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

Q23.1 A neutral atom is one that has no net charge. This means that it has the same number of electrons orbiting the nucleus as it has protons in the nucleus. A negatively charged atom has one or more excess electrons. Whenthe 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 the comb, and is repelled. The clothes dryer rubs dissimilar materials together as it tumbles the clothes. Electrons are transferred from onekind 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 the sparks.


23.6 23.7



To avoid making a spark. Rubber-soled shoes acquire a charge by friction with the floorand 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 field due to the charged rod induces charges on near and far sides of the sphere. The attractive Coulomb force of the rod on the dissimilarcharge 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 rod and the sphere, leaving both the rod and the sphere like-charged. This results in a repulsive Coulomb force. All of the constituents ofair 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 accumulate large amounts of excess charge on an object in a humid climate. During a North American winter, the cold, dry air allows accumulation ofsignificant excess charge, giving the potential (pun intended) for a shocking (pun also intended) introduction to static electricity sparks. 1

Q23.5 Q23.6



Electric Fields


Similarities: A force of gravity is proportional to the product of the intrinsic properties (masses) of two particles, and inversely proportional to the square of the separation distance. Anelectrical 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 between elementary particles is vastly stronger than the gravitational force. No. The balloon induces polarization of the molecules in the wall, so...
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