For an ideal battery ( r =0), closing the switch in the figure (Figure 1)does no

Question

For an ideal battery (r=0), closing the switch in the figure (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 1.5 V battery has an internal resistance 0.7 and that the resistance of a glowing bulb is 6.0 .

A. What is the current through bulb A when the switch is open?

Express your answer with the appropriate units. Already got .22A

B. What is the current through bulb A after the switch has closed?

Express your answer with the appropriate units.

C. By what percent does the current through A change when the switch is closed?

Explanation / Answer

A) when current is open.

Equivalent resistance of circuit = R + r = 6 + 0.7 = 6.7 ohm

I =e /R = 1.5 / 6.7 = 0.224 A

B) when switch is closed , now A and B are in parallel,

equivalent of these two is R' then

1/R' = 1/6 + 1/6

R' = 3 ohm

Req = R' + r = 3.7 ohm

Ibat = e /Req = 1.5 / 3.7 = 0.405 A

this current will divide in A and B

current through A = 0.405/2 =0.203 A

c) change = 0.224 - 0.203 = 0.021 A

% Change = 0.021 x 100 / 0.224   = 9.4 %

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