STRAIGHTERLINE PHY250 GENERAL PHYSICS I GRADED
EXAM 1 (NEW VERSION JUNE 2024)
MODERN PHYSICS (PHYS 250)
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: - (ANSWER)g'(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. - (ANSWER)NOTICE 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? -
(ANSWER)f = 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)? - (ANSWER)f = 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. - (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. - (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. - (ANSWER)False
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. - (ANSWER)True
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. - (ANSWER)False
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. - (ANSWER)True
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? - (ANSWER)Power = (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) - (ANSWER)Aluminum
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? - (ANSWER)The stopping potential depends only on frequency, work function,
and change of electron, NOT on intensity!
Stationary Charges - (ANSWER)constant E-field, no magnetic (B)-field
(We don't see charges glow in the dark)
Charges moving at a constant velocity - (ANSWER)Constant current through wire creates a B-
field
but B-field is constant. (We don't SEE DC.)
Accelerating charges - (ANSWER)changing 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 - (ANSWER)Stimulated 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 - (ANSWER)Absorption
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