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Let us now use this realistic view of a battery in a simple circuit containing a battery and a single resistor. The electric potential difference across the resistor is the same as between the two terminals of the battery. How would we find the current that is leaving the battery? Voltage across the terminals of the battery Voltage drop across the resistor same If the internal resistance is much smaller than the load resistance it can be considered negligible. Ifthentherefore we can assume

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Example: What is the emf of a real battery with an internal resistance of 0.1 , that supplies a current of 0.05 A to a load of 200 ? What would the emf be if the load was 2 ? Negligible compared to load For R = 2 : r is not negligible in this case! For R = 200 : You must use your judgment to decide when a quantity can be considered negligible. When in doubt don’t neglect!

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The circuits we will be considering next will be circuits containing different combinations of resistors. Resistors in Series: The same current must pass through each resistor There is a voltage drop across each resistor Do not confuse with capacitors in series!

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Resistors in Parallel: Each resistor has one end connected to the positive terminal and each has one end connected to the negative terminal providing the same electric potential difference across each resistor. The current supplied by the battery is distributed through each of the resistors. The current has a “choice” of paths. Do not confuse with capacitors in parallel!

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Consider two identical resistors wired in series (one behind the other). If there is an electric current through the combination, the current in the second resistor is 1. equal to 2. half 3. smaller than, but not necessarily half the current through the first resistor.

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As more identical resistors R are added to the parallel circuit shown here, the total resistance between points P and Q 1. increases. 2. remains the same. 3. decreases.

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If the four light bulbs in the figure are identical, which circuit puts out more light? 1. I. 2. The two emit the same amount of light. 3. II.

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The three light bulbs in the circuit all have the same resistance. Given that brightness is proportional to power dissipated, the brightness of bulbs B and C together, compared with the brightness of bulb A, is 1. twice as much. 2. the same. 3. half as much.

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Charge flows through a light bulb. Suppose a wire is connected across the bulb as shown. When the wire is connected, 1. all the charge continues to flow through the bulb. 2. half the charge flows through the wire, the other half continues through the bulb. 3. all the charge flows through the wire. 4. none of the above

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The circuit below consists of two identical light bulbs burning with equal brightness and a single 12 V battery. When the switch is closed, the brightness of bulb A 1. increases. 2. remains unchanged. 3. decreases.

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Two light bulbs A and B are connected in series to a constant voltage source. When a wire is connected across B as shown, bulb A 1. burns more brightly. 2. burns as brightly. 3. burns more dimly. 4. goes out.

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Example: Examine the circuit given and determine the following. a)Equivalent resistance for the circuit. b)Current through each resistor. c)Voltage across each resistor. d)Power dissipated by each resistor. a) b) and c)

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b) and c) continued d)

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©1997 by Eric Mazur Published by Pearson Prentice Hall Upper Saddle River, NJ 07458 ISBN 0-13-565441-6 No portion of the file may be distributed, transmitted.

©1997 by Eric Mazur Published by Pearson Prentice Hall Upper Saddle River, NJ 07458 ISBN 0-13-565441-6 No portion of the file may be distributed, transmitted.

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