Learning Library

# The Right-Hand Rule for Magnetic Force

What do you think will happen when you bring the magnet near the electron beam? What will happen if you bring the opposite pole of the magnet to the electron beam?

### Materials

• Cathode ray tube (and accompanying power source)
• Horseshoe magnet

Safety: Be sure to follow all the safety precautions recommended by the manufacturer of the cathode ray tube.

### Procedure

1. Following all safety precautions, turn on the power supply to the cathode ray tube (CRT).
2. Slowly bring the horseshoe magnet close to the CRT so that the CRT is in between the poles of the magnet. Note your observations.
3. Flip the magnet around so that the poles are on the opposite sides of the CRT than in step 2. Note your observations.

### Results

Bringing the magnet close to the electron beam will cause the beam to bend. The right hand rule will help you make sense of the fields in play.

### Why?

The cathode ray tube carries current in the electron beam when the power source is activated. Flowing current, which is the flow of electrons, has the ability to create its own magnetic field. Current flows from negative to positive, so based on how your power source is set up, you can determine which way the current is flowing. Using the basic right-hand-rule, you can figure out the direction of the magnetic field created by the current. If you make a “thumbs up” sign with your right hand, the thumb points in the direction of the current, and the curve of the fingers show which way the magnetic field is oriented.

When you bring the magnet close to the cathode ray tube, the magnetic fields will interact with each other and the electron beam will bend. The direction it bends is dependent on the orientation of the magnet poles on either side of the cathode ray tube.

A more complicated right hand rule (RHR) is Fleming’s RHR, which describes the motion or force in which something moves. It is useful for understanding the direction of various players in electromagnetism, since they interact at right angles. The direction of the thumb is the direction of the force, the direction of the index finger indicates the direction of the magnetic field, and the direction of the middle finger is the direction of the electric current.

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