Black & White TV & Magnetism


Magnets exert forces on moving charges such as electrons. To make a television picture, an electron gun at the back of the TV's picture tube shoots a beam of electrons at the inside of the front screen. These electrons hit a phosphor coating on the screen and cause the phosphor to glow. Two electromagnets inside the television exert forces on the electron beam and deflect it from left to right across the screen, and from top to bottom, up and down the screen. The rapidly moving electron beam “paints” the television image on the screen. As the beam scans the screen, the beam’s intensity gets stronger and weaker to make the bright and dim places on the screen.

The strong magnet you place on the screen also exerts forces on moving electrons. Your magnet deflects the electron beam, distorting the picture. Once you remove the magnet, the picture returns to normal.

You can use the “right hand rule” to predict which way the electron beam will move. Magnetic fields are defined to come out of the north pole of a magnet and to go into the south pole. First, you need to be facing the oscilloscope or TV tube (or imagine yourself facing it). Point the fingers of your right hand in the direction of the magnetic field (in our experiments, that’s downward), then point your thumb in the direction of the moving charge (toward the front of the screen). The trick here is to rotate your hand at the wrist so that your hand stays flat. As long as you’ve kept your hand flat, your palm will point in the direction of the magnetic force on a positive charge (in this case, to the right). For negative charges, the force will be in the direction opposite your palm (in this case, to the left). This allows you to predict that the electron beam is deflected to the left when you hold the north pole of a magnet over the oscilloscope.

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