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# Chapter 21 - Chapter 21 7 Picture the Problem A battery for...

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Chapter 21 7. Picture the Problem : A battery for a pacemaker is rated for a certain number of ampere-hours. Strategy: Convert the number of ampere-hours to coulombs of charge, then use equation 21-1 to determine the time required for the specified average current to deliver that much charge. Solution: 1. (a) Find from equation 21-1: 2. (b) Solve equation 21-1 for : Insight: The long life span of the battery is helpful in order to minimize the frequency of invasive surgery required to replace the batteries! The major risk, however, is the toxicity of the chemicals used to make the battery. 11. Picture the Problem : A copper wire of known dimensions has an intrinsic resistance. Strategy: Use table 21-1 together with equation 21-3 to determine the resistance of the wire. Solution: Apply equation 21-3 , using from table 21-1 and : Insight: This is a large resistance, and copper wires are often used for electric power and communication lines. However, the choice of a larger diameter can make a huge difference. For instance, if the diameter of the wire were 3.0 mm, the resistance would only be 23 Ω, a reduction by a factor of 30! 12. Picture the Problem : The section of copper wire between a bird’s feet has an intrinsic resistance. If any current is flowing in the wire, there is a small potential difference across the two ends of the wire section. Strategy: Ohm’s Law (equation 21-2) gives the relationship between potential difference, current, and resistance for any circuit element. In this problem we first apply equation 21-3 and the data from table 21-1 to find the resistance of the wire section, and then apply Ohm’s Law to find the potential difference. Solution: 1. (a) Combine equations 21-2 and 21-3 to find V : 2. (b) Since V is directly proportional to the separation L of the bird’s feet, the answer to part (a) will increase if L increases. Insight: Landing on a high voltage wire will not kill a bird because the potential difference between its feet is small. However, if another part of the bird’s body gets too close to a grounded conductor, a spark could leap from the bird’s body to the grounded conductor and kill the bird. 13.

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Chapter 21 - Chapter 21 7 Picture the Problem A battery for...

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