Seng Kwang Tan

04 January

Homopolar Motor 1

Seng Kwang Tan 16 Electromagnetism, Demonstrations, Featured

A homopolar motor is a simple electric motor that does not require the use of a commutator. The electric current flows in a fixed direction within the wires of the motor. The following are instructions on how to construct this simple teaching tool that can be used to demonstrate how a motor works, as well as teach concepts such as Fleming’s left-hand rule and $$\mathbf{F}= I\mathbf{l \times B}$$.

Materials

  1. Copper wire (about 22 cm)
  2. Small neodymium magnets (1 or 2)
  3. 1.5 V AA-size battery
  4. Base with either another magnet or a iron surface, such as the head of an iron nail

Procedure

  1. Make a V-shaped bend in the middle of the copper wire, with about 0.5 cm on both sides of the V-shape. Bend the copper wire into a rectangular loop using the dimensions shown below. Homopolar Motor Diagram
  2. Tip: You may use the edge of a wooden block as a guide to bend the copper wires at right angles. A pair of wooden blocks can also be used to flatten the rectangular loop if you press them together tightly with the loop in between.
  3. Mount the neodymium magnet(s) onto the magnet or iron base.
  4. Hook the wires at the base around the magnets.
  5. Place the AA-sized battery with the protruding end on the magnet(s).
  6. Complete the electric circuit by placing the V-shaped end of the rectangular loop onto the flat end of the battery and watch the loop spin.
  7. Be careful not to keep the current flowing for too long as the battery and wire can get very hot.

Science Explained

  1. A force acts on a current if it is placed in a magnetic field. This force is what causes the motor to spin about its axis.
  2. To apply Fleming’s left hand rule, observe from the diagram below how the magnetic field bends around the magnet and its direction with respect to the direction of current flow. How do you think the loop will spin?homopolar motor field lines

If you are having difficulty making this version of the homopolar motor, try the other design for the homopolar motor made using a screw.

23 December

Cardboard Boomerang

Seng Kwang Tan 06 Motion in a Circle, Demonstrations 0 Comments

A indoor boomerang can be constructed using 3 strips of cardboard put together. Throwing it may require some practice though but when you get the hang of it, it can inject great fun into your lesson. You can explore using different types of material to get the best boomerang.

Materials

  1. Cardboard about 1 mm thick, of suitable rigidity
  2. Staples
  3. Scissors
  4. Rubber band or tape for added weight

Procedure

Cardboard boomerang for science demonstration
Cardboard strip with a slit cut

  1. Cut 3 equal rectangular strips of cardboard measuring 12 cm x 2.5 cm. You may like to trim the sharp corners on one of the ends of each strip.
  2. Cut a slit of 1.5 cm along the middle of each strip, on the untrimmed end.
  3. Join the strips together at the slits, the angle between two adjacent strips being 120 degrees.

    cardboard boomerang
    3 strips of cardboard overlapping each other
  4. One side of the slit should overlap another so that it looks like the above:
  5. Staple the overlapping centre together.
  6. The boomerang is ready for use! Throwing the boomerang is done by holding onto one of the wings. The boomerang should be almost vertical, at an angle of about 10o. With a flick of the wrist, spin the boomerang as it leaves the hand. The direction of spin should be toward the side that is tilted up.

Science Explained
A boomerang requires a centripetal force to cause it to fly in a circular path back to the thrower. This centripetal force comes from the lift that the wings generate as they cut through the air.

08 November

Tuning a Guitar using Resonance

Seng Kwang Tan 10 Oscillations, Demonstrations

There are many ways to tune a guitar. Many musicians would have tuned a string instrument using a tuning fork at some point. However, the conventional method of tuning with a tuning fork is by listening to beats while adjusting the tension of the string. The tuning fork is of a known frequency which corresponds to a note. For instance, 440 Hz corresponds to an A-note. When the A-note string is slightly out of tune, such as having a frequency of 438 Hz, the resulting sound pattern (called beats) will have a frequency that is the difference between the two frequencies, i.e. 2 Hz. Hence, the aim of tuning by listening to beats is to adjust the tension of the string until the beats disappear.

An alternative method, which is the one we shall attempt in this demonstration, is to run the vibrating tuning fork along the E-string (this first from the top) until you reach the bridge between the 5th and 6th frets. You should expect to hear a loud resonating sound there. Otherwise, adjust the tension until you do.

All the other strings are tuned with respect to that first string.

Explanation

Resonance is the phenomenon where the frequency of the tuning fork (driving frequency) is equal to the frequency of the string (natural frequency) and maximum energy is transferred from the tuning fork to the string. The string will hence oscillate with the maximum amplitude.

 

03 November

Water Bender

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

A thin stream of water can be easily bent using a plastic comb or ruler which was previously rubbed with wool. This demonstrates the attractive forces between unlike charges.

Materials

  1. Plastic ruler
  2. Wool
  3. Water from a tap

Procedure

  1. Turn on the faucet for the thinnest stream of water with a consistent flow.
  2. Rub the plastic ruler with the wool.
  3. Place the part of the ruler which was rubbed near the stream of water without touching.

Science Explained

Water molecules are polar in nature, which means that one side (where the oxygen atoms are) is more negative while another side (where the hydrogen atom is) is more positive. When wool is rubbed with plastic, it deposits electrons on the ruler.

The electrons will remain on the plastic as it is a poor conductor of electricity. When placed near the stream of water, the water molecules reorientate themselves such that the positive pole of each molecule is now nearer to the ruler than the negative pole.

The resulting attractive forces are stronger than the repulsive forces as the forces between charges decrease when the distance apart increases.

02 November

Electromagnet

Seng Kwang Tan 16 Electromagnetism, Demonstrations, IP4 15 Electromagnetism

Materials

  1. Insulated wire (about 1 m in length)
  2. Iron nail (at least 5 cm in length)
  3. 1.5 V battery
  4. Adhesive tape
  5. Small metal paper clip

Procedure

  1. Test that the iron nail is not already magnetised by trying to pick up the metal paper clip with it.
  2. Strip the two ends of the wire off its insulation. Leave about 1 cm bare on each end.
  3. Coil the wire around the iron nail, pushing each coil tightly together, to make a solenoid. Make sure you leave about 5 cm free at each end of the wire in order to connect the battery to the solenoid.
  4. If there is excess wire, make a second layer of coils around the first layer.
  5. Connect the ends of the wire to the terminals of the battery.
  6. Test the solenoid now by picking up the paper clip.