Physics Demo Number: 008

Approximate Run Time: 10 min.

2x4 Beam Moved By Electrostatic Forces

Demo Description

Move a freely-hanging large beam using the strength of the electrostatic force from a charged plastic rod.

Then show the gravitational attraction of a lead brick on the beam does not come close to the attractive force of a charged plastic rod on the beam.

For a greatly abbreviated, but quicker, way of showing the strength of the electrostatic force, a soda can may be pulled into rolling motion on the lecture table by the charged plastic rod.

 

Scientific Principles

  • Electrostatic forces  can be stronger than gravitational forces.

Equipment

  • Large wooden beam with dedicated suspension components

  • Large floor stand with Horizontal Mounting Rod

  • Plastic rod and Fur

  • Lead Brick (sealed with white tape)

  • Soda Can

 

Equipment Location

  • The beam lives hanging from its suspension components in the rear hall of 4228

  • The stand lives in rear hall across from [H-1] column.

  • The plastic rod, fur, lead brick, soda can and horizontal mounting rod for the beam are in Kit(008) on [H-4-4] opposite to stored beam.








Instructions

The photos show a large wooden beam with an eye-bolt at its center of mass so that it hangs nicely balanced from a loop of flat tubular cotton string. The string loop is attached to a horizontal support rod on a stand by means of two carabiners and a high-tech, low-friction swivel .

This arrangement allows for a rapid set up, with the beam being instantly in neutral rotational equilibrium about a vertical axis through the eye-bolt.

One may then charge a rubber rod with a piece of fur and show the strength of the electrostatic force very nicely by pulling the beam into rotational motion with the charged rod.

(Note: One of the long skinny balloons (2" x 60") in Kit (036) on [D-4-2] ,inflated to about 24"-30" long, seems to be a good 'rod' for charging and then using to attract the beam. Probably adds more pizazz to the demo's eye appeal.)

Note that one may bring a heavy mass (white wrapped lead brick) up very close to the beam without being able to pull the beam into rotational motion.

One may also push the beam into motion with tiny mechanical pulses from a light string or with one’s lungs. Thus the suspension is clearly able to respond to tiny forces.

So we have a graphic indication of the electrostatic force being much stronger than the gravitational force, unless at least one mass is huge.

In other words we cannot equal the vertical pull of the earth on the beam with the simple charged rod, but we can exert a much larger horizontal force on the beam with the charged rod than with the horizontal gravitational force of the lead brick.

SAFETY NOTE: Arrange for the horizontal support rod to be vertically directly above one of the stands tripod feet to prevent the whole apparatus tipping over easily. The following picture from a top view shows this state of affairs, with the support rod actually a little rotated to the right for clarity.
  
Writeup created by David A. Burba
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