a. Without making a single calculation, it should now be apparent from previous examples that the total resistance is less than 1.6 kΩ and very close to this value because of the magnitude of the other resistance levels:
R_T=\frac{1}{\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3} }=\frac{1}{\frac{1}{1.6k\Omega }+\frac{1}{20k\Omega }+\frac{1}{56k\Omega } }\\[0.5cm] =\frac{1}{625\times 10^{-6}+50\times 10^{-6}+17.867\times 10^{-6}}=\frac{1}{692.867\times 10^{-6}}\\ and R_T=1.44k\Omega
b. Applying Ohm’s law gives
I_s=\frac{E}{R_T }=\frac{28V}{1.44k\Omega }=19.44mA
Recalling that current always seeks the path of least resistance immediately tells us that the current through the 1.6 kΩ resistor will be the largest and the current through the 56 kΩ resistor the smallest.
Applying Ohm’s law again gives
I_1=\frac{V_1}{R_1}=\frac{E}{R_1}=\frac{28V}{1.6k\Omega }=17.5mA\\[0.5cm] I_2=\frac{V_2}{R_2}=\frac{E}{R_2}=\frac{28V}{20k\Omega }=1.4mA\\[0.5cm] I_3=\frac{V_3}{R_3}=\frac{E}{R_3}=\frac{28V}{56k\Omega }=0.5mA
c. Applying Eq. (6.11) gives
P_E=EI_s (watts, W) (6.11)
P_E=EI_s=(28V)(19.4mA)=543.2mW
d. Applying each form of the power equation gives
P_1=V_1I_1=EI_1=(28V)(17.5mA)=490mW\\[0.5cm] P_2=I^{2}_{2}R_2=(1.4mA)^2(20k\Omega )=39.2mW\\[0.5cm] P_3=\frac{V^{2}_{3} }{R_3}=\frac{E^2}{R_3}=\frac{(28V)^2}{56k\Omega }=14mW
A review of the results clearly substantiates the fact that the larger the resistor, the less is the power absorbed.
e. P_E=P_{R_1}+P_{R_2}+P_{R_3}\\[0.5cm] 543.2mW=490mW+39.2mW+14mW=543.2mW (checks)