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In a meter bridge, initially the balance point is found at 50 cm. When the resistance in the left gap is increased by 5 ohms, the balance point shifts to 60 cm. Calculate the original resistances in t

Physics

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Answer

10 Ω and 10 Ω\text{10 } \Omega \text{ and 10 } \Omega

Step-by-step solution

5 steps
  1. Understand the meter bridge principle.

    In a meter bridge, the balance condition is given by the ratio of resistances being equal to the ratio of the corresponding lengths:

    R1R2=L1L2\frac{R_1}{R_2} = \frac{L_1}{L_2}

    Since the total length of the wire is 100 cm100 \text{ cm}, if L1L_1 is the length from one end, then L2=(100−L1) cmL_2 = (100 - L_1) \text{ cm}.
    So, the formula becomes:

    R1R2=L1100−L1\frac{R_1}{R_2} = \frac{L_1}{100 - L_1}
  2. Apply the initial condition.

    Initially, the balance point is at L1=50 cmL_1 = 50 \text{ cm}.
    Substituting this into the formula:

    R1R2=50100−50\frac{R_1}{R_2} = \frac{50}{100 - 50} R1R2=5050\frac{R_1}{R_2} = \frac{50}{50} R1R2=1\frac{R_1}{R_2} = 1

    This implies that R1=R2R_1 = R_2. Let's call this original resistance RR. So, R1=RR_1 = R and R2=RR_2 = R.

  3. Determine which resistance was increased.

    The balance point shifts from 50 cm50 \text{ cm} to 60 cm60 \text{ cm}. This means L1L_1 increased.
    For L1L_1 to increase, the ratio R1R2\frac{R_1}{R_2} must increase.
    If R1R_1 is increased, then R1+5R2\frac{R_1 + 5}{R_2} would be greater than R1R2\frac{R_1}{R_2}, causing L1L_1 to increase.
    If R2R_2 is increased, then R1R2+5\frac{R_1}{R_2 + 5} would be less than R1R2\frac{R_1}{R_2}, causing L1L_1 to decrease.
    Since the balance point shifted from 50 cm50 \text{ cm} to 60 cm60 \text{ cm} (an increase in L1L_1), it means the resistance R1R_1 (the one corresponding to L1L_1) was increased by 5Ω5\Omega.

  4. Apply the final condition.

    The new resistance in the first gap is R1′=R+5R_1' = R + 5.
    The resistance in the second gap remains R2′=RR_2' = R.
    The new balance point is L1′=60 cmL_1' = 60 \text{ cm}.
    Substituting these values into the meter bridge formula:

    R1′R2′=L1′100−L1′\frac{R_1'}{R_2'} = \frac{L_1'}{100 - L_1'} R+5R=60100−60\frac{R + 5}{R} = \frac{60}{100 - 60} R+5R=6040\frac{R + 5}{R} = \frac{60}{40}
  5. Solve for RR.

    Simplify the ratio 6040\frac{60}{40}:

    R+5R=32\frac{R + 5}{R} = \frac{3}{2}

    Cross-multiply:

    2(R+5)=3R2(R + 5) = 3R 2R+10=3R2R + 10 = 3R

    Subtract 2R2R from both sides:

    10=3R−2R10 = 3R - 2R R=10 ΩR = 10 \, \Omega

    The original resistances were R1=RR_1 = R and R2=RR_2 = R.
    Therefore, the original resistances are 10 Ω10 \, \Omega and 10 Ω10 \, \Omega.

    The original resistances are 10 Ω and 10 Ω\text{10 } \Omega \text{ and 10 } \Omega.

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