Physics 3LB Final

Ideal Meters Problem. What internal resistance is ideal for a voltmeter? What internal resistance is ideal for an ammeter?

Zero for ammeter infinity for voltmeter

Assume that a particular person's skin is found to have a resistance of 1 x 105 Ω when dry and 1400 Ω when wet. How much current will flow into the body on contact with a 120 V household AC outlet with wet and dry skin. Please round your answers to one decimal place in mA

1.2 mA (dry skin) 85.7 mA (wet skin)

Semilog graph problem. Semilog paper is a convenient tool for analyzing data that changes exponentially. The scale on the y axis of the paper is logarithmic. Since the logarithm of an exponential function is a linear function, exponentially varying data appear as a straight line on semilog paper. The figure above is a semilog plot of a voltage that decays exponentially with time. What is the voltage of the t = 0 (leftmost) point? Please write your answer with one decimal place.

12 V

Oscilloscope trigger problem. The 'trigger' settings determine when the trace begins. Suppose you have displayed a sine wave with V = Vosin(ωt) on the screen, where V o =5V and ω/2π=5kHz. Assume further that the signal triggers the scope when V=2.5V and ωt=30 degrees. EXAMPLE: Suppose you double the amplitude of the wave without changing any oscilloscope settings. What are V and ωt now when the scope is triggered? [Answer: the level hasn't changed so the scope still triggers when V=2.5V. V = 2.5 = 10sin (ωt); ωt=sin−1(2.5/10)=14.4775121851511 degrees.] Assume that you reduce V o back to 5V and switch the trigger slope polarity from '+' to '-' without changing the trigger level dial. What are V (express your answer with 1 decimal place) and ωt (express your answer without decimal places) when the scope is triggered now?

2.5 V 150 Degrees

DC versus AC problem. Suppose your DC power supply is set to 15 V and the vertical setting on the oscilloscope is at 5 V/div. How many divisions will the trace deflect if you connect the output of the power supply to the scope with the "A input" switch set on DC? How many divisions if the switch is on AC? Please write your answer with integer values.

3 divisions for DC 0 divisions for AC

Internal resistance problem. Suppose that in Figure 3.3, Vo=5V, R1=150kΩ, R2=450kΩ, and Ri=1.4MΩ. Initially, the voltmeter is disconnected. EXAMPLES: What is the voltage across R2? [Answer: 3.75 V.]What is the voltage across R2 with the meter connected? [Answer: 3.47107438016529 V.] QUESTION: If the internal resistance of the meter were 15 M Ω, what is the voltage across R2? Please write your answer with two decimal places.

3.72 V

Maximum voltage problem. Resistors are assigned a power rating. If the power rating is exceeded significantly, the resistor will overheat and crack. Suppose you have a 10 k Ω, 0.25 watt resistor. What is the maximum voltage it can safely handle? Please write your answer with one decimal place.

50 V

Temperature change time base problem. Suppose V = 24 V, I = 0.1 A, for water: mw = 51 gm, c_w = 4.18 J/gm *K-1, for resistor: m_r = 8 gm, and c_r = 3.7 J/gm *K-1. If the water is initially at room temperature, how long will it take for the water to heat up 5ºK? (Hint: dT/dt is approximately equal to Δ T / Δt .) Please write your answer with one decimal place in seconds.

505.8 sec

Thermistor calibration problem. Suppose you calibrate a thermistor by inserting it first in ice water, then in boiling water. In ice water, the thermistor reads -36 mV and in boiling water it reads 209 mV. Next you measure the temperature of an unknown sample and read 107 mV. What is the temperature in ºC? Please write your answer with one decimal place.

58.4 ºC

Maximum current problem. If the current on your power supply exceeds 500 mA it can damage the supply. Suppose the supply is set for 37 V. What is the smallest resistance you can measure? Please write your answer with integer value.

74 Ohms

Oscilloscope amplitude and frequency problem. Study the above graph. The "volts/div" dial is set to 2 volts/div and the "time/div" dial is set to 5 msec/div. What is the peak-to-peak amplitude of the displayed signal (express your answer without decimal places)? What is the frequency in kHz (express your answer with 3 decimal places, write the leading zero before the decimal point)?

8 V 0.0667 Hz

Oscilloscope time base problem. You are asked to display a 1 kHz sine wave signal on the screen. Since the period T is related to the frequency f by the formula T = 1 / f, the period of this wave is 1/(103/S)=1mS, so a reasonable setting for the "Time/div" dial is 1 mS/div. But suppose you select an inappropriate setting for the Time/div dial by mistake. What would you see if you set the dial on 1 μS/div? 100 mS/div?

At 1 μS/div the wave would look a horizontal line At 100 mS/div you would see many (nearly) vertical lines

Can you receive a shock severe enough to prevent you from releasing the wire from an ordinary outlet with dry hands? (Hint: Refer to reading 1.)

False

Ammeter and voltmeter circuit problem. Study the circuit above. Which of the following formulas correctly relates resistance to current and voltage?

I [ R + (RvR / (R+RV) ) ]

Oscilloscope channel addition problem. Read the following oscilloscope settings carefully. Suppose you display a 1 kHz, 2 V sine wave in channel A and a DC, 1V signal in channel B with sensitivities set on 1V/div. You select 'Add' so that the two signals are combined and you readjust the position so the trace is in the middle of the screen. If you switch the signal in channel B from DC to AC, what will happen? (Draw a picture if it helps.)

The trace jumps down 1 division