Month: March 2014

010 How did those electrons slip through the magnetic shield (at 00:21) ?

The force experienced by a moving charge is dependent on the angle between directions of v and B. (F = qvBsinθ)


This implies that a moving charge experiences zero force if it is moving along the magnetic field.


In this video, it was shown that the electrons which were headed directly into the pole of the magnet suffered no deflection, while the electrons which were headed around the hole were deflected away.

0 CRT_screen._closeup

We are lucky that the Earth’s magnetic field is not oriented with its pole pointing directly at the Sun. If not, we would be defenseless against the solar flare.

009 What is the polarity of the magnet at 00:24, 00:36 and 00:48?

Firstly, we note that an electron beam coming towards the CRT screen is equivalent to a conventional current moving towards the back of the CRT.

magnetic field

Secondly, we note that if the electrons are moving directly towards the north pole of a magnet, they should see a somewhat outward radial field.  Conversely, if they are moving towards the south pole of a magnet, they are traveling towards a somewhat inward radial field.


Thirdly, we need to know our Fleming’s Left Hand Rule.

So, if the north pole of a magnet is pressed into the CRT screen, the electrons coming towards the north pole should be deflected in a clockwise direction (use Fleming’s Left Hand Rule). This was the situation at 00:36 of the video.

On the other hand, if the south pole of a magnet is pressed into the CRT screen, the electrons should be deflected in an anti-clockwise direction (use Fleming’s Left Hand Rule). This situation occurred at 00:24 and 00:48 of the video.

008 Can magnets change the color of light?

A color CRT (Cathode Ray Tube) is coated with three different phosphors which emit red, green, and blue light respectively. These colored pixels are often arranged in configurations as shown below.Three electron guns (one for each primary color) direct electrons towards their intended pixels to form the intended image.

0 CRT_screen._closeup

Being negatively charged, moving electrons in a magnetic field will experience a magnetic force. As such, a magnet near CRT will deflect the electron beams away from their original intended paths, resulting in the electrons hitting a pixel at a different location and color from the original intended pixel.

007 Bulbs together light up faster? Bulb on its own light up slower?

The resistance of a filament increases with temperature. So a bulb that is already lit up has a larger resistance than a cold bulb.


In this video, when the 10 bulbs are all lit up from cold together, because they grow hot together at the same rate, their resistances are the same at any one time. So the potential difference across each bulb is always 1.2 V (12 V /10 bulbs).

00Picture1When 1 bulb is lit up from cold, with all the other 9 already hot, the initial potential difference across the cold bulb is much less than 1.2 V. This is because the other 9 hot filaments, with their larger resistances, has more than 1.2 V across each of them, as the potential divider principle dictates.

The pd across the 10th bulb will however eventually grow to become 1.2 V, when the bulb has heated up and its resistance becomes the same as all the other bulbs.

Compared to a bulb with 1.2 V across right from the beginning, a bulb with pd increasing slowly towards 1.2 V takes a longer time to light up.

005 What does the ringing say? (about projectile motion)

The force of gravity is the only force acting on the ball when it was in the air. Since gravity is constant and acts vertically downward, the vertical motion is uniform acceleration motion at g = 9.81 m s-2 (downward), whereas the horizontal motion is constant speed motion.


The fact that the horizontal speed of a projectile is constant is confirmed in the video by the regular spacing of the chimes, as the ball crosses the regularly spaced vertical lines at equal time intervals.

006 In which direction does fluid pressure act?

From the way the “water skin” “explodes” away, we can infer the direction in which the water was pressing against the balloon. It looks like at every point on the balloon surface, the pressure forces were acting perpendicular to the balloon surface.

We should be careful in distinguishing between PRESSURE and PRESSURE FORCES. At the microscopic level, because each fluid molecule pushes against all the surrounding molecules, pressure acts in all directions. At the macroscopic level, when a fluid encounters a surface, it exerts a pressure force perpendicularly into the surface.

Because pressure “acts in all directions”, pressure is not considered to be a vector quantity (which should act in only ONE direction). The pressure force acting on a surface, on the other hand, acts in one particular direction, and is considered to be a vector quantity. Meaning in the formula F=P.A, F is vector whereas P is scalar.

004 What is the cloud made of?

Initially, the bottle contains alcohol vapour, which is invisible to our eyes.

00Picture1When the pressurized air in the bottle is suddenly allowed to expand, it undergoes what is called an adiabatic process. During an adiabatic expansion, a gas does work to expand against the external pressure with negligible heat transfer from its surrounding. When a gas expenses its internal energy this way, its temperature drops. The chill in the bottle causes the alcohol vapor to condense into visible alcohol clouds.


002 How can a 2V EMF outsine a 3V EMF?


While a battery provides the EMF (E) to push current through the external circuit, it also comes with its own internal resistance (r). The amount of EMF spent to push current through its own internal resistance is directly proportional to the current passing through the battery (Ir). So the amount of EMF left to push current through the external circuit, called the terminal potential difference (Vt), decreases whenever the battery is made to pump out larger current.

In this video, as we connect more and more bulbs (in parallel) across the battery, we draw larger and larger current from the battery. This must lead to a larger and larger potential difference across the internal resistance of the battery, which implies a smaller and smaller terminal pd, which dims the bulbs.

There is also a simple explanation why the 2V battery could “outshine” the 3V. The 2V battery may have a lower EMF, but by having a smaller internal resistance, it provided a higher terminal pd to the external circuit.

001 Why does the electron beam jump over the LDR (Light Dependent Resistor)?



The circuit used in this demo is a simple potential divider consisting of an LDR (Light dependent resistor) and a fixed resistor. The potential difference across the fixed resistor is monitored on the CRO.

By facing the LDR towards the CRO screen, whenever the CRO trace arrives near the LDR, it illuminates the LDR and reduces its resistance. The potential difference across the fixed resistor thus increases. (This can be deduced by either (1) noting that the current in the circuit has increased or (2)the potential divider principle.) The CRO trace thus “jumps” over the LDR.

In a way, the CRO has ended up measuring its own brightness.