It’s physics demo day, and here we have the old “television picture distorted by a magnetic field” trick. Many of you may have observed this phenomenon directly, or even perpetrated this electromagnetic prank yourself. However, let’s use the experiment to clarify some basic electromagnetic principles that are fundamental to the universe in which we live, as well as excellent for small talk at cocktail parties.
We’ll highlight two major points which illustrate the interconnected nature between electrical and magnetic phenomenon:
1) All magnetic fields are created by moving electrical charge.
2) An electric charge moving through an external magnetic field (created by other moving charges) may experience a magnetic force — which depends on its velocity and the direction of its motion relative to the magnetic field.*
Cathode-ray-type television screens, like the one in the video, create an image on the screen by accelerating charged particles (electrons) using an electric field. The electrons then pass between two steering coils made of copper wire. Currents running through the coils produce magnetic fields. Therefore, as the electrons pass between the coils and through their respective magnetic fields, they are deflected to specific locations on the screen. One coil controls horizontal motion, and the other vertical motion. When the charges hit the screen they cause the phosphorescent coating to glow.
Because the picture is formed by steering the charges with the electromagnetic coils, any additional external field in the nearby vicinity is going to alter the field between the coils. This is going to affect the electron deflection, resulting in a distorted, fun-house-esque picture on the screen. It’s also important to note that a strong enough magnet can permanently magnetize the copper coils resulting in a permanently distorted picture. Bear that in mind if you ever try this at home.
*It might interest you to know that the magnetic force acting on moving charges does not act in the direction of the magnetic field or even in the direction of the motion of the charge, but perpendicular to both. For the mathematically inclined, this relationship can be expressed by the cross product
qv X B where
q is the magnitude of the charge,
v is the velocity of the charge, and
B is the magnetic field vector. If the charge is not moving it will not experience a magnetic force. If it is moving parallel to the field, it still won’t experience a force. The charge must have some component of its velocity perpendicular to the field for a force to be exerted, and in turn that force will be perpendicular to both v and B.
Adam Weiner is the author of Don’t Try This at Home! The Physics of Hollywood Movies.