Pre-lecture activity on Quantum Physics

Current Balance Demonstration/Experiment

Photo above showing the wire frame where current will pass through. The frame is completely between the poles of the u-shaped magnet (North is red) during experiment.

Photo above shows that the wire frame is not in contact with the magnet. 

The video of the experiment is embedded below or http://youtu.be/nkS7wCCzGiY. You may pause the video at each current reading to obtain the following data. The balance is zeroed to measure ony the gain/loss in mass of magnet.

Questions: What is the direction of the force on the wire frame and the magnet? Why is the magnet getting "heavier"? When the current is reversed (http://youtu.be/hnU4g4NBXXo), why is the magnet getting "lighter"?

The electronic balance gives reading in grammes. The magnetic force is obtained by multiplying the balance reading by 10 N/kg (or 9.8 N/kg if you want to be accurate). Since

F = BIL

Plot a graph of magnetic force F against current I in the wire frame. If a linear trend is obtained, the gradient will be BL where L is measured to be 4.0 cm. 

Gradient = 4.466 mN/A = 0.004466 N/A

Therefore B = 0.004466/0.040 = 0.11 T

This value is reasonable because the magnetic flux density of the magnet should be much greater than Earth's intrinsic magnetic field (~10 nT) and flux density of more than 1 T is hard to obtain in common school lab.

Demonstrations and questions on electrons in electric or magnetic field

Demo 1 - Increasing deflection plates potential difference

Questions:

• What do you notice about the electron beam when the deflection plates p.d. is increased?
• Explain your answer to the previous question.

Demo 2 - Increasing accelerating potential  

Questions:

• What do you notice about the electron beam when the accelerating p.d. is increased?
• Explain your answer to the previous question.

Demo 3 - Electron beam in uniform magnetic field while accelerating potential is increased

Questions:

• What do you notice about the electron beam in uniform when the accelerating potential is increased?
• Explain your answer to the previous question.

Demo 4 - Electron in increasing magnetic field

Questions:

• What do you notice about the electron beam in uniform field when magnetic flux density is increased?
• Explain your answer to the previous question.

Demo 5 - Electrons enter the magnetic field not at right angle (Helical motion)

Questions:

• What do you notice about the electron enter the magnetic field not at right angle?
• Explain your answer to the previous question. The video below may help you.

How does a potentiometer work?

Feynman on kinetic model

This week I am doing work and energy with my Y3 students. We started with the concept of work and went on to the activity bouncing ball activity shown below.

Bouncing Ball Activity

Explain:

• Describe the conversion of energies during the motion from point of release to just before hitting the surface 
• Explain why the returned height after each bounce is less

Students' explanation on why the ball has a lower height after each bounce

It is a rather common for learners to associate any lost in mechanical energy as "being converted to heat". It is probably because of the fact that they have experienced the warmness due to phenomena such as rubbing their palms, hitting a nail, etc.

Qualitative understanding of the energy exchange during the bounce

What really happens requires the understanding of the following concepts:

• Temperature is a macroscopic observable/measurable of the average kinetic energy of random molecular motion
• The contact between the ball and the floor increases the random molecular motion at the expense of the ordered mechanical energy. This causes the temperature of the ball and floor to rise or we say that the internal energy of the system increases.
• Assuming that the ball and floor is at thermal equilibrium to the surrounding, the increase in their temperatures result in a net transfer of heat to the surrounding, i.e. the warmness we feel if we are to touch them (the temperature difference is likely to be less than what we can perceive in reality).

In addition, we can also apply the same thinking to conclude that the movement of the ball through the air also increases the internal energy of both the ball and the surrounding air.

Feynman's explanation below (from about 1min45sec to 3min15sec) really sums it for learners and most importantly encourages them to enjoy thinking about science.