# f10_oct12 - Equilibrium A balance between opposing...

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Equilibrium A balance between opposing influences Consider a can with a hole in its bottom, held under a faucet. Is it possible for the level of water in the can to come to an equilibrium? What are the opposing influences? What would happen if you turned up the flow of water into the can? Equilibrium A balance between opposing influences Consider a can with a hole in its bottom, held under a faucet. Is it possible for the level of water in the can to come to an equilibrium? What are the opposing influences? What would happen if you turned up the flow of water into the can? The level of the water in the can would rise. If the can was very tall, would the water level just keep on rising or would it come to a new equilibrium level? A new equilibrium If one influence changes, we could have a new equilibrium if that made the other influence change. If the flow of water into the can increased, the water level would rise. That would increase the pressure in the can causing the flow out of the can to increase too. If the flow out rose to equal the flow in, the water level would stop rising. We would have a new equilibrium. This is an example of a stable equilibrium. A rock in space Consider a black rock orbiting the Sun. Energy is flowing into the rock because it is absorbing sunlight. How does energy flow out of a rock in space? A. by radiation from the rock B. by conduction into the surrounding space C. by reflecting sunlight

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A rock in space Consider a black rock orbiting the Sun. Energy is flowing into the rock because it is absorbing sunlight. How does energy flow out of a rock in space? A. by (infrared) radiation from the rock B. by conduction into the surrounding space C.by reflecting sunlight The formula is P = ! T 4 x A The temperature will come to an equilibrium if the power flowing into the rock equals that flowing out. Calculating the rock’s temperature To calculate the temperature of the rock orbiting the Sun, we need to write down the formulas for the energy going into the rock and the energy going out each second. Power going in is the flux of sunlight multiplied by the area of the side of the rock facing the Sun. P in = F sunlight x A face Power going out depends on the temperature of the rock and its total surface area. P out = ! T 4 x A surface In equilibrium, P out = P in Do the math If P out = P in : For a sphere, A face / A surface = ! . The answer comes out to 279 K, or 6 o C, or 42 o F. 4 4 4 surface face sunlight surface face sunlight face sunlight surface A A F T A A F T A F A T ! = = = Different distances from the Sun
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## f10_oct12 - Equilibrium A balance between opposing...

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