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.
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.
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.
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.
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.
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
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.
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