# Homework Help: I'm prepairing for university entrance examination and need help

1. Feb 17, 2012

### rockfromyou

1. The problem statement, all variables and given/known data
i have some multiple choice questions that i'm not sure about the answer,therefore i need help here. Could anyone suggest the choice of each question with a brief explanation to help me.
Question 1. The oscillation of a pendulum is simple harmonic when the amplitude is small compared to its length. This is because :
A. a long pendulum losses less energy
B. the restoring force is proportional to displacement at small angles.
C. friction of the pivot is smaller when the pendulum is long.
D. it has a higher length to mass ration
---my ans: B: i just know the requirement of a simple harmonic oscillation pendulum is its small angle ( smaller than 0.1 rad)
Question 2: A coil of wire stores energy in the form of
A. charge
B. potential difference
C. magnetic field
D. A and B
E. A and C
---my ans: C: i'm confused between A and D because i think charge is not energy, is it wrong?

Question 3. A sinusoidal alternating current is fullwave rectified. The rectified current will produce in the same load
A. the same power
B. 0.71 times the power
C. 1.41 times the power
D. half the power
E. twice the power
---my ans: B

Question 4: The property of a transparent material that makes it useful as material for a lens is
A. refraction
B. diffraction
C. polarization
D. dispersion
E. absorption
---my ans: D

Question 5: Heat is used to change a liquid to its gaseous state at the same temperature as the liquid. This is because of the property of the liquid called
A. heat capacity
B. latent heat
C. specific heat capacity
D. conductivity
E. boiling point
---my ans: E

Question 6: two metal plates carrying charge Q is separated by a distance d. The capacitance of the resultant charged capacitor is then decreased by moving the plates. The energy stored in the capacitor
A. stays the same
B. increases
C. decreases
D. stays the same if the plates are moved parallel to each other.
E. decreases if the plates are moved parallel to each other.
---my ans: B. i think when the capacitor is decreased, as a result, the energy is decreased because its energy is proportional to C^-1

Question 7: an opaque object 10 cm wide casts a shadow when placed in a beam of light but has little effect on a beam of sound emitted by a small source of frequency 500Hz. This is because
A. sound is a pressure wave whereas light is an electromagnetic wave
B. sound travels much more slowly than light
C. sound waves are longitudinal whereas light waves are transverse
D. sound waves have a much longer wavelength than light waves
E. the power per unit area in a beam of sound is much lower than that in a beam of light
---my ans: D

2. Relevant equations

3. The attempt at a solution