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Unformatted text preview: NAME LAB PARTNERS Station Number Electric Circuits: Part I Experiment 7 INTRODUCTION In this and the next experiment (Electric Circuits: Part 2) you will study both qualitatively
and quantitatively the behavior of electric circuits. Using circuit diagrams, you will construct
circuits consisting of wires, resistors, switches, and batteries or power supplies. One of the most
difﬁcult aspects of this work is that the circuit diagram may bear little resemblance to the shape
of the real circuit. The circuit diagram is usually drawn with straight lines while the circuit is
connected with wires that are most likely not straight. The purpose of this experiment is to
acquaint you with some of the elementary aspects of electric circuits, resistance, Ohm's law,
short circuits, and the differences between conductors and insulators. THEORY A metal may be considered to consist of ﬁxed positive charges and electrons (negative
charges) that are free to move around between the positive charges. Ordinarily, although the
electrons are constantly moving, their motion is random so that no electron really moves very far
from some equilibrium position. When a battery or other source of electromotive force (emf) is
connected to a conductor (a wire, for example), the electrons move in response to the electric
ﬁeld. In the absence of other forces, the electrons would continue to accelerate in accordance
with Newton's second law of motion. The electrons, however, undergo collisions which act as a
resistance to their motion. Therefore, the electrons reach a terminal or drift velocity. (This
situation is analogous to the motion of a parachutist, who, without air resistance, would continue
to accelerate. The air resistance causes her to reach a terminal velocity a short time after opening
the parachute.) This resistance of a piece of metal to the ﬂow of charge is determined partly by
its resistivity p. The average electric current I is deﬁned to be the rate of change of charge with
respect to time (I = Aq/At). For a cylindrical conductor (a wire) having length L and cross sectional area A, the resistance R to the ﬂow of charge is given by
R = pL/A. (1) For many metallic conductors the potential difference V, current I, and resistance R are related
by the simple equation V = IR, a relationship know as Ohm's law. In general, most metals are good conductors of electric current while nonmetals are not good conductors. In poor conductors the electrons are strongly bound to the positive charges so that
few (or even none) of the electrons are free to move. Such materials are called nonconductors or
insulators. l. EXPERIMENT NO. 7 Using one low voltage power supply (follow the setup procedure described by your
instructor), one light bulb (not in the socket), and two wires, ﬁnd a way to light the bulb.
Describe the path of the current. Set the voltage to 5 volts. If the light bulb is represented by a resistor symbol, the wires by a straight lines, and the
power supply by a battery symbol, draw a circuit diagram for the connections that cause the
bulb to light. The power supply you use may be considered to be an "electronic battery". Reconstruct this circuit with the bulb in the socket. Interpose several materials (coins, keys,
people, pencils, etc.) into the circuit and use the brightness of the bulb to classify each object
as a conductor or an insulator. Set the voltage to 5 volts. Set up each of the following circuits to investigate the brightness of each bulb as compared to
a standard. The standard circuit is shown below; it should be retained for comparison
purposes. You may use the ﬁxed 5volt output for this. R (bulb) ['15 power supply 44 Compare the brightness of each bulb in each of the following circuits to the standard. This
comparison will have to be qualitative. Be sure both power supplies are set to the same voltage.
power supply power supply
Brightness Brightness 4. With the bulbs connected as shown, connect a wire
around one of the bulbs. Describe what happens and
try to explain your observation using the discussion
of resistance from the theory section. power supply Connect a wire around the bulb on the standard circuit and observe what happens. Draw the
circuit diagram and explain your observations. Each of these is an example of a "short" circuit. 5. Connect the slidewire resistance board as shown in the ﬁgure. Obtain values for I and V for
currents ranging from 0.05 A to 0.50 A in increments of 0.05 A and record them in the table.
Note that a digital multimeter (DMM) is used for measuring both current and potential
difference. Instructions are provided on the use of the DMM. 45 power supply Current 0.... GD Voltage 200 cm Plot a graph of V versus I with V as the ordinate and I as the abscissa. Calculate the slope of
this graph and show your calculations on the graph. Also record the value of the resistance
stamped on the 2—meter resistance board. This value is the standard resistance. Slope = Standard resistance = What does the slope you calculated represent? Calculate the percent error in the measured value of the resistance. percent error = . With the circuit still connected as in Part
5 and the current set to 0.5 A, record the
voltage readings with the voltmeter con
nected across lengths of wire of 40 cm,
80 cm, 120 cm, 160 cm, and 200 cm.
Calculate the resistance of each length of
wire using V=IR and complete the table. Voltage Resistance Plot a graph of the resistance versus length with resistance as the ordinate and length as the
abscissa. Calculate the slope of the graph. Show the calculations on the graph paper. Slope = Write the equation that relates the slope to the crosssectional area of the wire and its
resistivity. Equation QUESTIONS 1. A conducting wire of length L 0 and diameter d0 has resistance R0. A second wire of the same
material has length 2L0 and diameter 2d0. What is the resistance of the second wire? 2. For the circuit shown, which bulbs will go out when a wire is f a 0
connected between points a and b, b and c, c and d, e and f?
Explain your answers. b B
D C 47 3. A voltmeter and an ammeter are connected to measure the current through and potential
difference across a one meter long aluminum wire. The current is 0.4 amperes and the
potential difference is 6.0 volts. If the wire is replaced by a 1.25meter long aluminum wire
with the same crosssectional area, and the potential difference across the wire is increased to
8.0 volts, what is the new current? ...
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This note was uploaded on 02/20/2012 for the course PHYS 1101 taught by Professor Lowellwood during the Fall '10 term at University of Houston.
 Fall '10
 LOWELLWOOD

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