OBJECTIVE
To demonstrate Ohms Law and to show its various forms.
To become familiar with DC voltmeters and ammeters.
DISCUSSION
Electrical resistance is the opposition to the flow of electrical current in a circuit and
is dependent on many factors. Copper wire, though considered a good conductor
of electric current, does offer some resistance. A German physicist, George Simon
Ohm (1787-1854) discovered that the ratio of voltage to current was constant for a
given metal conductor of specified length and cross-sectional area. This ratio is
known as resistance and is expressed in units of ohms, in his honor.
Ohms Law is often referred to as the foundation of circuit analysis and can be
expressed by the formula:
(1)
where: E = The potential difference from one end of a resistance element to the
other (measured in volts).
I = The electrical current through the same resistance element (measured
in amperes).
R = The resistance of the same element (measured in ohms).
Two other useful expressions can be derived from equation (1); they are:
(2)
E = IR
(3)
In order to produce a current, a voltage must firs exist across the resistance.
Early experimenters in electricity recognized the fact that an electric current was a
movement of charges along a conductor. The direction of the flow of current was not
known and unfortunately, it was arbitrarily chosen to be from a positively charged
body to a negatively charged body (positive to negative) and this convention has
been so firmly established that it is still in use. Thus, the conventional direction or
positive direction of current flow is taken to be from positive to negative even though
it is now known that the direction of electron flow, which actually constitutes and
electric current, is from negative to positive.
5-1
Electric power systems of which this program is a part use conventional current
flow for electric current. In this conventional system, current flows from a positive
to a negative terminal.
A volt is the unit of electrical pressure or potential. Voltage is measured by using a
voltmeter. Voltmeters have a high internal resistance and are always connected in
parallel with a circuit or component such as a resistor. See Figure 5-1.
Figure 5-1.
Note that the polarities marked on the meter terminals must be observed to obtain
a positive (up-scale) meter reading. If the connections are reversed, the pointer will
deflect in the negative direction.
The ampere is the unit of electric current. Current is measured by using an ammeter.
Ammeters have low internal resistance and are always connected in series with a
circuit or component such as a resistor. See Figure 5-2.
Figure 5-2.
5-2
The same note about the voltmeter polarity applies to an ammeter. Polarity must be
maintained for proper pointer deflection.
EQUIPMENT REQUIRED
Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the
list of equipment required to perform this exercise.
PROCEDURE
CAUTION!
High voltages are present in this Experiment! Do not make or
modify any banana jack connections with the power on unless
otherwise specified!
1. Using your ohmmeter, measure the resistance between the terminals of the
200 V dc voltmeter.
R=
2. Measure the resistance of the 2.5 A dc ammeter.
R=
3. Measure the resistance of the 500 mA dc.
R=
4. Does the voltmeter have an appreciably higher internal resistance than the
current meters? Can you explain why?
Yes
No
5. Using your Resistive Load, DC Voltmeter/Ammeter and Power Supply,
connect the circuit shown in Figure 5-3. Be careful to observe instrument
polarities. Make certain that the power supply switch is open, the on-off
indicator lamp is extinguished and the variable voltage output control is
turned fully counterclockwise. The power supply voltmeter switch should be
in the DC position and the meter should indicate zero volts. (7 is the positive
and N the negative terminals, for the variable DC voltage output of your
power supply).
5-3
Figure 5-3.
6. Turn on the power supply. Slowly advance the voltage output control knob
(clockwise) until the 0-200 V dc voltmeter - across the 300 load-indicates
20 V dc. The current flowing through your circuit is indicated by the
0-500 mA dc milliammeter. Record this current in the space provided in the
table. Repeat for each of the voltages listed in Table 5-1. Return the voltage
to zero and turn off the power supply switch. (Do not disconnect your
circuit).
VOLTS
E
0
20
40
60
AMPS
I
Table 5-1.
Figure 5-4.
5-4
80
100
120
7. Plot the recorded currents (at the listed voltages) of Table 5-1 on the graph
of Figure 5-4.
