Electric Forces

Straws and Pens


Q1: When you first pull the straw out of the paper there is a static electric charge present. Where do you think this charge originated from?

One conclusion is that friction between the paper and straw caused a separation of charge leaving one item positively charged and the other negatively charged.

Q2: The Coulomb force (attractive or repulsive force between charged objects) between the paper and the straw allows you to initially attract the paper and "lift" it off of the table. What must be true of the relative sizes of Coulomb force  and force due to Gravity for the paper to be lifted by static electricity? 

The Coulomb force must be greater than the force due to gravity in order to lift the paper.

Q3: Why won't the straw pick up the paper after you wipe the straw through your bare hand a few times? 

The straw has an electric charge because of an excess or lack of electrons. As you wipe the straw with your hand electrons are transferred between the your hand and the straw, leaving the straw electrically neutral.

Q4: What does rubbing the plastic pen or the straw over fabric do to the pen and straw? 

The friction created by rubbing the pen against another surface causes electrons to be transferred, leaving the pen or straw electrically charged. 

Q5: Why does the straw "spin" when you hold the end of the pen near it? What would happen if you rubbed the pen over a different material, like rubber or leather and then brought the pen near the straw? See the table of triboelectronegativities. 

Depending on the kind of charge gained by the pen, the end of the straw will either be attracted or repelled by the end of the pen causing the straw to spin. Depending on the relative triboelectronegativities of the two materials the pen will either gain or lose electrons. The straw was most likely negatively charged. Rubbing the pen on fabric most likely left it with a negative charge. Under these conditions the straw and pen would repel each other. Rubbing the pen on rubber would most likely leave it with a positive charge. Under this condition the straw and pen would attract each other. 

Q6: Draw a free-body diagram of the paper while it is being lifted by the straw. Label the Coulomb force and force due to gravity and indicate their relative sizes

Fc = Fg  for lift at a constant velocity

Fc > Fg  for acceleration

Advanced Question: What must be the minimum electric force present for you to lift the paper if you know its mass is 0.15 grams? 

The paper is accelerated upward therefore there must be a net force acting on it. The Coulomb force must be greater then the force due to gravity.

Fc >Fg

Fg = mg = (.15 g)(1 kg / 1000 g)(9.8 m/s2) = 1.47 x 10-3 N

Fc must be greater than 1.47 x 10-3 N

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