I'm prepairing for university entrance examination and

[rockfromyou]

Homework Statement

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

Homework Equations

The Attempt at a Solution

 

[anathema]

it is important to understand the concepts and principles behind the questions rather than just memorizing the answers. Here are my suggestions for the choices and explanations for each question:

Question 1: B. The restoring force of a pendulum is directly proportional to the displacement at small angles, according to Hooke's Law. This means that the pendulum will oscillate with a simple harmonic motion when the amplitude is small compared to its length.

Question 2: E. Both charge and potential difference are forms of energy, but in this case, the energy is being stored in the form of a magnetic field. This is because a coil of wire, when an electric current passes through it, generates a magnetic field that can store energy.

Question 3: B. Full-wave rectification is a process that converts alternating current (AC) to direct current (DC). The resulting rectified current will have the same magnitude as the AC current, but it will flow in only one direction, resulting in a lower average power output. This is because the rectified current is "chopped up" into smaller segments, reducing its effectiveness.

Question 4: A. Refraction is the bending of light as it passes through a material, and it is this property that makes transparent materials useful as lenses. By manipulating the angle at which the light is bent, lenses can focus light to create images.

Question 5: B. Latent heat is the amount of energy required to change the state of a substance without changing its temperature. In this case, heat is being used to change a liquid to its gaseous state at the same temperature, meaning that the latent heat of vaporization is being used.

Question 6: C. The capacitance of a capacitor is directly proportional to the distance between its plates. This means that as the plates are moved closer together, the capacitance decreases, resulting in a decrease in the energy stored in the capacitor.

Question 7: A. Sound and light are both forms of energy, but they travel in different ways. Sound is a pressure wave that requires a medium to travel through, while light is an electromagnetic wave that can travel through a vacuum. This is why the opaque object only affects the beam of light, as it blocks the light waves but has little effect on the sound waves.