Name:_________________________
Homework #3: Basic Electricity--Ohm’s Law and The Power Law
EE-1322: Survey of Electrical and Electronic Devices
Assigned: Wednesday, September 3, 2014
Due: Wednesday, September 10, 2014
[Each question is worth four points unless noted otherwise.]
1. Briefly explain your understanding of AC and DC?
2. Briefly describe the nominal terminal voltages found in the following battery types?
Lithium ion:
Lithium 123:
Lead-acid wet cell:
Carbon-zinc dry cell
Nickel-metal hydride:
3. Briefly describe automotive voltage?
4. Briefly describe household voltage?
5. Briefly explain your understanding of elements connected in series, and elements
connected in parallel?
For each of the following problems, sketch a simple schematic, label the known and
unknown quantities, and use the proper law (or laws) to perform the required
computations.
Laws: Ohm’s Law (I=V/R), the Power Law (P=VxI), Kirchhoff’s Voltage Law (the way
the batteries are connected matches the way the light bulbs or resistors are connected) and
Kirchhoff’s Current Law (the current that flows into a connection equals the current that
flows out of the connection).
Your calculated results should be rounded off to about three significant figures; for
example, 0.nnn, n.nn, nn.n, or nnn.
6. A small resistor exhibits a current flow of 1.5 amps when connected to a six-volt
battery. What is the resistance value in ohms? How much power does the resistor
dissipate? (Your schematic should show a 6V battery and a resistor.)
7. What battery voltage is needed to cause a current of 0.5 amps to flow through a 24
resistor? (Your schematic should show a battery, a resistor, and the indicated current
flow.)
8. A 10 resistor is connected to a 3V battery. What current will flow in the resistor?
(Your schematic should show a 3V battery and a 10 resistor.)
9. An automobile headlight draws about 6 amps. Draw a schematic showing two
headlights in parallel connected to an automotive battery. Label the currents on the
schematic. How much power does each headlight dissipate, and how much power is the
battery supplying?
10. A hair dryer is rated at 1400 watts. How much current will flow in that hair dryer
when it is running? (Your schematic should show an AC voltage source and a resistor (or
other made-up symbol) for the hair dryer.)
11. How much current flows in a 50 watt light designed for recreational vehicles using a
12 V DC electrical system? (Your schematic should show a 12 V battery and a light bulb
dissipating 50 W.)
12. How much current flows in a 50 watt light bulb intended for household use? (Your
schematic should show a 120 V AC source and a light bulb dissipating 50 W.)
13. The how much power is lost (or dissipated) in a 4 resistor when the current through
the resistor is 0.5 amps? (Your schematic should show a resistor with the indicated
current.)
14. How much power is lost (or dissipated) in a 4 resistor when the current through the
resistor is 5 amps? (Your schematic should show a resistor with the indicated current.)
15. A twelve-volt battery is connected to a 6 resistor. How much power will be
dissipated by the resistor? (Your schematic should show a battery and a resistor.)
16. When starting an automobile, the current that is initially supplied by the battery is
limited only by the total internal resistance of the battery in series with resistance of the
connecting cables, the starter switch, and the resistance of the wire in the starter motor.
[When resistors are in series, the total resistance is the sum of the individual resistance
values.] If this resistance value is about 17.4 milliohms (0.0174 ohms), how large is the
initial starter current? Your schematic should show a battery, a switch, and a resistor.
Volt, ampere, watt and ohm values can be large or small.
The Power Law and Ohm’s Law handles them all!
17. The wireless microphone transmitter that I use in class is powered by a 9 volt carbonzinc dry battery. I measured the current to be about 35 mA. How much power does the
wireless microphone use? What value of resistance would draw the same current when
connected to the 9 volt battery? (Conversion factor: 1 mA = 0.001 A) (Your schematic
should show a battery and a resistor.)
18. How much current will flow if we connect an 18 K resistor to a 9 volt battery?
Give the answer in amperes, milliamperes, and microamperes! (Your schematic should
show a battery and a resistor.)
Conversion factors:
1 K = 1,000
m
m
mA =
Ohm’s law gives the answer in amperes if the resistance is in ohms and the voltage is in volts.
K means 1000, so move the decimal three places to the right to convert 18K ohms to ohms.
The word milli means 1/1000 and micro means 1/1,000,000. Move the decimal three places to the
right to convert amperes to milliamperes. Move it three more places (six places total) to convert to
microamperes.
Most light bulbs have their voltage and current ratings (or their voltage and power
ratings) stamped somewhere on the base or printed on the bulb envelope. For the purpose
of answering these questions, the flashlight bulb voltage ratings in questions 19-21 would
be taken from the following values: 1.5 volts, 2.0 volts, 3.0 volts, 4.5 volts, 6.0 volts, or
7.5 volts.
19. Sketch a schematic showing four carbon-zinc AA batteries in series connected to
three flashlight bulbs in series? Include a switch that will turn all lights on or off. What
is the voltage rating of the individual flashlight bulbs, assuming identical light bulbs?
How much current flows through each battery if the bulb current is 0.1 A?
20. Sketch a schematic showing four carbon-zinc AA batteries in parallel connected to
three flashlight bulbs in parallel? Include a switch that will turn all lights on or off. What
is the voltage rating of the individual flashlight bulbs? How much current flows through
each battery if the bulb current is 0.1 A?
21. Sketch a schematic showing four carbon-zinc AA batteries in series connected to
three flashlight bulbs in parallel? Include a switch that will turn the lights on or off.
What is the voltage rating of the individual flashlight bulbs? How much current flows
through each battery if the bulb current is 0.1 A?
For the following questions 22, 23, and 24, the opencircuit voltage of a brand-new, fresh out-of-the-box AA
carbon-zinc battery is about1.600 volts. The internal
resistance of the battery is about 0.25 ohms. The
schematic symbol for this real battery is a 1.600 volt ideal
battery in series with 0.25 ohms as shown to the right.
22. How much current will flow in a short-circuit wire when the battery is shorted, and
how much power will be dissipated in the wire? Shorting the battery means taking a wire
and connecting the battery + and – terminal together with a wire of essentially 0 ohms
resistance. (Your schematic should include the battery with internal resistance and
shorting circuit.)
23. Assume that a 1.75 ohm resistor is connected to the battery. How much current will
flow through the 1.75 ohm resistor, and how much power is dissipated in it? (Your
schematic should include the battery with internal resistance and an external 1.75 ohm
resistor.)
24. Assume that a variable resistor R whose resistance can be continuously varied from 0
ohms to 50,000 ohms is connected to the battery. Show how the current through the
variable resistor and the power dissipated in the variable resistor varies as a function of its
resistance. (Your schematic should include the battery with internal resistance, and the
proper symbol for a variable resistor.) [Note: you likely will need extra paper to answer
this challenge question.][8 points total]