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Chapter 25 Problem Solns

Chapter 25 Problem Solns - Chapter 25 Electric Circuits 19...

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Chapter 25: Electric Circuits 19) If you accidentally leave your car headlights (current drain 5 A) on for an hour, how much of the 12-V battery’s chemical energy is used up? Answer: Recall from Physics 8a that Energy = Power * Time Power = I*V = 5 A * 12 V = 60 Watts Energy = Power * Time = 60 Watts * (1 hour) * (3600 secs/1 hour) = 216,000 Joules Note: How do we know the power without knowing resistance? Well actually we are given resistance because we are given the current and the voltage. And remember from ohm’s law that resistance, current, and voltage are all connected. If we know current and voltage then we can find resistance. 21) What resistance should be placed in parallel with a 56-k uni2126 resistor to make an equivalent resistance of 45-k uni2126 ? Answer: For parallel resistors 1 R total = 1 R 1 + 1 R 2 . Generally we know the two resistances and we do not know R total . In this problem we know R total and one of the Resistances but not the other one. So we have 1 45k uni2126 = 1 56k uni2126 + 1 R => 1 R = 1 45k uni2126 - 1 56k uni2126 => 1 R = 56k uni2126 45k uni2126 × 56k uni2126 - 45k uni2126 45k uni2126 × 56k uni2126 => 1 R = 11 2520k uni2126 => R = 2520k uni2126 11 = 229 k uni2126
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28) What is the current through the 3- uni2126 resistor in the circuit of Fig. 25.28? Hint: This is trivial. Can you see why? Answer: We can find the current through a resistor using Ohm’s Law. V= I * R We know the Resistance and we want to find the Current. Looking at Ohm’s Law if we know the Resistance and the Voltage then we can find the Current. Well we know from Kirchoff’s rules that the net Voltage around any closed loop must equal 0. If we look at the loop containing only the 6 volt battery and the 3 ohm resistor it becomes clear that the voltage drop across the 3 ohm resistor must be 6 Volts. Knowing this I = V R = 6 V 3 uni2126 = 2 Amps. Note: This problem might look complicated but remember that Kirchoff’s rules and Ohm’s Law always hold. These are your fallback equations that you will need to understand for circuit problems.
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