Physics Current percent change question?

For an ideal battery (r = 0 Ω), closing the switch in (Figure 1)does not affect the brightness of bulb A. In practice, bulb A dims just a little when the switch closes. To see why, assume that the 1.50 V battery has an internal resistance r = 0.30 Ω and that the resistance of a glowing bulb is R = 8.00 Ω.  

The figure shows an electric circuit which contains a battery with the emf epsilon and the internal resistance r, two bulbs and a switch. Bulbs A and B are connected in parallel. The opened switch is placed in series with the bulb B.The circuit is two parallel paths with a bulb down each path and a switch closing off the path to bulb BPart A: What is the current through bulb A when the switch is open? This answer was .181 APart B: What is the current through bulb A after the switch has closed? This answer was .174 APart C: By what percent does the current through A change when the switch is closed? This is where I am stuck, I've tried -3.86,-3.87,3.86,3.87,4.02, and -4.02 and they are all wrong.

1 Answer

  • 1 month ago

    Why not show us your working?  Then you would know why the answers are right or wrong.  a) total resistance 8.30 ohm total current 1.50 / 8.30 ~ 0.181.  b) total resistance = 4.30 ohm current through A is half the total = 1.50/(4.30 * 2) ~ 0.174

    the percentage change is (0.1744186/ 0.1807229 - 1)*100 = - 3.49 % ( I kept all answers fully accurate rather than the rounded off values.

    Whenever you have a subtraction to do you lose accuracy very easily through ANY tiny change in the values.

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