Month: February 2013

28 February

Ionisation of Air to Remove Static Electric Charges

13 Electric Fields, Demonstrations

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.

27 February

Magnetic Shielding

16 Electromagnetism, Demonstrations, IP4 15 Magnetism

Materials

  1. Bar magnet
  2. Paper clip (stainless steel)
  3. Plastic clipboard
  4. Steel bookend

Procedure

  1. Before the demonstration, make sure that the paper clip is not already magnetised by touching it with the steel bookend. If it gets attracted to the bookend, get a new paper clip.
  2. For the demonstration, first show that the paper clip can be picked up by a bar magnet in direct contact.
  3. Next, place the bar magnet on the plastic clipboard and try to pick up the paper clip with the clipboard in between them. Show the audience that the paper clip is attracted.
  4. Finally, place the bar magnet on the steel bookend and attempt to pick up the paper clip with the bookend between them. The paper clip will not be picked up.

Science Explained

Iron and steel are examples of ferromagnetic materials that have their magnetic domains aligned with an external magnetic field when placed in that field. This strengthens the magnetic field but also serves to concentrate the field lines within the ferromagnetic material itself, such that very little of the magnetic field penetrates the “shield”.

 

26 February

Heat Capacity of Water

09 First Law of Thermodynamics, Demonstrations

Water has a high specific heat capacity of about 4200 J kg-1 K-1. When a little bit of water is placed in a balloon, it is able to absorb a significant amount of heat from a candle flame and hence prevent the balloon from bursting.
Materials
  1. Two balloons
  2. Two candles
  3. Lighter
Procedure
  1. In this demonstration, one balloons is filled with about 3 tablespoons of water and then inflated.
  2. Another balloon is inflated to the same size as the first to serve as a control.
  3. Both balloons are then placed vertically over two identical candles. Adjust the balloons such that the distance from balloon to candle is the same for both setups. You can use retort stands to clamp the balloons in place if you have them.
  4. Light the candles with the balloons temporarily removed. The flames will have to touch the bottom of the balloon when they are placed back over the candles.
  5. Observe the balloon without water burst first.
  6. The air gushing out from the exploding balloon may put out the other candle.
  7. If you like, you can keep the balloon with water over the flame for a longer duration. The balloon still will not burst until a long time later.
25 February

Homopolar Motor 2

16 Electromagnetism, Demonstrations

This video demonstrates how a simple homopolar motor is made using a screw and a small neodymium magnet. The simplest possible motor one can make, it can be used to teach concepts at various levels. For lower secondary students, they can learn about conversion of energy forms while upper secondary students can learn about magnetic forces and Fleming’s left-hand rule.

Materials

  1. 10 cm wire
  2. 1.5 V battery
  3. iron screw
  4. neodymium magnet

Procedure

  1. Attach the neodymium magnet to the head of the screw.
  2. Attach the tip of the screw to one end of the battery such that the screw hangs below the battery. The screw will remain attached to the battery as the magnetic force from the neodymium holds them together.
  3. Hold one end of the wire on the top terminal of the battery and allow the other end of the wire to touch the side of the screw or the magnet. Watch the screw spin.

(Link: Alternative design for the homopolar motor)

16 February

Electromagnetism Lecture

16 Electromagnetism, Blog

I enjoy lecturing on topics like Superposition and Electromagnetism in the GCE A-level syllabus as they lend themselves well to the use of fun demonstrations that I can perform in front of the audience.

One of the recent demonstrations that I did was to demonstrate the measurement of the magnetic force acting on a wire and to show that the force can be inverted when the current is reversed. The magnitude of the force can be shown to be consistent with the relationship $$F = BIl \sin \theta$$, where $B$ is the magnetic flux density, $I$ is the current within the wire, $l$ is the length of the wire and $\theta$ is the angle between the wire and the magnetic field. This can be illustrated by independently varying one of the 4 variables and observing the change in force.

The setup is also a good for a demonstration to illustrate Fleming’s Left-Hand Rule.

Meanwhile, here’s a video I made to show what I did:

16 February

Measuring the Force on a Current-Carrying Conductor

16 Electromagnetism, Demonstrations

The force acting on a current-carrying conductor is given by $$F =BIl sin \theta$$ where B is the magnetic flux density, I is the current, l is the length of the wire and $\theta$ is the angle between the current and the magnetic field. It can be measured in directly using a weighing scale as described below:

Materials

  1. An electronic weighing scale
  2. Two neodymium magnets
  3. Plasticine
  4. Batteries
  5. 40 cm long wire

Procedure

  1. Place some plasticine between two neodymium magnets to hold them together with each magnet having a different pole facing up (north for one and south for the other).
  2. Place the magnets on the weighing scale.
  3. Connect the 40 cm wire across the opposite terminals of the batteries.
  4. Hold the wire horizontally and place it between the two magnets so that the current runs perpendicularly to the magnetic field lines.
  5. Using Fleming’s left-hand rule, one can predict whether the measured weight will increase or decrease. If the magnetic force acting on the wire is upward, by Newton’s 3rd law, the reaction force acting on the magnets is downward and the measured weight will increase, and vice versa.
  6. Flipping the current around in the opposite direction will yield opposite results.