Demonstrations

Physics demonstrations for all.

18 May

Polarization Using Sunglasses and a Computer Screen

Seng Kwang Tan 11 Wave Motion, Demonstrations 0 Comments

Using a pair of polarizing sunglasses, you can demonstrate the effects of polarization together with a computer screen which is also polarizing. When the axes of polarization of the two polarizing screens are rotated, the brightness alternates between bright and dark.

Light coming from a computer screen is usually polarized. In the video below, when polarized light passes through another polarizer, the intensity of the light is given by Malus’ law:

$$I = I_o cos ^2{\theta}$$

where $$\theta$$ is the angle between the two axes of polarization and $$I_o$$ is the original intensity of the unpolarized light.

Only the components of electric field vectors in electromagnetic radiation that are parallel to the axis of polarization of a polarizing filter will be permitted through. Those electric field components that are perpendicular to the polarization axis are blocked by the filter.

malus-law

Hence, the amplitude of a vector A that passes through is given by $$A = A_o \cos{\theta}$$. Since intensity is proportional to the square of amplitude ($$ I \propto {A^2}$$), we have Malus’ law.

The purpose of having polarizing filters in sunglasses and computer screens is to cut out glare due to light from other sources.

30 March

Boiling under Reduced Pressure

Seng Kwang Tan 08 Temperature and Ideal Gases, Demonstrations, IP4 18 Thermal Properties of Matter

With the help of a simple manual vacuum pump that is used to keep food fresh, we can demonstrate the effect of a reduced pressure on the boiling point of water. This leads students to a discussion on what it takes to boil a liquid and a deeper understanding of the kinetic model of matter.

Materials

  1. Vacuum food storage jar with hand-held vacuum pump
  2. Hot water

Procedure

  1. Boil some water and pour them into the jar such that it is half filled. This is necessary as hand-held vacuum pumps are not able to lower pressure enough for boiling point to drop to room temperature.
  2. Cover the jar with the lid and draw out some air with the vacuum pump.

Explanation

When water boils, latent heat is needed to overcome the intermolecular forces of attraction as well as to overcome atmospheric pressure. Atmospheric air molecules would prevent a significant portion of the energetic water molecules from escaping as they will collide with one another, and cause them to return beneath the liquid surface.

Removal of part of the air molecules within the jar lowers the boiling point of water because less energy is needed for molecules to escape the liquid surface.

29 March

Egg out of Flask

Seng Kwang Tan 08 Temperature and Ideal Gases, Demonstrations, IP3 05 Density and Pressure

In a previous demonstration, we put a boiled egg into a flask with a mouth narrower than the egg. The challenge is now to remove the egg from the flask without breaking it.

Materials

  1. Flask
  2. Egg
  3. Water
  4. Bunsen burner or candle

Procedure

  1. Pour some water into the conical flask.
  2. Invert the flask quickly over a tray such that the egg seals the mouth of the flask, preventing the water from coming out.
  3. Light a flame and place the part of the flask with water over the flame. This will help prevent the heat from cracking the flask.
  4. Place a tray under the mouth of the flask as the egg slides out to prevent a mess.

Explanation

The flame heats up the air and the water in the flask. The heated air expands while some of the water vapourizes. With the increase in amount of gas and temperature, the pressure within the flask increases.

29 March

Egg into Flask

Seng Kwang Tan 08 Temperature and Ideal Gases, Demonstrations, IP3 05 Density and Pressure

This classic physics demonstration is used to show the effects of pressure difference between the atmosphere and a cooling volume of air. With a set of clean apparatus, you can even have the egg for a snack after that.

Materials

  1. Hard-boiled Egg
  2. Flask or glass bottle with mouth smaller than the egg
  3. Paper measuring about 2 cm by 5 cm
  4. Lighter

Procedure

  1. Peel the hard-boiled egg.
  2. Light the piece of paper and drop it into the flask.
  3. Place the peeled egg on the mouth of the flask such that the egg seals the flask.
  4. Observe the egg being sucked in while the flame dies.

Explanation

When the burning paper enters the flask, it causes the air within the flask to heat up and expand, with some escaping from the flask. When the egg seals the flask, the flame dies as the paper is about to be burned up while oxygen is also running out.

The air then cools down and the pressure within the flask drops. The pressure due to the atmosphere acting downward on the egg is then greater than that acting upward due to the pressure of the cooling air. This pushes the egg into the bottle.

08 March

Single Slit Diffraction using Fingers

Seng Kwang Tan 12 Superposition, Demonstrations

This demonstration requires no material other than your own fingers. Hold your index and middle fingers close to each other, leaving a small slit between them about 1 mm in width.

Look through the slit into a source of light such as the window or a lamp. You will need to look with one eye up close to the slit. Warning: do not look directly at the sun.

You will be able to see a number of vertical dark lines between the fingers.

diffraction and interference pattern,

Science Explained

So where do these vertical lines come from? They are dark fringes caused by destructive interference of light when it diffracts through your finger tips.

This phenomenon can be explained using Huygens’ principle. Huygens pictures every point on a primary wavefront as a source of secondary wavelets and the sum of these secondary waves determines the form of the wave at any subsequent time. Hence, each of these secondary wavelets can interference with one another.

Constructive interference takes place when the difference in path lengths between two coherent waves is an integer multiple of the wavelength. This is when the resultant wave is the brightest. Destructive interference occurs when that difference in path length is a half-integer of the wavelength (e.g. $\frac{1}{2}\lambda$, $\frac{3}{2}\lambda$, $\frac{5}{2}\lambda$, etc.) and gives a dark fringe.

The alternating bright and dark fringes is a diffraction pattern, which becomes observable by the eye looking through the slit.

28 February

Ionisation of Air to Remove Static Electric Charges

Seng Kwang Tan 13 Electric Fields, Demonstrations, IP4 10 Static Electricity

Materials

  1. Wool
  2. PVC pipe or plastic comb
  3. String
  4. Lighter

Procedure

  1. Hang the string from an elevated position. Leave the bottom end free.
  2. Rub the PVC pipe with wool. This deposits negative charges, or electrons, onto the surface of the PVC pipe.
  3. Place the side of the pipe that is rubbed near the string. You should notice the string being attracted  towards the PVC pipe.
  4. Holding the PVC pipe still while attracting the string, light a flame using the lighter and place it in between the string and pipe. You should observe the string falling back to its original position.

Science Explained

When air is ionised with the help of a flame, it serves as a conducting medium through which static electric charges can escape from a surface.