Electrical Circuits... A junction is a point where two or more wires (or lines i

Question

Electrical Circuits...

A junction is a point where two or more wires (or lines in a circuit diagram) connect. For example, Kirchhoff's Junction Law states that the sum of currents entering a junction must equal the sum of currents leaving the junction. To apply Kirchhoff's Junction Law the direction of the current in each wire must be designated. Consider the following circuit. Since conventional current flows from + to - the current I from the power supply enters junction "a" from the left. Kirchhoff's Junction Law therefore requires that the sum of the currents flowing through R_1 and R_2 equal current I. A student measures the potential difference across R_1(600 Omega) and recordsV_1 = 5.8 Plus-Minus 0.2 V. A measurement of the current through R_2 (300 Omega) gives I_2 = 20 Plus-Minus 1 mA. Determine the current through R_1 with its uncertainty. Determine the current I with its uncertainty. Compare your value of I (with uncertainty) to a measured value of 31 Plus-Minus 1 mA.

Explanation / Answer

a)

I1 = V/R1 = 5.8 / 600 = 9.6 mA (+/- 0.4 mA)

b)

I = I1 + I2 = 9.6 + 20 = 29.6 mA (+/- 1.4 mA)

c)

theoretically we get I =  29.6 mA (+/- 1.4 mA) whose '+' uncertainity is equal to the ,measured value.

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