Category: 15 Electromagnetism

900 Magnetic Pump

We have current flowing across the mercury (from one ring to the other), which is sitting in a magnetic field. So there is a F=BIL magnetic force. Or we can think the moving electrons (that constitute the current) experience a F=Bqv magnetic force. This force is directed perpendicular to the current (and magnetic field), which turns out to be along the circumference of the ring. The mercuy is thus pumped in a merry-go-round fashion.

Why should the metal lattice chase after/get dragged by the electrons?

099 Simple Motor



First note that an electric current is flowing from the external wire, through the magnet, into the screw.

This current is sitting in the magnetic field of the magnet. (In fact, it is the field inside the magnet)

There is thus a F=BIL force acting on that part of the magnet which carries the current. This results in a net torque about the centre of mass of the magnet, which causes the magnet to rotate.

Alternatively, you can think of it as charges in the magnet moving in the magnet’s magnetic field. You will then be thinking of F=Bqv forces acting on the moving charges, which will result in the same spinning motion.

093 Levitron

Apparently, Levitrons come with a ring magnet in the base. The copycat Levitron I found in the lab actually placed 4 bar magnets at the 4 corners of the base (all with N-pole facing up).


Nevertheless, I think the magnetic field formed should be similar to the one shown below. I didn’t occur to me that the magnetic field of a ring magnet is so interesting: as in our situation, because of the “switch” in magnetic field lines, the magnetic top is repelled when it is above a certain height, but attracted when it is nearer to the ring magnet.


This explains the trouble I had in getting the Levitron to work. The region of space where equilibrium is possible is very tiny. And it is an unstable equilibrium that can be destroyed by the slightest disturbance. Hence the spinning is crucial in keeping the spinning top in the upright orientation.

Further reading:

035 What makes the heart spin?

A current carrying conductor placed (perpendicularly) in a magnetic field will experience a magnetic force. (F=BIL)


Most of the action is taking place at the bottom of the heart near the magnet. The right half of the heart experiences a magnetic force into the screen, whereas the left half of the heart experiences a magnetic force out of the screen. These two forces form a couple which rotates the heart in an anti-clockwise direction (seen from the top).

024 Why does this DC motor not need any commutator?


When electric current passes through the coil in a magnetic field, the magnetic forces (F=BIL) produce an anti-clockwise torque which turns the DC motor.


When the coil crosses over to the other side, the magnetic forces produce a clockwise torque instead which would slow down the rotation. This is of course undesirable.

Usually, a commutator is used to reverse the current (and thus magnetic forces on either sides of the coil) during each cross over to keep the torque in the same direction.


What this video showed is a cheap alternative to the commutator: The coil’s insulation was sandpapered away only on one side. So when the coil flips over, it stops making electrical contact with the power supply. Without any current in the coil, there is no magnetic force. So the coil only experiences an anti-clockwise torque for half a cycle, and zero torque for the other half. Now we have a commutator-less DC motor that works by working by working only half the time.

Note: To be accurate, the connection is switched off completely only when the coil is horizontal. So the clockwise torque is not completely zero, but it is decreased enough for the design to work.

010 How did those electrons slip through the magnetic shield (at 00:21) ?

The force experienced by a moving charge is dependent on the angle between directions of v and B. (F = qvBsinθ)


This implies that a moving charge experiences zero force if it is moving along the magnetic field.


In this video, it was shown that the electrons which were headed directly into the pole of the magnet suffered no deflection, while the electrons which were headed around the hole were deflected away.

0 CRT_screen._closeup

We are lucky that the Earth’s magnetic field is not oriented with its pole pointing directly at the Sun. If not, we would be defenseless against the solar flare.

009 What is the polarity of the magnet at 00:24, 00:36 and 00:48?

Firstly, we note that an electron beam coming towards the CRT screen is equivalent to a conventional current moving towards the back of the CRT.

magnetic field

Secondly, we note that if the electrons are moving directly towards the north pole of a magnet, they should see a somewhat outward radial field.  Conversely, if they are moving towards the south pole of a magnet, they are traveling towards a somewhat inward radial field.


Thirdly, we need to know our Fleming’s Left Hand Rule.

So, if the north pole of a magnet is pressed into the CRT screen, the electrons coming towards the north pole should be deflected in a clockwise direction (use Fleming’s Left Hand Rule). This was the situation at 00:36 of the video.

On the other hand, if the south pole of a magnet is pressed into the CRT screen, the electrons should be deflected in an anti-clockwise direction (use Fleming’s Left Hand Rule). This situation occurred at 00:24 and 00:48 of the video.

008 Can magnets change the color of light?

A color CRT (Cathode Ray Tube) is coated with three different phosphors which emit red, green, and blue light respectively. These colored pixels are often arranged in configurations as shown below.Three electron guns (one for each primary color) direct electrons towards their intended pixels to form the intended image.

0 CRT_screen._closeup

Being negatively charged, moving electrons in a magnetic field will experience a magnetic force. As such, a magnet near CRT will deflect the electron beams away from their original intended paths, resulting in the electrons hitting a pixel at a different location and color from the original intended pixel.