Teaching Resources

Physics teaching resources

23 October

Electroscope

Seng Kwang Tan 14 Electric Fields, Demonstrations, IP4 10 Static Electricity

An electroscope is a device that can be used to detect or measure the amount of charge in its vicinity. One of the earliest electroscopes is the gold-leaf electroscope which was invented by a British clergyman Abraham Bennet. This is a cheaper model of the leaf electroscope made using aluminum foil.

Materials

  1. Paper clip
  2. Aluminum foil
  3. Modelling clay
  4. Glass bottle with a narrow neck
  5. Steel or brass sinker

Procedure

  1. Cut two strips of aluminum foil measuring 2 cm by 0.5 cm.
  2. Straighten the paper clip before bending both ends to make two hooks. Hang the paper clip using one hook from the sinker.
  3. Pierce each aluminum strip at one end through the other hook of the paper clip, leaving it to hang from the hook.
  4. Place the paper clip and aluminum strips inside the bottle. If the sinker is smaller than the neck of the bottle, use some modeling clay to keep it in place.
  5. Now you can test the electroscope by rubbing a comb with some wool and placing it near the paper clip.

Science Explained

Negative charges (electrons) are deposited on the comb by rubbing with wool. When the comb is placed near the sinker without touching, the negative charges in the sinker are repelled. As glass is an electric insulator, the only way for them to go is downwards onto the aluminum strips. Both strips are now negatively charged and will repel each other. The extent of their repulsion is dependent on the amount of charge on the comb and its distance from the electroscope.

22 October

Oersted’s Experiment

Seng Kwang Tan 17 Electromagnetic Forces, Demonstrations, IP4 15 Electromagnetism

Hans Christian Oersted showed that an electric current can affect a compass needle in 1820. This confirms the direct relationship between electricity and magnetism, which in turn, paved the way for further understanding of the two. The direction of the magnetic field can be changed by flipping the wire around, which suggests that the direction of the magnetic field is dependent on the direction of current flow.

Materials

  1. 1.5V Battery
  2. Wire
  3. Compass

Procedure

  1. Place the compass on a horizontal surface.
  2. Connect the wire to both ends of the battery.
  3. Place the middle of the wire directly over the compass, parallel to the initial orientation of the needle.
  4. Observe the needle deflect to one direction.
  5. Now flip the wire over so the current flows in the opposite direction and place it over the compass again.
  6. The needle will deflect in the other direction.
  7. Additionally, you can place the compass on top of the wire now.

Science Explained

A current will carry with it its own magnetic field. The magnetic field lines form concentric circles around the wire so that the field points in one direction above the wire and the opposite direction below the wire. Using the right-hand grip rule, where one holds his hands as though he is gripping something with his thumb pointing in the direction of current flow, his fingers will curl in a way as to indicate the direction of the magnetic field. This is also the direction in which the needle deflects.

13 July

Angular Displacement – 2011 A-level question

Seng Kwang Tan 07 Circular Motion

A disc rotates clockwise about its centre O until point P has moved to point Q, such that OP equals the length of the straight line PQ. What is the angular displacement of OQ relative to OP?

A.   $\frac{\pi}{3}$ rad

B.   $\frac{2\pi}{3}$ rad

C.   $\frac{4\pi}{3}$ rad

D.   $\frac{5\pi}{3}$ rad

Click to view answer

Answer: D.

The triangle OPQ is equilateral, so the angle $\angle QOP$ = 60° or $\dfrac{2\pi}{6}=\dfrac{\pi}{3}$ rad.

As OQ is displaced clockwise from OP, angular displacement $\theta = 2\pi – \dfrac{\pi}{3} = \dfrac{5\pi}{3}$ rad.

31 January

How Does Siphoning Work?

Seng Kwang Tan 02 Force and Moments, Demonstrations, IP3 05 Density and Pressure 0 Comments

A siphon operates through the combined effects of gravity and air pressure, which work together to move liquid from a higher elevation to a lower one. Gravity is the primary force driving the flow, as it pulls the liquid from the higher container down through the siphon tube to the lower container. The liquid’s potential energy, due to its elevated position, is converted into kinetic energy as it flows downward.

