire in a uniform magnetic field

rotates 30° as shown. The area of the loop is .180 m2 and B =

.0750 T. Find:

a. the change in flux through the loop.

b. the average emf induced if the change takes 4.17 ms.

ans: – .00181 Wb, .434 V

Sec. 12 – Interference.

Read: Ch. 27 sec. 3 – 7.

A. 1. (5.5 pts) What is the minimum distance between two points which can be resolved 29 km away

using a telescope with a 4.00 cm diameter objective lens? (Assume λ = 580 nm.)

ans: 51.3 cm

2. (4.5) A material having an index of refraction of 1.30 is used to coat a piece of glass (n = 1.50). What

should be the minimum thickness of this film if it is to minimize reflection of 500 nm light:

ans: 96.2 nm

B. Light of wavelength 500 nm is incident normally on a diffraction grating. If the third order maximum

of the diffraction pattern is observed at 32.0, (a) what is the number of rulings per centimeter for the

grating? (b) Determine the highest order maxima that can be observed in this situation.

ans: 3.53 x 103, 5

C. 1. (2 pts) An oil film on water appears brightest at the outer regions where it is thinnest. From this

information, what can you say about the index of refraction of oil relative to that of water?

2. (8) A pair of narrow parallel slits separated by .250 mm are illuminated by green light (λ = 546.1 nm).

The interference pattern is observed on a screen 1.20 m away from the plane of the slits. Calculate the

distance from the central maximum to the first bright fringe on either side.

ans: 2.62 mm

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D. 1. (1 point) This is an enlarged cross-sectional view of a camera lens. Light

reflected from the top of the coating interferes with light reflected from the

bottom. Is the interference constructive or destructive?

2. (2) As it evaporates, the thinnest spot on a soap bubble appears black just

before it breaks, indicating the reflected rays are about 180° out of phase. Why

doesn’t a path difference of nearly zero make it 0°? (Shiny instead of dark?)

3. (7) The second-order maximum in a single slit-diffraction pattern is 1.40 mm from the center of the

central maximum. The screen is 80.0 cm from a slit of width .800 mm. Calculate the wavelength.

ans: 560 nm

E. 1. (2 points) i. In the equations for single slit interference, what does “a” stand for?

ii. In the grating equation, how is “d” related to the number of lines per centimeter?

2. (2 points) All rays shown are approximately perpendicular to the soap

film, which is one wavelength thick. Do rays A and B interfere

constructively, destructively or something in between?

3. (6) Waves in a ripple tank travel from sources 8.00 cm apart to a

wall 14.0 cm from them. If the first order maxima hit the wall

directly in front of the sources, what is the wavelength? (Hint: The

wall is too close for mλ = d sinθ to be valid.)

ans: 2.12 cm

F. (review) An electron is near two ionized atoms as shown. Atom A has

lost two electrons, atom B lost three. (e stands for 1.60 x 10–19

C.) If the

force on the electron from atom A is twice the force from B, how far is A

from the electron?

ans: 1.91 x 10 –9

m

Sec. 13 – Atoms. Solids.

Read: Ch. 29 first half page of sec. 3, sec 5 & 6.

Ch. 30 sec. 3, “Lasers” p. 1203 – 1205, sec. 6, just skim sec. 9.

(The text does not cover solids. I can refer you to other books if you would like.)

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A. 1. (2 pts) Is the center layer of a pnp transistor doped with donors or acceptors? What about the outer

layers?

2. (2 pts) If matter has a wave nature, why is this not observable in our daily experiences?

3. (6) An FM radio transmitter has a power output of 150 kW and operates at a frequency of 99.7 MHz.

How many photons per second does the transmitter emit?

ans: 2.27 x 1030

B. A beam of electrons is incident on a slit 3.50 nm wide, forming a diffraction pattern on a screen 20.0

cm beyond. If the first side maximum is 3.1 cm from the pattern’s center, what is the speed of the

electrons?

ans: 2.04 x 106 m/s

C. 1. (2 pts) Can the electron in the ground state of hydrogen absorb a photon of energy (a) 10.0 eV? (b)

14.0 eV?

2. (8) In hydrogen, when electrons dropping into the ground state give off a 94.96 nm spectral line, what

value of n do they fall from?

ans: 5

D. 1. (3 pts) In an LED, electrons at the bottom of the conduction band combine with holes at the top of

the valence band releasing an amount of light energy equal to the gap between the two bands. In an LED

emitting green light with a wavelength of 520 nm, how much energy is the gap between the bands?

ans: 3.82 x 10-19

J (or 2.38 eV)

2. (2 pts) A common type of transistor is a pnp or npn sandwich, whose center layer is very thin. Explain

how this acts as a "valve" for electric current.

