17 Electromagnetic Forces

20 March

Magnetic Shielding

Seng Kwang Tan 17 Electromagnetic Forces, Demonstrations, IP4 14 Magnetism 0 Comments

I made this rather simple video this morning showing a physics demonstration on the effect of magnetic shielding. A paper clip is shown to be attracted to a magnet. A series of objects are placed in between, such as a plastic ruler, a steel ruler, a steel bookend, and some coins of different alloys.

It is interesting to note the types of material that provide magnetic shielding and those that do not. There is even a distinction between the types of steel, which is an alloy containing iron. Ferritic steel is magnetic while austenitic steel is not.

The theory behind magnetic shielding is that the flat magnetic material will direct the field lines of the magnet along its plane instead of allowing them to pass through, thus depriving the paper clip of a strong enough magnet field to keep it flying.

01 September

Diamagnetism

Seng Kwang Tan 17 Electromagnetic Forces, Demonstrations, IP4 14 Magnetism 0 Comments

I didn’t want to spend money on buying a piece of pyrolytic graphite and large neodymium magnets so I made do with what I have to make the following video. While diamagnetism is not in the A-level physics syllabus, it’s good for students to know that there are other classifications of magnetic materials.

What we study in our syllabus is ferromagnetism, which is exhibited by materials such as iron, cobalt and nickel. Some pencil leads are paramagnetic (weakly attracted to magnets) while others such as the one in the video are diamagnetic (repelled by magnets).

I bought my neodymium magnets from DX.com and the shipping to Singapore takes about 3 weeks, so you might want to factor that time in if you want to get some for your lessons. These magnets are great for other demonstrations such as homopolar motors and Newton’s nightmare.

27 February

Magnetic Shielding

Seng Kwang Tan 17 Electromagnetic Forces, Demonstrations, IP4 14 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”.

 

25 February

Homopolar Motor 2

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

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

Seng Kwang Tan 17 Electromagnetic Forces, 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

Seng Kwang Tan 17 Electromagnetic Forces, 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.