Microsoft Word - GRR-CP2-16

# Microsoft Word - GRR-CP2-16 - Chapter 16 ELECTRIC FORCES...

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635 Chapter 16 ELECTRIC FORCES AND FIELDS Conceptual Questions 1. (a) Gravity and the electric force are both long range, proportional to the inverse distance squared, and proportional to the product of the magnitudes of the masses or charges, respectively. (b) The gravitational force is always attractive—there is only one kind of mass. The electric force can be either attractive or repulsive depending on the charges of the two objects. The electric force is in general much stronger than the gravitational force, the latter only becomes significant with very massive objects. 2. This proposition would not work because even with small net charges some objects would be observed to repel each other, which does not happen with gravity. To account for the weight of an object one may say, for example, that the Earth is slightly positively charged and the object negatively charged. A slightly positively charged object should then be repelled from the Earth and fall upward. Furthermore, increasing the charge on an object would increase the force from the Earth, but the weight is not observed to change by increasing an object’s charge. 3. The clothes transfer some charge to one another as they rub together in the dryer causing static cling between parts with opposite charges. A charged piece of clothing may also cling to something that is uncharged due to polarization. When the clothes are slightly damp they do not hold a charge very long because the moisture allows the charge to leak off more quickly. We would expect clothes to cling more when made of different materials, since some materials more readily give up or take on electrons by rubbing than others. Rubbing of two such different materials would therefore result in a greater transfer of charge than rubbing of two objects of the same material (an affinity to give up or take on electrons). 4. In equilibrium, charges are not in motion. Thus, the electric field inside any conductor in equilibrium must be zero, or else charges would move inside the conductor as a result. If the electric field inside the conductor is zero, there cannot be any net charge distributed inside the conductor, or else there would be an electric field produced by these charges. Therefore, any net charge on a conductor in equilibrium is found on the outside surface. 5. Electric field lines point in the direction of the electric field. Near a positive charge the electric field points outward away from the charge, so the field lines emanate from a positive charge. Near a negative charge the electric field points inward toward the charge, and so do the field lines. Thus, field lines must begin on positive charges and end on negative charges. 6. (a) Whenever there is an external electric field, the mobile charges in the conducting box move in response in such a way as to make the electric field inside the box zero.

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## This note was uploaded on 04/07/2009 for the course PHY PHY2140 taught by Professor Talagala during the Spring '09 term at Wayne State University.

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Microsoft Word - GRR-CP2-16 - Chapter 16 ELECTRIC FORCES...

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