Physics 313 assignment for Thursday, Aug. 23:

Topics: current/voltage/power, voltage dividers, Thévenin model, analog and digital meters

Reading: (much of this material should be review)

Diefenderfer and Holton (D&H) 1.1-1.9, 1.11-1.13, 6.1-6.4

Problems to hand in:

1) Refer to D&H Ch. 1 Figure A (bottom of p. 19). Which of the three resistors dissipates the most power, and how much power does that resistor dissipate? Show your reasoning.

2) You want to use a voltmeter with an impedance of R_V and an ammeter with an impedance of R_A, along with a DC power supply of voltage V, to determine the resistance of a resistor (true value=R).

a) Sketch a circuit diagram for each of the two possible configurations you could use if you wanted to make simultaneous measurements of voltage and current in order to find R.

b) For each of these configurations, derive an expression for the measured resistance—that is, use R_m=V_m/I_m, where V_m is the measured voltage (which might not equal the power supply voltage V!) and I_m is the measured current. Your results for R_m should depend only on R, R_V , and R_A.

c) What percentage error in measuring R is made with each configuration if R_V = 10 R and R_A = 0 (ideal ammeter, not-so-ideal voltmeter)? What if R_V = ∞ and R_A = 0.1 R (ideal voltmeter, not-so-ideal ammeter)?

3) Consider this voltage divider circuit:

		Vout

a) If V_in=15.00 V, what is V_out?

b) If a load of 1 k is attached, what is the new V_out?

4. Reading questions: a) In determining the Thévenin equivalent circuit, what does V_TH refer to?

b) For an accurate measurement, should the input impedance of a voltmeter be much lower or much higher than the impedance of the resistor across which it is placed?

c) An analog meter measures current directly, but it can also be used to measure voltage and resistance by adjusting its internal circuitry (for example, measuring the current through a known resistor gives voltage). What does a digital meter measure directly?