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Using Norton's theorem, calculate the current flowing through the 11.2 Ω resistor in FIGURE 2. Also calculate the voltage drop across this resistor.

Physics

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Answer

0.1976 A\text{0.1976 A}

2.213 V\text{2.213 V}

Step-by-step solution

4 steps

To calculate the current and voltage drop across the 11.2 Ω11.2 \, \Omega resistor using Norton's theorem, we follow these steps:

  1. Find the Norton Current (INI_N)

    To find INI_N, we remove the load resistor (Rload=11.2 ΩR_{load} = 11.2 \, \Omega) and place a short circuit across its terminals. Let the terminals be B and ground. This means the voltage at terminal B is 0 V0 \, \text{V}.
    The circuit now consists of the 20 V20 \, \text{V} source, R1=20 ΩR_1 = 20 \, \Omega, R2=12 ΩR_2 = 12 \, \Omega, and R3=8 ΩR_3 = 8 \, \Omega.
    Let Node A be the junction between R1R_1, R2R_2, and R3R_3.
    When terminal B is shorted to ground, R2R_2 is connected between Node A and ground, and R3R_3 is also connected between Node A and ground. Thus, R2R_2 and R3R_3 are in parallel.
    The equivalent resistance of R2∥R3R_2 \parallel R_3 is:

    R23=R2×R3R2+R3=12 Ω×8 Ω12 Ω+8 Ω=96 Ω220 Ω=4.8 ΩR_{23} = \frac{R_2 \times R_3}{R_2 + R_3} = \frac{12 \, \Omega \times 8 \, \Omega}{12 \, \Omega + 8 \, \Omega} = \frac{96 \, \Omega^2}{20 \, \Omega} = 4.8 \, \Omega

    The total current (ItotalI_{total}) flowing from the source is through R1R_1 and then through the parallel combination of R2R_2 and R3R_3:

    Itotal=VR1+R23=20 V20 Ω+4.8 Ω=20 V24.8 Ω=200248 A=2531 AI_{total} = \frac{V}{R_1 + R_{23}} = \frac{20 \, \text{V}}{20 \, \Omega + 4.8 \, \Omega} = \frac{20 \, \text{V}}{24.8 \, \Omega} = \frac{200}{248} \, \text{A} = \frac{25}{31} \, \text{A}

    This current ItotalI_{total} reaches Node A and splits between R2R_2 and R3R_3. The Norton current INI_N is the current flowing through the short circuit, which is the current that would flow through R2R_2 in this configuration (since the short is across the terminals where RloadR_{load} was, and R2R_2 is connected to that node). Using the current divider rule:

    IN=Itotal×R3R2+R3=2531 A×8 Ω12 Ω+8 Ω=2531 A×820=2531 A×25=1031 AI_N = I_{total} \times \frac{R_3}{R_2 + R_3} = \frac{25}{31} \, \text{A} \times \frac{8 \, \Omega}{12 \, \Omega + 8 \, \Omega} = \frac{25}{31} \, \text{A} \times \frac{8}{20} = \frac{25}{31} \, \text{A} \times \frac{2}{5} = \frac{10}{31} \, \text{A} IN≈0.3226 AI_N \approx 0.3226 \, \text{A}
  2. Find the Norton Resistance (RNR_N)

    To find RNR_N, we turn off the independent voltage source (replace it with a short circuit) and look into the terminals where the load resistor was connected (terminals B and ground).
    With the voltage source shorted, R1R_1 is connected from Node A to ground. R3R_3 is also connected from Node A to ground. So, R1R_1 and R3R_3 are in parallel.

    R13=R1×R3R1+R3=20 Ω×8 Ω20 Ω+8 Ω=160 Ω228 Ω=407 ΩR_{13} = \frac{R_1 \times R_3}{R_1 + R_3} = \frac{20 \, \Omega \times 8 \, \Omega}{20 \, \Omega + 8 \, \Omega} = \frac{160 \, \Omega^2}{28 \, \Omega} = \frac{40}{7} \, \Omega

    Now, looking from terminals B to ground, R2R_2 is in series with the parallel combination of R1R_1 and R3R_3.

    RN=R2+R13=12 Ω+407 Ω=12×7+407 Ω=84+407 Ω=1247 ΩR_N = R_2 + R_{13} = 12 \, \Omega + \frac{40}{7} \, \Omega = \frac{12 \times 7 + 40}{7} \, \Omega = \frac{84 + 40}{7} \, \Omega = \frac{124}{7} \, \Omega RN≈17.7143 ΩR_N \approx 17.7143 \, \Omega
  3. Calculate the current through the load resistor (IloadI_{load})

    Now we connect the load resistor Rload=11.2 ΩR_{load} = 11.2 \, \Omega in parallel with the Norton equivalent circuit (INI_N in parallel with RNR_N). We use the current divider rule:

    Iload=IN×RNRN+RloadI_{load} = I_N \times \frac{R_N}{R_N + R_{load}} Iload=1031 A×1247 Ω1247 Ω+11.2 ΩI_{load} = \frac{10}{31} \, \text{A} \times \frac{\frac{124}{7} \, \Omega}{\frac{124}{7} \, \Omega + 11.2 \, \Omega}

    Convert 11.2 Ω11.2 \, \Omega to a fraction: 11.2=11210=56511.2 = \frac{112}{10} = \frac{56}{5}.

    Iload=1031 A×12471247+565=1031 A×1247124×5+56×735I_{load} = \frac{10}{31} \, \text{A} \times \frac{\frac{124}{7}}{\frac{124}{7} + \frac{56}{5}} = \frac{10}{31} \, \text{A} \times \frac{\frac{124}{7}}{\frac{124 \times 5 + 56 \times 7}{35}} Iload=1031 A×1247620+39235=1031 A×1247101235I_{load} = \frac{10}{31} \, \text{A} \times \frac{\frac{124}{7}}{\frac{620 + 392}{35}} = \frac{10}{31} \, \text{A} \times \frac{\frac{124}{7}}{\frac{1012}{35}} Iload=1031 A×1247×351012=1031 A×124×51012I_{load} = \frac{10}{31} \, \text{A} \times \frac{124}{7} \times \frac{35}{1012} = \frac{10}{31} \, \text{A} \times \frac{124 \times 5}{1012} Iload=620031372 A=15507843 AI_{load} = \frac{6200}{31372} \, \text{A} = \frac{1550}{7843} \, \text{A} Iload≈0.1976 AI_{load} \approx \boxed{\text{0.1976 A}}
  4. Calculate the voltage drop across the load resistor (VloadV_{load})

    Using Ohm's Law:

    Vload=Iload×RloadV_{load} = I_{load} \times R_{load} Vload=15507843 A×11.2 Ω=15507843 A×565 ΩV_{load} = \frac{1550}{7843} \, \text{A} \times 11.2 \, \Omega = \frac{1550}{7843} \, \text{A} \times \frac{56}{5} \, \Omega Vload=310×567843 V=173607843 VV_{load} = \frac{310 \times 56}{7843} \, \text{V} = \frac{17360}{7843} \, \text{V} Vload≈2.213 VV_{load} \approx \boxed{\text{2.213 V}}
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