3. (5) In the Bohr model of hydrogen, what is the wavelength of the electron in

a. The ground state, n = 1? (The atom’s radius is .0529 nm.)

b. A state where n = 2? (The atom’s radius is .212 nm.)

ans: .332 nm, .666 nm

E. 1. (3 pts) A solid state diode is a p-n junction. Explain why current flows through it easily in one

direction, but not the other way.

2. (7) The hydrogen lines in the visible part of the spectrum are those for which the electron falls into the n

= 2 level. If light from a hydrogen lamp falls on a CdS crystal (energy gap = 2.42 eV), which of these

visible lines are absorbed, and which are transmitted? (Just as in a single atom, an electron in a solid can

only jump from one allowed energy to another.)

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ans: The n = 3 to n = 2 line is transmitted, all others absorbed.

F. (Review) 1. (2 pts) a. In a series RLC circuit, what is the possible range of values for the phase angle?

b. What is the phase angle when the inductive reactance equals the capacitive reactance?

2. (8) The region shown is inside a solenoid 40.0 cm long with 2000

turns. Both the solenoid and the wire inside it carry 12.0 A. Find the

magnitude of the force on the 3.00 cm straight wire.

ans: .0136 N

Sec. 14 – Nuclei.

Ch. 31 sec. 1, 3 – 6. Ch. 32 sec. 2, just skim sec 5 & 6.

A. 1. (2 pts) The decay of 235

U to 207

Pb (by a series of steps) is used by geologists to date rocks. Rock A

contains 5 mg of 235

U and 5 mg of 207

Pb. Rock B contains 10 mg of 235

U and 20 mg of 207

Pb. Rock C

contains 30 mg of 235

U and 10 mg of 207

Pb. Which is oldest?

2. (8) Using the fact that the atomic mass of 56

26Fe is 55.934 940, find its binding energy per nucleon.

ans: 8.79 MeV/nucleon

B. 1. (2 points) Explain the difference between radiation sickness and cancer.

2. (8) A freshly prepared sample of a certain radioactive isotope has an activity of 10.0 mCi. After 4.00 h,

its activity is 8.00 mCi.

a. Find the half life.

b. What will the activity be 8.00 h after it was fresh?

ans: 12.4 h, 6.39 mCi

C. 1. (3 pts) What is an alpha particle? What is a

beta particle? What is a gamma ray?

2. (1) By what process does the Sun generate its

energy?

3. (6) Determine which decays can occur

spontaneously. Show why each answer is correct.

(a) 40

20Ca e+ +

4019K

(b) 99

44Ru 42He +

9542Mo

(c) 144

60Nd 42He +

14058Ce

ans: Only (c) can occur.

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D. 1. (2 points) The mass of any nucleus is

slightly less than what you get by adding the

individual masses of the protons and neutrons

that make it up. Explain why.

2. (2) What is one difference between a neutron

and a neutrino?

3. (6) Using this graph, estimate the energy

released when a nucleus of mass number 200 is

split into two nuclei each of mass number 100.

ans: about 180 MeV, depending on how

you estimate tenths of an MeV on the

graph.

E. 1. (2 pts) A sample of an isotope with a half-life of 5 years presently has an activity of .80 mCi. Ten

years from now, its activity will be _______mCi. Twenty-five years from now, it will be _______mCi.

2. (2) What is given off by the fission of uranium that can go on to cause another fission event?

3. (6) Fill in the blanks for the unknown nuclide X, and answer the question.

(a) 𝑋 → 𝑁𝑖 + 𝛾2865 −

− Is X also a Ni nucleus?

(b) 𝑃𝑜 → 𝑋 + 𝛼−−

84215 Is X also a Po nucleus?

(c) 𝑋 → 𝑃𝑏 + 𝑒− + �̅�82208

−− Is X also a Pb nucleus?

ans: 𝑋2865 yes, 𝑋82

211 no, 𝑋81208 no.

F. (Review) 100 m from the speakers, the sound level at an outdoor concert is 90.0 dB. Assuming open

space in between, how many decibels are there outside a home 5000 m away? (A little over three miles.)

ans: 56.0 dB