Physics Laboratory
Last update: 2005.5.9
Experiment 6. Ohm’s Law
Caution for TA
Purpose of Experiment
By measuring voltage on a circuit consisted of resistances, electromotive forces and current
flowing the circuit by means of a voltmeter and an ammeter, confirm Ohm’s law and
Kirchihoff’s law, and obtain the values of resistances and learn resistance color codes.
Outline of Experiment
• Have full knowledge of principles of an ammeter and a voltmeter.
• Measure resistances by use of a voltmeter and an ammeter.
• Guess values of resistances obtained from parallel circuits and series circuits
Experimental Methods
These equipments are prepared in the laboratory. (Parentheses mean the number of them.)
dc power supply (0-10V) (1)
digital tester(1)-------------------------------------(뭔지 모르겠음)
resistance connecting box(1)
resistances(6)
If you need more equipments, inquire to your teaching assistant or the experiment preparation
room (19-114), or prepare them for yourself. And also you need to know the static
current(voltage) sources.
Bellows are the recommended and standard experimental methods
1) Plug in an unknown resistance in the series connection part of the resistance
connecting box. In this time being aware of that the yellow lines are the road that the
electric wire pass through, connect each of +,– terminals of the DC power supply with
that of the resistance connecting box. In this time be sure that the voltmeter is
connected in parrellel and the ammeter is connected in series. When you don’t use an
ammeter, connect a wire between the ammeter terminals of the resistance connecting
box not to open the circuit. Then turning on the DC power supply, apply adequate
voltages and currents. Measure the voltage and the current to obtain the value of the
resistance. Repeat this procedure for 4 other resistances.
[video]
2) Connecting two resistances that you know the values of by performing 1) with the box
in series, measure the voltages and currents to know the total resistance value of the 2
series resistances.
[video]
Make a parallel circuit with the Known 2 resistances,
do the same experiment for a parallel circuit.
[video]
3) Connecting 4 resistances that you know the values of by performing 1) with the box in
a way that two in series and two in parallel, obtain the total resistance value by using
the measured voltage and the current.
[video]
4) Below is a recommended format for the laboratory report.
(Measurement of Ohm;s law)
Voltage applied to resistance 1 = _______V, Current flowing resistance 1 =_______A
Value of resistance 1 (V/I) = _______ohm
Voltage applied to resistance 2 = _______V, Current flowing resistance 2 =_______A
Value of resistance 2 (V/I) = _______ohm
Voltage applied to resistance 3 = _______V, Current flowing resistance 3 =_______A
Value of resistance 3 (V/I) = _______ohm
Voltage applied to resistance 4 = _______V, Current flowing resistance 4 =_______A
Value of resistance 4 (V/I) = _______ohm
Total resistance of resistance 1,2 in series =_______(V/I)ohm
Total resistance of resistance 3,4 in parallel =_______(V/I)ohm
Total resistance of resistance 1,2 in series and resistance 3,4 in parallel =_______(V/I)ohm
Background Theory
(1) Ohm’s Law
The equation below holds for a voltage V for some resistant object and for a current I
flowing through the object. Ie, the current I is proportional to the applied voltage V
V = RI
(1)
with R is the resistance of the resistant object.
There are two ways to construct a circuit; a sires circuit, and a parallel circuit.
For a series circuit,
RS = R1 + R2 + R3 + …
(2)
For a parallel circuit
1/RS = 1/R1 + 1/R2 + 1/R3 + …
(3)
Here, because I = ∆q / ∆t, the unit of I is [coul/sec] = [ampere], because V = W/q, the
unit of V is [joule/coul] = [volt], and because R = V/I, the unit of R is [volt/ampere] =
[ohm].
(2) Kirchihoff’s Law
1) 1st Law
At any node(junction) of electrical circuit, the sum of currents flowing into the node
is equal to the sum of currents flowing out of the node. Ie;
It = 0
(4)
2) 2nd Law
The algebraic sum of all electromotive force E in a closed circuit is equal to the
algebraic sum of all voltage drops by all resistances in the same closed circuit. Ie;
The following table is the method of expressing resistances by colors and the model
of expressing the equipments by signs.
color
black
brown
red
orange
yellow
green
blue
violet
A
0
1
2
3
4
5
6
7
B
0
1
2
3
4
5
6
7
C
0
1
2
3
4
5
6
7
D
grey
white
colorless
silver
gold
8
9
8
9
8
9
20%
10%
5%
0.01
0.1
※ According to the ohm’s law, the voltage drops across the resistant object is
proportional to the current that flows through the resistant object. In other words, ohm’s
law holds.
Therefore if the resistance is constant in the range of all supplied voltages, the voltagecurrent graph would be straight. Devices having this property are called linear devices.
Since many devices are heated as the current is increasing, their resistance value changes,
these devices are called non-linear devices.
More Thinking
• Heat
emission from a filament
In a static state, because the temperature of the filament don’t change, all applied power is
emitted. The aspect of emitting energy is different depending on the emitting mechanism. In
other words, in the case of emission of heat by conduction, according to the Newton’s law of
cooling, the energy emitted per unit time is, by the formula
P = k(A/L)(T-T0)
(A1)
, proportional to the temperature difference between the filament and the surroundings,
inversely proportional to the distance with respect to the direction of heat flow, and
proportional to the cross-sectional area. The constant K is called thermal conductivity. In the
case of energy transfer by radiation, the energy emitted per unit time is, according to the
Stefan-Boltzmann law of radiation,
P = εσA(T4 - To4)
(A2)
, proportional to the difference of T4, with T is the temperature of the object, A the surface
area of the object, and ε is called emissivity, σ is called Stefan-Boltzmann constant(σ =
5.67x10-8W/m2·K4).
For the perfect radiating object, like a black body, ε is 1, for most objects 0<ε<1.
Intensity of light emitted from black body – reference E.Hecht general physics book.
Lights emitted from 3000 K filaments are almost infra red rays rather than visible rays.
In the visible region, because the intensity of light is almost the same regardless of the
wavelength, white lights, red rays are slightly stronger, are made.
The intensity of light radiated by a filament – reference E.Hecht general physics book
As the temperature of the filament increases(picture on your right), the intensity is increases
and the color becomes to be white.
When you see the upper two equations(equation A1, and A2), you may notice that, depending
on the way of heat transfer, the aspect of the temperature dependence of heat transfer power
is different. In other words, when the temperature of the filament is not quite different from
the room temperature, heat transfer mainly occurs by conduction(equation (A1)), however, as
the temperature increases, radiation(equation (A2)) is expected to be the main mechanism of
heat transfer. Therefore, from the power P applied to the filament and the temperature T of
the filament in a static state, drawing log-log graph of P versus (T-T0) or P versus(T4-T04)
gives you the information whether the energy transfer from the filament to the surroundings
is mainly occurs by conduction or radiation and the temperature ranges where each transfer
machnism is important.
References
• Static current(voltage) power supply
• Analysis of measurement data
• Aanlysis method based on the graph
• James Joule – the importance of precise measurement
• Georg Ohm – Ohm’s law given a cold reception
• Joseph Stefan – the creator of kinetic gas theory
• Ludwig Boltzmann – the theoretical physicist laid the foundation of statistical mechanics