Sample questions

1. A resistor is connected to a single dry cell (battery) and the length of time T required for the bulb to exhaust the battery is determined. Then two identical fresh batteries are connected in series to the same resistor. How long will the batteries last? Explain your reasoning and state any assumptions you are making.

2. Suppose you want to connect the light bulb (on the right, below) to the battery (on the left, below), using the voltmeter to measure the voltage across, and the ammeter to measure the current through, the light bulb. Draw lines representing the wires you would use to connect up these items.

3. a. What is the value of the resistor in the following circuit, in ohms.

b. How much power does the resistor dissipate, in watts?

4. Suppose you need a 1.0 volt full-scale voltmeter, and all you have on hand is a 100u A moving-coil meter with an internal meter resistance of 1000 ohms.

(a). Draw the diagram of a simple voltmeter circuit based on the 100u A meter.

(b) calculate the value(s) of any additional component(s) needed.

(c) What is the input resistance of your voltmeter?

5. a. What resistance will be measured in the following circuit from point A to point B (i.e. if an ohmmeter is connected between point A and point B)?

b. Which single resistor in this circuit has the largest voltage across it?

6. In the following circuit, which single resistor has the largest current flowing through it? Explain your reasoning.

5. Suppose you need a 85 Ohms (+/- 5%), 5 watt resistance, but all you have on hand is a 100 Ohms , (+/- 5%), 5 watt resistor and a wide selection of general purpose 0.5 watt resistors in the standard sequence of resistance values (10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91) over the range from 1 ohm to 22 M ohms, with a rated tolerance of +/- 10%. Therefore you have, for example, 2.2 Ohms , 22 Ohms , 220 Ohms , 2.2 KOhms and so on up to 22 MOhms .

(a) Why not just forget the 100 Ohms resistor and use a standard 8.2 Ohms resistor? Is that close enough (+/- 5%) to the goal of 85 Ohms ? What about the power rating?

(b) Show how you could combine the 100 Ohms , 5 watt resistor with one of the standard 0.5 watt resistors to obtain as close as possible to the desired result of 85 Ohms (+/- 5%), 5 watt rating. Is it possible to to get within +/- 5% of 85 Ohmsr ?

(c) How much power will actually be be dissipated in the 100 resistor when voltage is such that the entire assembly is dissipating a total of 5 watts?

6. Suppose you have a simple line-operated 18 volt DC power supply which can deliver a maximum of 0.5 Amps. You need to derive from this a regulated 10 volt output capable of delivering 150 mA of current. For this you have obtained a 10 volt, 5 watt Zener diode and a resistor. Draw the circuit diagram of the regulator circuit you would use, showing (a) where the 18 volt supply would connect , (b) where the 10 volt output would be taken, and (c) the values of any additional components needed.

7. A light emitting diode is to be powered from a 12 volt car battery with a current-limiting resistor in series with the diode.

a. Draw the schematic diagram showing how these parts would be connected. Make sure the polarity of the diode is correct.

b. Assume that the forward voltage drop of the diode is 2.0 volts and that its maximum power dissipation is 5.0 watts. Calculate the value of the current-limiting resistor, in ohms, and the current, in mA, that would be drawn by the battery in order to operate the diode at its maximum power.