Download Lab 18 Charge Flow - Measuring Electric Current

Lab 18: Charge Flow Rate – Measuring Electric Current
April 29, 2017
Set-up and Figure 1
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Lab 18: Charge Flow Rate – Measuring Electric Current
April 29, 2017
OVERVIEW:
Electric current is defined as the flow of electrons. The standard measure for current flow
in an electrical unit is the ampere or "amp".
One ampere is equal to one coulomb per second.
There are two types of current. Direct Current (DC) flows in one direction only.
Alternating Current (AC) periodically reverses its direction of current flow.
When making current measurements (either AC or DC) one must take care to place the
meter in the circuit correctly. To measure current, the meter must be placed in series with
the load. By doing so, the current that flows through the load is the same as that through
the meter.
.
Whenever the size of a circuit current is small (less than 0.1 amp), typically the units of
milliamps are used. Since “milli” means 1/1000th , there are 1000 milliamps in 1 amp.
Therefore a small current measured in amps can be converted into milliamps by moving
the decimal point three places to the right, and using the prefix “milli”. For example,
0.050 A is converted into 50 milliamps (mA).
When measuring an unknown value of current, the meter range switch should be set to
its highest range first. This will help to protect the meter from too much current. You can
always lower the range setting, if the current does not register on the meter. In this
experiment, the current will not exceed 0.9 A (900 mA).
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Lab 18: Charge Flow Rate – Measuring Electric Current
April 29, 2017
OBJECTIVE: Use analog and digital multimeters to measure DC current in an electrical
circuit.
EQUIPMENT:
10-ohm, 50 Watt Resistance Heater
Circuit Panel and Easel
High Current AC/DC Power Supply
Digital Multimeter (DMM)
SPST Knife Switch
Universal Lead Set
Analog Multimeter
PROCEDURE:
Be sure that the power supply is turned off.
Connect the circuit as shown in Figure 2,
Follow the directions below:
A) 1. Use a black banana-banana lead to
connect the negative terminal of the power
supply to the multimeter "common"
(ground) connection.
2. Use a red banana-banana lead to connect
the positive terminal of the power supply to
the heater as shown in Figure 3.
3. Use two banana-spade leads to connect
the switch to the resistance heater and to
the "2A” or “10A” input of the digital
multimeter (refer to Figure 2)
4. Check the settings:
Power supply set for 0-24 V DC.
Power supply set to measure volts. Circuit Switch
is open.
Digital Multimeter set on DC.
Digital Multimeter set to "2A” or “10A” range
(to correspond to the input jack).
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Lab 18: Charge Flow Rate – Measuring Electric Current
April 29, 2017
B) CURRENT MEASUREMENTS WITH THE DIGITAL MULTIMETER
CAUTION: If the power is left on for extended periods of time, the resistance heater will
become extremely hot. Do not touch it.
Trial 1:
1. Turn on the power supply. Adjust the output to 9.0 V.
2. Turn on the digital multimeter and close the switch. Readjust the power supply, if
necessary, to read the designated voltage while the current is flowing.
3. Read the current value from the multimeter. Record this value in the data table.
4. Turn off the power supply and digital multimeter.
Trial 2:
6. Repeat steps 1 - 4 with a setting of 6.0 volts on the power supply.
Trial 3:
7. Repeat steps 1 - 4 with a setting of 3.0 volts.
C) MEASURING CURRENT WITH THE ANALOG MULTIMETER
1. Disconnect the DMM. Be sure the meter is off.
2. Connect the black analog meter lead to the common (-) connection on the meter, and
connect the red lead to the + connection.
3. Insert the analog meter probes into the back of the banana plugs which were connected
to the digital meter. The analog meter will now be connected in series with the heater,
replacing the digital meter.
4. Set the meter. function to +DC, & the range control to 500 mA (Simpson meter) or 300
mA (ECI meter).
Trial 4:
5. Set the power supply to measure volts.
6. Turn on the power supply, and adjust it for 1.0 volts.
7. Close the switch. Re-adjust the power supply, if necessary, to read the designated
voltage while the current is flowing.
8. Read the analog meter value of current, and record it in the data table.
9. Open the switch, and turn off the power supply.
Trial 5:
11. Repeat steps 5 - 9 for 2.0 volts.
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Lab 18: Charge Flow Rate – Measuring Electric Current
April 29, 2017
Trial 6:
12. Repeat steps 5 - 9 for 3.0 volts.
13. Discuss the questions on the back of the lab form with your partners, and fill in the
answers that you agree upon. If you cannot agree on a specific question, ask for help from
the instructor.
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Lab 18: Charge Flow Rate – Measuring Electric Current
LAB 18: MEASURING CURRENT
OBJECTIVE:
April 29, 2017
Date_________
SCHEMATIC DIAGRAM OF THE CIRCUIT:
Trial
#
DATA TABLE 1
Power Supply
Type of
Voltage
meter
V (volts)
(V)
1
Digital
2
Digital
3
Digital
4
Analog
5
Analog
6
Analog
Meter Reading:
I (amperes)
(A)
Resistance:
R =V/I
()
AVERAGE VALUE OF R (TO NEAREST WHOLE NUMBER): _____________ 
Did the ratio remain approximately constant? ________
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Lab 18: Charge Flow Rate – Measuring Electric Current
April 29, 2017
LAB 18 ANALYSIS:
1. Describe what happens to the load current when the resistance is held constant and the
voltage is increased.
2. The ratio of voltage to current is the "resistance" of the load. What resistance is marked
on the heater you used in this experiment? note: volts/amps = ohms ()
HEATER RESISTANCE:_____________
3. Calculate the percentage difference (%) between the measured resistance (average of
your ratios) to the value marked on the heater.
%
4. When a meter is used to measure current in a circuit, how is it connected to the load
(series or parallel)? _______________
Compare this to the connections when the meter will be used to measure voltage:
______________________________
5. When using a meter to measure the current in a circuit of unknown resistance, why is
the meter range always set to the highest value when the switch is first turned on?
USE THE 5-STEP METHOD TO SOLVE THE FOLLOWING PROBLEMS:
6. A 110 volt heating element in an electrical water heater is rated at 20.0 amperes. A)
How many coulombs of charge pass through the element in 45.0 seconds? B) How
many electrons does this represent?
7. In an electric motor, operating at a constant speed, 3.40 x 104 coulombs of charge have
moved through the motor, with a steady current of 35.5 amperes. Find how much time
elapsed while the motor operate.
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