Conductors and Electric Fields in Static Equilibrium Quiz

PDF page 855: downed line touching the car. Conductors contain free charges that move easily. When excess charge is placed on a conductor or the conductor is put into a static electric field, charges in the conductor quickly respond to reach a steady state called electrostatic equilibrium.

PDF page 856: FIGURE 18.16 When an electric field is applied to a conductor, free charges inside the conductor move until the field is perpendicular to

PDF page 856: the surface. (a) The electric field is a vector quantity, with both parallel and perpendicular components. The parallel component ( ) exerts a force ( ) on the free charge , which moves the charge until . (b) The resulting field is perpendicular to the surface. The free charge has been brought to the conductor’s surface, leaving electrostatic forces in equilibrium.

PDF page 857: FIGURE 18.18 The mutual repulsion of excess positive charges on a spherical conductor distributes them uniformly on its surface. The

PDF page 857: resulting electric field is perpendicular to the surface and zero inside. Outside the conductor, the field is identical to that of a point charge at the center equal to the excess charge.

PDF page 857: A near uniform electric field of approximately 150 N/C, directed downward, surrounds Earth, with the magnitude increasing slightly as we get closer to the surface. What causes the electric field? At around 100 km above the surface of Earth we have a layer of charged particles, called the ionosphere. The ionosphere is responsible for a range of phenomena including the electric field surrounding Earth. In fair weather the ionosphere is positive and the Earth largely negative, maintaining the electric field (Figure 18.20(a)). In storm conditions clouds form and localized electric fields can be larger and reversed in direction (Figure 18.20(b)). The exact charge distributions depend on the local conditions, and variations of Figure 18.20(b) are possible. If the electric field is sufficiently large, the insulating properties of the surrounding material break down and it becomes conducting. For air this occurs at around N/C. Air ionizes ions and electrons recombine, and we

PDF page 857: A near uniform electric field of approximately 150 N/C, directed downward, surrounds Earth, with the magnitude increasing slightly as we get closer to the surface. What causes the electric field? At around 100 km above the surface of Earth we have a layer of charged particles, called the ionosphere. The ionosphere is responsible for a range of phenomena including the electric field surrounding Earth. In fair weather the ionosphere is positive and the Earth largely negative, maintaining the electric field (Figure 18.20(a)). In storm conditions clouds form and localized electric fields can be larger and reversed in direction (Figure 18.20(b)). The exact charge distributions depend on the local conditions, and variations of Figure 18.20(b) are possible. If the electric field is sufficiently large, the insulating properties of the surrounding material break down and it becomes conducting. For air this occurs at around N/C. Air ionizes ions and electrons recombine, and we

PDF page 857: A near uniform electric field of approximately 150 N/C, directed downward, surrounds Earth, with the magnitude increasing slightly as we get closer to the surface. What causes the electric field? At around 100 km above the surface of Earth we have a layer of charged particles, called the ionosphere. The ionosphere is responsible for a range of phenomena including the electric field surrounding Earth. In fair weather the ionosphere is positive and the Earth largely negative, maintaining the electric field (Figure 18.20(a)). In storm conditions clouds form and localized electric fields can be larger and reversed in direction (Figure 18.20(b)). The exact charge distributions depend on the local conditions, and variations of Figure 18.20(b) are possible. If the electric field is sufficiently large, the insulating properties of the surrounding material break down and it becomes conducting. For air this occurs at around N/C. Air ionizes ions and electrons recombine, and we

PDF page 857: A near uniform electric field of approximately 150 N/C, directed downward, surrounds Earth, with the magnitude increasing slightly as we get closer to the surface. What causes the electric field? At around 100 km above the surface of Earth we have a layer of charged particles, called the ionosphere. The ionosphere is responsible for a range of phenomena including the electric field surrounding Earth. In fair weather the ionosphere is positive and the Earth largely negative, maintaining the electric field (Figure 18.20(a)). In storm conditions clouds form and localized electric fields can be larger and reversed in direction (Figure 18.20(b)). The exact charge distributions depend on the local conditions, and variations of Figure 18.20(b) are possible. If the electric field is sufficiently large, the insulating properties of the surrounding material break down and it becomes conducting. For air this occurs at around N/C. Air ionizes ions and electrons recombine, and we

PDF page 858: So far we have considered excess charges on a smooth, symmetrical conductor surface. What happens if a conductor has sharp corners or is pointed? Excess charges on a nonuniform conductor become concentrated at the sharpest points. Additionally, excess charge may move on or off the conductor at the sharpest points. To see how and why this happens, consider the charged conductor in Figure 18.21. The electrostatic repulsion of like charges is most effective in moving them apart on the flattest surface, and so they become least concentrated there. This is because the forces between identical pairs of charges at either end of the conductor are identical, but the components of the forces parallel to the surfaces are different. The component parallel to the surface is greatest on the flattest surface and, hence, more effective in moving the charge. The same effect is produced on a conductor by an externally applied electric field, as seen in Figure 18.21 (c). Since the field lines must be perpendicular to the surface, more of them are concentrated on the most curved parts.

PDF page 858: So far we have considered excess charges on a smooth, symmetrical conductor surface. What happens if a conductor has sharp corners or is pointed? Excess charges on a nonuniform conductor become concentrated at the sharpest points. Additionally, excess charge may move on or off the conductor at the sharpest points. To see how and why this happens, consider the charged conductor in Figure 18.21. The electrostatic repulsion of like charges is most effective in moving them apart on the flattest surface, and so they become least concentrated there. This is because the forces between identical pairs of charges at either end of the conductor are identical, but the components of the forces parallel to the surfaces are different. The component parallel to the surface is greatest on the flattest surface and, hence, more effective in moving the charge. The same effect is produced on a conductor by an externally applied electric field, as seen in Figure 18.21 (c). Since the field lines must be perpendicular to the surface, more of them are concentrated on the most curved parts.

PDF page 858: On a very sharply curved surface, such as shown in Figure 18.22, the charges are so concentrated at the point that the resulting electric field can be great enough to remove them from the surface. This can be useful. Lightning rods work best when they are most pointed. The large charges created in storm clouds induce an opposite charge on a building that can result in a lightning bolt hitting the building. The induced charge is bled away continually by a lightning rod, preventing the more dramatic lightning strike. Of course, we sometimes wish to prevent the transfer of charge rather than to facilitate it. In that case, the conductor should be very smooth and have as large a radius of curvature as possible. (See Figure 18.23.) Smooth surfaces are used on high-voltage transmission lines, for example, to avoid leakage of charge into the air. Another device that makes use of some of these principles is a Faraday cage. This is a metal shield that encloses a

PDF page 858: On a very sharply curved surface, such as shown in Figure 18.22, the charges are so concentrated at the point that the resulting electric field can be great enough to remove them from the surface. This can be useful. Lightning rods work best when they are most pointed. The large charges created in storm clouds induce an opposite charge on a building that can result in a lightning bolt hitting the building. The induced charge is bled away continually by a lightning rod, preventing the more dramatic lightning strike. Of course, we sometimes wish to prevent the transfer of charge rather than to facilitate it. In that case, the conductor should be very smooth and have as large a radius of curvature as possible. (See Figure 18.23.) Smooth surfaces are used on high-voltage transmission lines, for example, to avoid leakage of charge into the air. Another device that makes use of some of these principles is a Faraday cage. This is a metal shield that encloses a