Act10_sol - Period 10 Activity Sheet Solutions: Electric...

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37 Period 10 Activity Sheet Solutions: Electric Charge and Force Activity 10.1: How Do Electric Charges Exert Forces? a) Evidence of Electric Forces: 1) Your instructor will show you how to give a Styrofoam ring an electric charge by rubbing it between two pieces of foam. What happens when you place the charged ring on the table and hold the foam square above the ring? The ring moves toward the foam square because the electric charge on the ring is attracted to the electric charge on the foam. 2) Rub a plastic rod with a second piece of foam to give the rod an electric charge. Charge the Styrofoam ring again as you did in part 1). Float the Styrofoam ring above the rod. Why does the ring float? The ring moves away from the rod because the electric charge on the rod repels the electric charge on the ring. A repulsive force between the charges on the rod and on the ring causes the ring to float above the rod. b) Electric Force and Gravitational Force : 1) We have learned that the force of gravity is always an attractive force. Based on your experiments in part a), is the electrical force always attractive? How do you know? The electric force can be attractive or repulsive. The Styrofoam ring is attracted to foam square on which it was rubbed. But when the rod is rubbed with foam, the rod repels the Styrofoam ring. 2) The Styrofoam ring you floated in part a) has a mass of 0.0065 grams or 6.5 x 10 -6 kg. a) When the ring floats, what two forces act on the ring? A repulsive electric force between the rod and the ring pushes up on the ring. The attractive gravitational force pulls down on the ring. b) Calculate the amount of electrical force that supports the ring when it floats. With the information given, you cannot directly calculate the electrical force. However, when the ring floats (does not move vertically), the amount of electrical force pushing up on the ring is equal to the amount of gravitational force pulling down on it. In this case, F electric = F grav = M g = 6.5 x 10 -6 kg x 9.8 m/s 2 = 6.4 x 10 -5 N 3) If you floated a ring that had twice the mass but kept the same charges on the rod and ring, would the ring float higher above the rod, closer to the rod, or at the same distance from the rod? _ closer to the rod _ Why? If the mass doubles, the force of gravity pulling the ring down also doubles. Therefore, the ring floats closer to the rod. Because the distance between the ring and the rod has now decreased, the electric force between the ring and the rod increases to balance the additional gravitational force. c) Group Discussion Question: When you rub the plastic rod with foam, where does the charge on the rod come from? Do you “create” charge when you rub the rod? Does the piece of foam end up with a charge?
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38 Rubbing the rod with the foam transfers negative electrons in the atoms of the foam onto the rod, giving the rod a net negative charge. Having lost some negative charge, the foam now has a net positive charge.
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This note was uploaded on 06/11/2011 for the course PHYSICS 103 taught by Professor Staff during the Spring '10 term at Ohio State.

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Act10_sol - Period 10 Activity Sheet Solutions: Electric...

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