[Physics Form 5] Cathode Rays

Question:
How are cathode rays formed?

Answer:

• The 6V source first heats up the filament, which, in turn, heats up the cathode.
  
  
• The cathode thus exhibits thermionic emission and produces thermal electrons.
  
  
• The EHT exerts a strong electric field between the cathode and the anode, causing the thermal electrons to accelerate, and travel at a high speed and in a straight line towards the anode.
  
  
• The beam of fast-moving electrons generated is known as cathode rays.

[Physics Form 5] Power Of Resonance

Resonance is what happens when the vibrations produced by one object come into alignment with those another. An apparatus set-up is shown as below.

When the radio is switched on, the diaphragm is observed to vibrate. A few moments later, the polystyrene ball starts to oscillate. It oscillates as long as the radio switched on. When the radio is switched off, the amplitude of oscillating polystyrene ball decreases and eventually becomes zero.

Q & A

1. Q: How is the polystyrene ball able to oscillate when no apparent external force is present to displace and release it?
   A: The vibration of the loudspeaker produces energy, which is transferred to the air molecules in front of it. The energy is then transferred from one air molecule to another until it reaches the polystyrene ball. When the polystyrene ball receives the energy, it is displaced from its equilibrium position, thus setting it into oscillation. This shows that the oscillating system transfers its energy to its surrounding medium.
   
   
2. Q: Why is a polystyrene ball used? What if the polystyrene ball is substituted with a lead bob?
   A: Polystyrene is lighter or less massive than most materials. If a lead bob is used, no oscillation will be observed. A lead bob, with a relatively larger mass, generates larger inertia. Hence, the small amount of energy transferred by the loudspeaker is not able to overcome the inertia and displace the bob to set it into oscillation.

Did You Know?

• The ocean-like sound we hear when we place a seashell to our ears is due to an effect of resonance.
  
  
• Resonance can also damage properties. The collapse of the Tacoma Narrows Bridge in Washington in 1940 was a result of resonance. The frequency of the wind was equal to the natural frequency of the bridge, thus setting the structure into vibration, and causing it to collapse.

[Physics Form 5] Rendezvous Point

Situation:

Two waves, Wave 1 and Wave 2, of the same frequency, wave length and amplitude, approach each other.

1. State the principle applied in the phenomenon above.
   Principle of superposition.
   
   
2. Define the principle.
   When two coherent waves travelling in opposite directions meet, the resultant displacement is the vector sum of the displacements of the two waves at the point of interference.
   
   
3. What is happening at Y and what is X?
   At Y, where two pulses of the waves overlap, constructive interference occurs. X is the resultant pulse of the overlap.
   
   
4. What is happening at U and what is V?
   At U, where a pulse of a crest and a pulse of a trough overlap, destructive interference occurs. V is the resultant pulse of the overlap. It shows zero displacement; a state of calmness or no wave pulse.
   
   
5. What is expected after the meeting of Wave 1 and Wave 2?
   The two waves travel in opposite directions along their original paths, unaffected by each other.

Note: Interference only occurs at the meeting point between two pulses of waves.

[Physics Form 5] The Superpose

Situation 1:
Two circular water waves of the same frequency that are in phase, or of a constant phase difference, are produced adjacent to each other.

These two waves, such as S₁ and S₂ in the below diagram, are known as coherent waves.

Due to their close proximity, the wavefronts of the waves overlap. This sets off the phenomenon of interference.

Shown in the diagram is the plan view, or view from the top, of the overlapping waterfronts. The full circle represents the crests or peaks while the dotted lines represent the troughs. The crests and troughs form circles of points that are in phase.

Question & Answer

1. What do line P and point L represent?
   Due to interference:
   All crests of S₁ and S₂ that are in phase overlap each other. The same goes to all troughs of S₁ and S₂. This reinforces the amplitude of the waves and is known as constructive interference, represented by line P.
   
   The meeting of a crest of S₁ and trough of S₂, and vice versa, causes the cancellation of the amplitude of the waves. This is known as destructive interference, which is represented by point L.
   
   
2. What can be done to analyse the phenomena occurring at P and L?
   By placing a floating object on the surface of water along line P and point L, the movement of object can be observed and the phenomena analysed.
   
   
3. How are the sources, represented by S₁ and S₂, produced using;
   
   • Water waves
     By using two circular dippers fixed to a motor - both the dippers will vibrate at the same frequency and be in phase, as dictated by the motor.
     
     
   • Light waves
     Light from a single bulb is incident on a double slit. Each slit acts as a secondary source of light that has the same frequency and phase.