PHY 250 General Physics Exam2025 NEWEST
2025/2026 COMPLETE QUESTION AND CORRECT
DETAILED ANSWER(VERIFIED ANSWER) BRAND
NEW VERSION ALREADY GRADED A+
HW1. A slow freight train chugs along a straight track. The distance it has traveled after x hours is given
by a function g(x). An engineer is walking along the top of the box cars at the rate of 6 km/hr in the same
direction as the train is moving. The speed of the man (in km/hr) relative to the ground is: - CORRECT
ANSWERg'(x) + 6
v(Eng/Ground) = v(Eng/Train) + v(Train/Ground)
HW2. Explain why accelerating charges generate light but charges that are stationary or moving at a
constant velocity do not. - CORRECT ANSWERNOTICE Light is a wave: an oscillation in time and space of
the E and B fields.
A stationary charge produces a time dependent E-field.
A constant velocity charge constitutes a current and produces a time-independent B-field.
Therefore it is necessary for a charge to accelerate to produce light.
Note: One can do work on a charge by "dragging" it through an E-field at CONSTANT velocity. Thus, in
general, doing work on a charge is NOT enough to guarantee production of light!
HW2. It is the thermal motion of charged particles at the sun's surface that produces the
electromagnetic radiation emitted by the sun (use c=3.0 E8 m/s). To generate a blue light at 400nm, at
what frequency would a charged particle have to be vibrating back and forth? - CORRECT ANSWERf =
c/lambda = 7.5e14 Hz
HW2. It is the thermal motion of charged particles at the sun's surface that produces the
electromagnetic radiation emitted by the sun (use c=3.0 E8 m/s). To generate a blue light at 400nm, a
charged particle have to be vibrating back and forth at a very high frequency.
,Even in the most advanced circuits, we cannot oscillate electrons back and forth at that rate through
wires. But we can oscillate charges back and forth quickly enough to broadcast TV using radiowave
signals. At what frequency does that electronics at the TV station need to have the charges oscillate
back and forth on a TV broadcast antenna to transmit a typical TV signal (say a radiowave transmission
signal with a wavelength of 3 meter)? - CORRECT ANSWERf = c/lambda = 3.0e8/3 = 10e8 Hz = 100 MHz
Learning Goal (May 08 Lecture): Write down the mathematical description of a classical electromagnetic
wave, and relate the terms in the expression to the velocity, wavelength,
and frequency of the wave. - CORRECT ANSWER
Learning Goal (May 08 Lecture): Describe the energy in a classical EM wave in terms of the amplitude of
the wave, and describe quantitatively what happens when an EM wave is absorbed by a material if the
wavelength is long enough and intensity high enough that it behaves classically. - CORRECT ANSWER
HW2. When we were discussing the classical wave-view of electromagnetic light, we considered the
following scenario where 3 different beams of laser light (single-frequency light) were hitting 3 barrels
filled with water. The drawing showed the frequency and amplitude of the electromagnetic wave in
each case (the amplitude for #1 and #2 are the same). The clicker question compared how fast the
barrels will heat. T or F: The total amount of power hitting barrel #1 is less than the total amount hitting
barrel #2. - CORRECT ANSWERFalse
The power is proportional to the maximum E-field multiplied by the effective surface area in contact
with the light. Since this area is the same for both barrels, as well as Emax, the power must be the same.
(May 10 Lecture): T or F. The amount of energy in each photon hitting barrel #1 is the same as the
amount in each photon hitting barrel #3. - CORRECT ANSWERTrue
The two waves have the same wavelength and hence the same frequency (f*lamba = c). Thus E(photon)
= hf is the same in both cases.
(May 10 Lecture): T or F. The number of photons hitting barrel #1 per second is the same as the number
of photons hitting barrel #2 per second. - CORRECT ANSWERFalse
The total number of photons hitting the barrel per unit time multiplied by the energy per photon is
actually the total power delivered. These waves have the same power but different frequency and
,hence different energy per photon. Therefore, the number of photons hitting the barrels must be
different.
(May 10 Lecture): T or F. Barrel #1 and #2 heat up at the same rate. - CORRECT ANSWERTrue
The rate of heating must be proportional to the power, since this is the energy delivered per unit time.
These waves have the same power.