8. Draw a smooth curve through these plotted points. Is the current directly
proportional to the voltage; (does the current double, triple, etc., when the
voltage doubles, triples, etc)?
Yes
No
9. Using the values of I and E from the table in procedure 6, calculate the
ratios of E/I in each case. Record your calculations in Table 5-2.
E
20
40
60
80
100
120
E/I
Table 5-2.
10. The average value of E/I is
.
Note that the ratio between the voltage applied across the resistor and the
current flowing through it is a constant value, called resistance.
11. You will now verify that the alternate form of Ohms Law (I = E/R) is valid.
Use the same circuit shown in Figure 5-3. Turn on the power supply and
adjust for 90 V dc as indicated on the voltmeter across the 300 resistor.
Measure and record the current through the 300 resistor.
Imeasured =
A dc
Return the voltage to zero and turn off the power supply switch.
Does Imeasured = E/R = 90/300?
Yes
No
12. You will now verify that the other alternate form of Ohms Law (E = I x R) is
valid. Use the same circuit shown in Figure 5-3. However, this time set the
resistance to 600 . Turn on the power supply and adjust the output voltage
until the current meter indicates 0.2 A. Measure and record the voltage
across the 600 resistance.
Emeasured =
V dc
Return the voltage to zero and turn off the power supply switch.
5-5
Does Emeasured = 1 x R = 0.2 x 600?
Yes
No
13. You will now measure the value o an equivalent resistance without the use
of your ohmmeter. Use the same circuit shown in Figure 5-3. Turn on the
power supply and adjust the output voltage for 60 V dc as measured on the
voltmeter across the resistance. Vary the resistance by using the switches
until approximately 0.3 A is indicated by the current meter. Readjust the
voltage control if necessary to maintain 60 V dc across the resistance.
a. Using Ohms Law and with the above voltage (60 V) and current (0.3 A),
calculate the equivalent resistance now in the circuit.
Requivalent = E/I = 60/0.3 =
Return the voltage to zero and turn off the power supply switch.
b. Using the formula for parallel resistance, and with the resistances you
have connected in parallel, calculate Requivalent :
Requivalent =
Is there good agreement between (a) and (b)?
Yes
No
14. Disconnect your circuit without disturbing the position of the resistance
switches. With your ohmmeter measure the equivalent resistance of
procedure 13.
Requivalent =
Is there good agreement between the ohmmeter measurement of Requivalent
and your calculated Requivalent of procedure 13 (b)? Explain.
5-6
REVIEW QUESTIONS
1. Using Ohms Law in its various forms, fill in the blanks in Table 5-3.
NO.
1
E (V)
6
I (A)
2
R( )
2
3
4
4
9
5
5
3
25
6
8
12
6
7
8
5
12
12
10
30
4
9
10
120
100
1000
0.1
Table 5-3.
2. A 3 A dc meter has a resistance of 0.1 . If it were accidentally connected
across a 120 V dc line, what would be the current through the instrument?
I=
A dc
What do you think would happen?
3. A 3 A dc meter has a resistance of 0.15 , and carries a current of 2 A. What is
the voltage across its terminals?
E=
V dc
4. A 0-150 V dc meter has a resistance of 150000 . What is the current through
the instrument when it is connected across a 120 V dc line?
I=
A dc
5-7
5. An experimenter accidentally touches a 240 V dc line. If his skin resistance is
10 000 , what value of current flows through his body?
I=
A dc
Is this dangerous?
Yes
No
6. An electroplating plant has bus-bars which carry up to 1000 A at 6 V direct
current. The surroundings are very wet with water and electrolyte. Should the
bus-bars be insulated, and if so, why?
7. Birds have been known to perch on 2300 V bare transmission lines without
apparent harm. Is this because of the very dry nature of their feet? Explain.
Yes
No
8. An ammeter having a scale 0-1 A dc and a resistance of 1
across a source of 300 mV. What will it indicate?
5-8
is connected
0
