Fisica 2

23
Electric Fields
CHAPTER OUTLINE
23.1 23.2 23.3 23.4 23.5 Properties of Electric Charges Charging Objects by Induction Coulomb’s Law The Electric Field Electric Field of a Continuous Charge Distribution Electric Field Lines Motion of Charged Particles in a Uniform Electric Field

ANSWERS TO QUESTIONS
Q23.1 A neutral atom is one that has no net charge. This means that it has the samenumber of electrons orbiting the nucleus as it has protons in the nucleus. A negatively charged atom has one or more excess electrons. When the comb is nearby, molecules in the paper are polarized, similar to the molecules in the wall in Figure 23.5a, and the paper is attracted. During contact, charge from the comb is transferred to the paper by conduction. Then the paper has the same charge as thecomb, and is repelled. The clothes dryer rubs dissimilar materials together as it tumbles the clothes. Electrons are transferred from one kind of molecule to another. The charges on pieces of cloth, or on nearby objects charged by induction, can produce strong electric fields that promote the ionization process in the surrounding air that is necessary for a spark to occur. Then you hear or see thesparks.

Q23.2

23.6 23.7

Q23.3

Q23.4

To avoid making a spark. Rubber-soled shoes acquire a charge by friction with the floor and could discharge with a spark, possibly causing an explosion of any flammable material in the oxygenenriched atmosphere. Electrons are less massive and more mobile than protons. Also, they are more easily detached from atoms than protons. The electric fielddue to the charged rod induces charges on near and far sides of the sphere. The attractive Coulomb force of the rod on the dissimilar charge on the close side of the sphere is larger than the repulsive Coulomb force of the rod on the like charge on the far side of the sphere. The result is a net attraction of the sphere to the rod. When the sphere touches the rod, charge is conducted between the rodand the sphere, leaving both the rod and the sphere like-charged. This results in a repulsive Coulomb force. All of the constituents of air are nonpolar except for water. The polar water molecules in the air quite readily “steal” charge from a charged object, as any physics teacher trying to perform electrostatics demonstrations in the summer well knows. As a result—it is difficult to accumulatelarge amounts of excess charge on an object in a humid climate. During a North American winter, the cold, dry air allows accumulation of significant excess charge, giving the potential (pun intended) for a shocking (pun also intended) introduction to static electricity sparks. 1

Q23.5 Q23.6

Q23.7

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Electric Fields

Q23.8

Similarities: A force of gravity is proportional to theproduct of the intrinsic properties (masses) of two particles, and inversely proportional to the square of the separation distance. An electrical force exhibits the same proportionalities, with charge as the intrinsic property. Differences: The electrical force can either attract or repel, while the gravitational force as described by Newton’s law can only attract. The electrical force betweenelementary particles is vastly stronger than the gravitational force. No. The balloon induces polarization of the molecules in the wall, so that a layer of positive charge exists near the balloon. This is just like the situation in Figure 23.5a, except that the signs of the charges are reversed. The attraction between these charges and the negative charges on the balloon is stronger than the repulsionbetween the negative charges on the balloon and the negative charges in the polarized molecules (because they are farther from the balloon), so that there is a net attractive force toward the wall. Ionization processes in the air surrounding the balloon provide ions to which excess electrons in the balloon can transfer, reducing the charge on the balloon and eventually causing the attractive...