Air pressure plays a crucial supporting role by maintaining the continuous flow of liquid. Atmospheric pressure on the liquid’s surface in the higher container pushes the liquid into the siphon tube. This pressure counteracts gravity’s pull that might otherwise cause the liquid to fall back into the higher container. As the liquid moves downwards, it creates a partial vacuum in the upper part of the tube, allowing atmospheric pressure to push more liquid into the tube, sustaining the flow.

Thus, a siphon can continue to operate as long as the outlet is lower than the liquid surface in the source container, the tube remains filled with liquid, and atmospheric pressure supports the flow.

15 January

Wave-Particle Duality of Electrons

Seng Kwang Tan 19 Quantum Physics, Teaching Resources 0 Comments

I find this video easy to understand and it may be useful for students to appreciate the wave property of matter and how it is observed via interference.  The video ends with a mind-boggling problem that when an attempt to detect the path of the electron, it goes back to behaving as a particle.

There’s a whole series of “What the Bleep” videos that you might want to check out also. Be careful though, the rabbit hole is pretty deep.

Quoting from another website on what could have happened to each electron and to make the problem clearer (and hence more confusing):

The possibilities are: 1) the electron went through the left slit; or 2) the electron went through the right slit; or 3) the electron went through both slits. For the sake of logical rigor, we should add the possibility that 4) the electron went through neither slit (that is, it found some other way to get to the back wall). Now, one problem with possibility number 3 — a single electron going through both slits — is that, in nature, there is no such thing as half an electron. So if we found half an electron at both slits, we would have something really new; but that has to be a distinct possibility, considering that, in order to create the apparent interference pattern, something would have to radiate from both slits.

How are we going to find out? Well, we are going to put an electron detector at each slit. The electron detectors at the slits will be devices to keep watch over the passage through the slit. Every time an electron (or part of an electron) goes through, the detector will give a holler, “Hey, an electron (or part of an electron) just went through.” In this way, we will be able to learn something about how the electrons get through the barrier in a double slit experiment.

As it turns out, when you put the electron detectors at the slits, the result is that the electron is always detected at one slit or the other slit. It is never found going through both slits. And it is never found going through neither slit. You send one electron through, you find it at one of the slits. We have eliminated possibilities number 3 (both slits) and number 4 (neither slit). The only results we find are possibilities number 1 (left slit) or number 2 (right slit), in equal proportions.

They call this phenomenon the measurement effect. When we measure something at the quantum level, the very act of measurement will have an effect on the thing itself.

This is a phenomenon that still has no classical explanation.

Even Richard Feynman called it “a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery.”

02 January

Water in an Inverted Cup

Seng Kwang Tan 02 Force and Moments, Demonstrations, IP3 05 Density and Pressure 0 Comments

This demonstration can be modified for use as a magic trick.

Materials:

  1. Glass of water
  2. Piece of cardboard that is larger than the mouth of the glass.

Procedure:

  1. Fill the glass up with water.
  2. Place the piece of cardboard over the mouth of the glass.
  3. Holding the cardboard against the mouth of the glass, invert the glass.
  4. Release the hand slowly.

Explanation

Water can remain in an inverted glass with the piece of cardboard underneath because atmospheric pressure is acting upward on the cardboard, holding it up together with the water. There is little air pressure within the g;ass, so the downward force acting on the cardboard is mainly the weight of the water, which is to the order of several newtons whereas atmospheric pressure exert an upward force of several thousand newtons.

Modification:

  1. Drill a small hole in a plastic cup, near the base.
  2. Seal the hole with your thumb and fill the cup with water.
  3. Place the cardboard over the mouth of the cup.
  4. Invert the cup together with the cardboard, while keeping your thumb over the hole.
  5. Using a magic word as the cue, shift your thumb slightly to allow a little air into the cup. This will cause the cardboard and water to fall. As the air pressure within the cup is equal to that of the atmosphere.