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SECTION 28.1 • Electromotive Force 861 Example 28.1 Terminal Voltage of a Battery A battery has an emf of 12.0 V and an internal resistance of (A) Find the current in the circuit and the terminal voltage of the battery. Solution Equation 28.3 gives us the current: and from Equation 28.1, we find the terminal voltage: " V # $ Ir # 12.0 V $ (3.93 A)(0.05 ') # To check this result, we can calculate the voltage across the " V # IR # (3.93 A)(3.00 ') # 11.8 V (B) Calculate the power delivered to the load resistor, the power delivered to the internal resistance of the battery, and Solution The power delivered to the load resistor is ! R # I 2 R # (3.93 A) 2 (3.00 ') # The power delivered to the internal resistance is ! r # I 2 r # (3.93 A) 2 (0.05 ') # 0.772 W 46.3 W 11.8 V ! 3.93 A I # ! R % r # 12.0 V 3.05 ' # Hence, the power delivered by the battery is the sum What If? As a battery ages, its internal resistance increases. Suppose the internal resistance of this battery Answer Let us connect the same 3.00-' load resistor to the and the terminal voltage is " V # $ Ir # 12.0 V $ (2.40 A) (2.00 ') # 7.2 V Notice that the terminal voltage is only 60% of the emf. The ! R # I 2 R # (2.40 A) 2 (3.00 ') # ! r # I 2 r # (2.40 A) 2 (2.00 ') # 11.5 W Notice that 40% of the power from the battery is delivered 17.3 W ! I # ! R % r # 12.0 V (3.00 ' % 2.00 ') # 2.40 A ! Interactive Example 28.2 Matching the Load Show that the maximum power delivered to the load resis- Solution The power delivered to the load resistance is 2 R, where I is given by Equation 28.3: When ! is plotted versus R as in Figure 28.3, we find that reaches a maximum value of 2 /4r at R # r. When R is large, there is very little current, so that the power I 2 R delivered to the load resistor is small. When R is small, 2 r as energy is delivered to the internal resistance. When R # r, these effects balance to give a maximum transfer of We can also prove that the power maximizes at R # r by differentiating ! with respect to R, setting the result equal ! ! # I 2 R # ! 2 R (R % r) 2 to zero, and solving for R. The details are left as a problem At the Interactive Worked Example link at http://www.pse6.com, you can vary the load resistance and internal resistance, r 2r 3r R ! max ! Figure 28.3 (Example 28.2) Graph of the power ! delivered by a battery to a load resistor of resistance R as a function of R. The power delivered to the resistor is a maximum when the load resistance equals the internal resistance of the battery. |