Sam Tet wish (SPM 2014)

Sam Tet wish 2014

SPM 2014 is around the corner.  

SMJK Sam Tet is a happy hunting ground for A, A, A, …… ,∞  .

May your blood, toil, tears, and sweat bear bountiful fruits.    

To all science students of Sam Tet in general, and to all students of 5S1, 5S2 & 5S7 in particular, may you all score A's in Physics.

And, to relay a message from the Physicists Hall of Fame:
"Befriend us, read our minds, understand our works, and we'll never fail you!"

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Calculating Wave Energy

Calculating the Energy Carried by a Wave

The diagram below shows the volume of a small slice of a medium carrying a wave
(longitudinal or transverse included).

What is the total amount of energy carried by a wave of speed v travelling through S, the

cross-sectional surface area in a certain time t?

 

The energy transported by a wave can be expressed by:

            E  =  2p²mf²A²    

where E is the energy of the wave, m the mass of a particle in the medium, f the frequency of the wave & A the amplitude of the wave motion.

The energy transported by a wave can also be expressed by:

            E  =  2p²rSvtf²A²    

where r is the density of the medium, S the cross-sectional surface area through which the

wave travels, v the speed of the wave, & t the time taken for the wave to travel the

   distance l.

Example 1:  A wave propagates from left to right with a speed of 3.0 ms^-1 through a medium of density 1.4 kg m^-3. Given that the frequency of the wave is 8.0 Hz. & the amplitude 60 cm. Determine the total wave energy that passes through a cross-sectional surface area of 5.0 m² during a time interval of 40 s.   

Answer:   E  =  2p²rSvtf²A² 

                      =  2  x  3.14²   x  1.4  x  5.0  x  3.0  x  40  x  8.0²  x  0.6²      

                      =  380,000 J  (2 significant figures)

Example 2:  The diagram below shows the graphical representation of 3 waves P, Q & R passing through the same medium during the same period of time.

(i)   Compare wave P & wave Q. Which one possesses more energy?

(ii)  Compare wave P & wave R. Which one possesses more energy?

Answer:  

(i)   Frequency of wave Q is 1.5 times that of wave P (3 Hz ¸ 2 Hz).

       There is factor of f² in the wave energy equation.

       Therefore, wave Q possesses more energy than wave P by a factor of 1.5² or 2.25.   

(ii)  Amplitude of wave R is 2 times that of wave P.

       There is factor of A² in the wave energy equation.

       Therefore, wave R possesses more energy than wave P by a factor of 2² or 4.   

Producing A Sound Wave

Using a Drum to Produce a Sound Wave

Prior knowledge: Longitudinal wave, compression & rarefaction.

See how a vibrating drum membrane produces a sound wave.
The longitudinal wave produced will propagate from left to right.

 

 

 

A sound wave can be represented by a sinusoidal graph as shown below.