Interactive Lecture Demonstration: Diffraction spectra of hydrogen, helium, and mercury

by Erees Queen Macabebe and Ivan Culaba
Department of Physics, Ateneo de Manila University

Spectral tubes for hydrogen, helium and mercury. The diffraction grating has 600 lines per mm.

Objective

Show that different wavelengths of light diffract at different angles.

Description of the Set-Up

This demonstration requires diffraction gratings, spectral tubes containing different gases and power supply for the tubes.

The spectral tubes contain different gases: hydrogen, helium and mercury. The diffraction grating used has a specification of 600 lines per mm

Demonstration

Attach a spectral tube to the power supply and observe the spectral lines emitted by the gas using the diffraction grating. Is the light emitted monochromatic? Is the spectrum continuous or is it composed of discrete lines?

Do the same for the other spectral tubes. Compare the emission spectra of the different gases.

Precaution

The power supply for the spectral tubes has a high voltage output. Do not touch its electrodes.

Hydrogen gas in a spectral tube

Helium gas in a spectral tube

Mercury gas in a spectral tube

Diffraction spectra of hydrogen

Diffraction spectra of helium

Diffraction spectra of mercury

Interactive Lecture Demonstration: Convection current in air

by Erees Queen Macabebe and Ivan Culaba
Department of Physics, Ateneo de Manila University

Candle, mosquito coil, matches, and a transparent box with two chimneys

Objective

Demonstrate convection of heat in air.

Description of the Set-Up

The set-up consists of a transparent box with two chimneys, a candle and a mosquito coil. The match is used to light the candle.

Demonstration

Light the candle and place it inside the box near the opening of one of the chimneys. Light the mosquito coil until it glows red then put off the flame. With the box closed, place the mosquito coil near the top of the second chimney.

Observe what happens to the smoke coming from the coil.

The lighted mosquito coil is placed near the opening of the second chimney.  Where did the smoke from the mosquito coil go? Why?

A lighted mosquito coil about to be placed on top of the chimney that has no candle

A lighted mosquito coil placed on top of the chimney that has no candle

Interactive Lecture Demonstration: Bouyancy of a medicine dropper

by Reese Macabebe and Ivan Culaba

Medicine dropper floating in the water inside a plastic bottle

Objective

Demonstrate that changes in the density of an object due to fluid pressure causes an object immersed in a fluid to float or sink.

Description of the Set-Up

The set-up consists of a 2-liter plastic bottle with screw cap, a medicine dropper and tap water.

Demonstration
Fill the 2-liter plastic bottle with tap water almost to the top. Draw water into the medicine dropper until it is approximately 3/4 full then drop the medicine dropper into the plastic bottle. The medicine dropper floats in the bottle just below the surface of the water. Tightly screw the cap into the bottle and squeeze the sides of the bottle.

What will happen to the dropper if you squeeze the bottle? If you release your grip on the bottle, what will happen to the dropper? movie

Note

For this demonstration, food coloring was added to the water inside the medicine dropper for distinction.

Interactive Lecture Demonstration: Bernoulli’s principle and airfoil

by Reese Macabebe and Ivan Culaba

Objective

Demonstrate one application of Bernoulli’s Principle: the lift on an airplane’s wing.

Description of the Set-Up

The set-up is a small wind tunnel. A high-speed fan, controlled by a VARIAC provides the air stream on the air foil, a miniature section of an airplane’s wing. The airfoil is tilted at an angle (the angle of attack) with respect to the direction of the air stream. A counterweight, which is not present in real wings, is attached to the airfoil so that the airfoil will lift at lower air speeds.

Demonstration

The airfoil is first set at its lowest position. The VARIAC is slowly adjusted to increase the voltage in the motor of the fan. What will happen to the airfoil when the speed of the air stream is increased? Why?

Precaution

1. Do not apply a voltage on the fan motor beyond 110 V otherwise it will be damaged.
2. Do not hold the fan nor put anything on the wind tunnel while it is in operation.

Interactive Lecture Demonstration: Bernoulli’s principle and objects suspended in air stream

by Erees Queen Macabebe and Ivan Culaba

Variac, styrofoam ball, and air blower

Objective

Demonstrate object suspension in an air stream.

Description of the Set-Up

The set-up consists of a blower, styrofoam ball, VARIAC, and extension wire for the blower.

Demonstration

Connect the blower to the VARIAC. The VARIAC controls the input voltage to the blower and thus, controls the rate of airflow from the blower.

Place the ball on the mouth of the blower. Switch on the blower and slowly adjust the VARIAC until the air stream lifts the ball above the blower.

What will happen if the ball is slightly pushed off the line of the air flow?

Tilt the blower to one side. What will happen to the ball? Why?

Balance the ball in the air stream and then move the blower toward a wall (try the corner of a room). What will happen to the ball?