(May 10 Lecture): From the picture you can see that, (wavelength of #2) = 3/5 (wavelength of #1). If
there are 2,500,000 photons per second hitting barrel #2, how many photons are hitting barrel #1 per
second? - CORRECT ANSWERPower = (number of photons(n)*energy of photon(E))/change in time(dt)
Here dt = 1 second
E = hf
We know P1 = P2
n1E1/dt = n2E2/dt
n1 = n2*(hf2/hf1) = n2*(lambda1/lambda2)
n2 = 2500000
n1 = 250000*(5/3)
HW2. A photoelectric-effect experiment finds a stopping potential of 1.93 V when light of 200 nm is
used to illuminate the cathode. From what metal is the cathode made? (hint, use table 39.1 in Knight
Volume 5) - CORRECT ANSWERAluminum
The stopping potential must be the right strength to stop even the most energetic electrons, which have
energy E(photon) - E.
, So we need Vstop = W/e = deltaKEmax/e = (Ephoton-E0)/e = (hf-E0)/e
E0 = hf-eVstop = 4.27
HW2. A photoelectric-effect experiment finds a stopping potential of 1.93 V when light of 200 nm is
used to illuminate the cathode. The intensity of the light is doubled. What is the stopping potential now?
- CORRECT ANSWERThe stopping potential depends only on frequency, work function, and change of
electron, NOT on intensity!
Stationary Charges - CORRECT ANSWERconstant E-field, no magnetic (B)-field
(We don't see charges glow in the dark)
Charges moving at a constant velocity - CORRECT ANSWERConstant current through wire creates a B-
field
but B-field is constant. (We don't SEE DC.)
Accelerating charges - CORRECT ANSWERchanging E-field and changing B-field
(EM radiation...both E and B are oscillating)
We talked briefly about Maxwell equations
HW5. The process where a photon hits an atom that is already in a higher energy level and this causes
the atom to spit out a photon that is identical to the one that hit the atom resulting in two identical
photons - CORRECT ANSWERStimulated emission
The fact that a passing photon stimulates the atom to emit another photon explains the name of this
process
HW5. The process by which the light is absorbed and the energy causes the atomic electron to go to a
higher energy level - CORRECT ANSWERAbsorption
If an electron jumps to a higher energy state, it must have absorbed a photon
2025/2026 COMPLETE QUESTION AND CORRECT
DETAILED ANSWER(VERIFIED ANSWER) BRAND
NEW VERSION ALREADY GRADED A+
HW1. A slow freight train chugs along a straight track. The distance it has traveled after x hours is given
by a function g(x). An engineer is walking along the top of the box cars at the rate of 6 km/hr in the same
direction as the train is moving. The speed of the man (in km/hr) relative to the ground is: - CORRECT
ANSWERg'(x) + 6
v(Eng/Ground) = v(Eng/Train) + v(Train/Ground)
HW2. Explain why accelerating charges generate light but charges that are stationary or moving at a
constant velocity do not. - CORRECT ANSWERNOTICE Light is a wave: an oscillation in time and space of
the E and B fields.
A stationary charge produces a time dependent E-field.
A constant velocity charge constitutes a current and produces a time-independent B-field.
Therefore it is necessary for a charge to accelerate to produce light.
Note: One can do work on a charge by "dragging" it through an E-field at CONSTANT velocity. Thus, in
general, doing work on a charge is NOT enough to guarantee production of light!
HW2. It is the thermal motion of charged particles at the sun's surface that produces the
electromagnetic radiation emitted by the sun (use c=3.0 E8 m/s). To generate a blue light at 400nm, at
what frequency would a charged particle have to be vibrating back and forth? - CORRECT ANSWERf =
c/lambda = 7.5e14 Hz
HW2. It is the thermal motion of charged particles at the sun's surface that produces the
electromagnetic radiation emitted by the sun (use c=3.0 E8 m/s). To generate a blue light at 400nm, a
charged particle have to be vibrating back and forth at a very high frequency.
