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Transcript
Lab 18: Charge Flow Rate – Measuring Electric Current April 29, 2017 Set-up and Figure 1 1 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). 2 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). 3 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. 4 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. 5 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? ________ 6 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. 7 8