Lab Methods - Voltage Amplifier

In a voltage amplifier, which of the following is NOT usually a result of introducing negative feedback?

A. Increased amplification
B. Increased bandwidth
C. Increased stability
D. Decreased distortion
E. Decreased voltage gain

(GR9677 #72)

Solution:

Negative feedback → noise cancellation.
Answer B, C, D, and E are benefits of negative feedback.

Positive feedback → increased amplification.

Answer: A

Thermal Physics - Heat Capacity

Questions 71-73

A system in thermal equilibrium at temperature T consists of a large number N₀ of subsystems, each of which can exist only in two states of energy E₁ and E₂, where . In the expressions that follow, k is the Boltzmann constant.

The internal energy of this system at any temperature T is given by . The heat capacity of the system is given by which of the following expressions?

A.

B.

C.

D.

E.

(GR9277 #72)

Solution:

Heat Capacity (at constant  volume), 



Answer: A

Special Relativity - Index of Refraction

The measured index of refraction of X-rays in rock salt is less than one. This is consistent with the theory of relativity because

A. Relativity deals with light waves traveling in a vacuum only
B. X-rays cannot transmit signal
C. X-rays photons have imaginary mass
D. The theory of relativity predates the development of solid-state physics
E. The phase velocity and group velocity are different

(GR8677 #72)

Solution:

Index of refraction, n = ^c/_v

For X-rays, n  1 → v  c (violates the theory of special relativity)
However, this is OK, because in this case, v is phase velocity, v_p.

v_p does not represent the particle velocity.
The physical speed of particle is represented by group velocity, v_g.

Answer: E

Classical Mechanics - Harmonic Oscillator

Two identical blocks are connected by a spring. The combination is suspended, at rest, from a string attached to the ceiling, as shown in the figure. The string breaks suddenly. Immediately after the string breaks, what is the downward acceleration of the upper block?

A. 0
B. g/2
C. g
D. √2 g
E. 2g

(GR0177 #72)

Solution:

In equilibrium (before the string is cut):
Block 1:

                                    …(Eq.1)

Block 2:
                  … (Eq.2)

After the string is cut:
, and
(minus sign    is because the direction is in downward or parallel with  )

(Eq.1) becomes:

                                                                   ...(Eq.3)

Submit (Eq.2) to (Eq.3):   

Since  

Answer: E