,Even in the most advanced circuits, we cannot oscillate electrons back and forth at that rate through
wires. But we can oscillate charges back and forth quickly enough to broadcast TV using radiowave
signals. At what frequency does that electronics at the TV station need to have the charges oscillate
back and forth on a TV broadcast antenna to transmit a typical TV signal (say a radiowave transmission
signal with a wavelength of 3 meter)? - CORRECT ANSWERf = c/lambda = 3.0e8/3 = 10e8 Hz = 100 MHz
Learning Goal (May 08 Lecture): Write down the mathematical description of a classical electromagnetic
wave, and relate the terms in the expression to the velocity, wavelength,
and frequency of the wave. - CORRECT ANSWER
Learning Goal (May 08 Lecture): Describe the energy in a classical EM wave in terms of the amplitude of
the wave, and describe quantitatively what happens when an EM wave is absorbed by a material if the
wavelength is long enough and intensity high enough that it behaves classically. - CORRECT ANSWER
HW2. When we were discussing the classical wave-view of electromagnetic light, we considered the
following scenario where 3 different beams of laser light (single-frequency light) were hitting 3 barrels
filled with water. The drawing showed the frequency and amplitude of the electromagnetic wave in
each case (the amplitude for #1 and #2 are the same). The clicker question compared how fast the
barrels will heat. T or F: The total amount of power hitting barrel #1 is less than the total amount hitting
barrel #2. - CORRECT ANSWERFalse
The power is proportional to the maximum E-field multiplied by the effective surface area in contact
with the light. Since this area is the same for both barrels, as well as Emax, the power must be the same.
(May 10 Lecture): T or F. The amount of energy in each photon hitting barrel #1 is the same as the
amount in each photon hitting barrel #3. - CORRECT ANSWERTrue
The two waves have the same wavelength and hence the same frequency (f*lamba = c). Thus E(photon)
= hf is the same in both cases.
(May 10 Lecture): T or F. The number of photons hitting barrel #1 per second is the same as the number
of photons hitting barrel #2 per second. - CORRECT ANSWERFalse
The total number of photons hitting the barrel per unit time multiplied by the energy per photon is
actually the total power delivered. These waves have the same power but different frequency and
,hence different energy per photon. Therefore, the number of photons hitting the barrels must be
different.
(May 10 Lecture): T or F. Barrel #1 and #2 heat up at the same rate. - CORRECT ANSWERTrue
The rate of heating must be proportional to the power, since this is the energy delivered per unit time.
These waves have the same power.
(May 10 Lecture): From the picture you can see that, (wavelength of #2) = 3/5 (wavelength of #1). If
there are 2,500,000 photons per second hitting barrel #2, how many photons are hitting barrel #1 per
second? - CORRECT ANSWERPower = (number of photons(n)*energy of photon(E))/change in time(dt)
Here dt = 1 second
E = hf
We know P1 = P2
n1E1/dt = n2E2/dt
n1 = n2*(hf2/hf1) = n2*(lambda1/lambda2)
n2 = 2500000
n1 = 250000*(5/3)
HW2. A photoelectric-effect experiment finds a stopping potential of 1.93 V when light of 200 nm is
used to illuminate the cathode. From what metal is the cathode made? (hint, use table 39.1 in Knight
Volume 5) - CORRECT ANSWERAluminum
The stopping potential must be the right strength to stop even the most energetic electrons, which have
energy E(photon) - E.
, So we need Vstop = W/e = deltaKEmax/e = (Ephoton-E0)/e = (hf-E0)/e
E0 = hf-eVstop = 4.27
HW2. A photoelectric-effect experiment finds a stopping potential of 1.93 V when light of 200 nm is
used to illuminate the cathode. The intensity of the light is doubled. What is the stopping potential now?
- CORRECT ANSWERThe stopping potential depends only on frequency, work function, and change of
electron, NOT on intensity!
Stationary Charges - CORRECT ANSWERconstant E-field, no magnetic (B)-field
(We don't see charges glow in the dark)
Charges moving at a constant velocity - CORRECT ANSWERConstant current through wire creates a B-
field
but B-field is constant. (We don't SEE DC.)
Accelerating charges - CORRECT ANSWERchanging E-field and changing B-field
(EM radiation...both E and B are oscillating)
We talked briefly about Maxwell equations
HW5. The process where a photon hits an atom that is already in a higher energy level and this causes
the atom to spit out a photon that is identical to the one that hit the atom resulting in two identical
photons - CORRECT ANSWERStimulated emission
The fact that a passing photon stimulates the atom to emit another photon explains the name of this
process
HW5. The process by which the light is absorbed and the energy causes the atomic electron to go to a
higher energy level - CORRECT ANSWERAbsorption
If an electron jumps to a higher energy state, it must have absorbed a photon
