Physics 8.02T 1 Fall 2001 Simple Circuits Experiment Introduction Our world is filled with devices that contain electrical circuits in which various voltage sources cause currents to flow. We use radios, telephones, computers, flashlights, irons, heaters, stoves, motors the list is long. Each of these contains circuits, which take electrical energy from some power supply and turn it into other forms: the light of a lamp, the heat of a stove, stereo sound or mechanical work. In general, electrical currents generate heat and magnetic fields and produce chemical effects. Any of these phenomena can be used to measure current. One of the simplest ways is to let the current flow through a coil of wire that is in a magnetic field and to measure the resulting torque on the coil by observing the deflection of a torsion spring. This is how your multimeter works. See http://web.mit.edu/8.02t/www/coursedocs/current/labs.htm for a link about multimeters. In this experiment you will use meters, batteries, power supplies, and resistors to make measurements to get some intuition and knowledge about circuit behavior. Equipment: You will use two size D 1.5 V batteries, the AC/DC Electronics Lab circuit board, a multimeter; three 33 Ω resistors; and wire leads. Figure 1: The AC/DC Electronics Lab Circuit Board
Physics 8.02T 2 Fall 2001 AC/DC Electronics Lab circuit board: Throughout the AC/DC Electronics Lab circuit board (see Figure 1 on previous page) there are springs. These allow you to easily connect wires or various circuit elements: In order to maintain two different springs at the same voltage, connect the two springs with a wire lead. In order to connect a circuit element (for example, a resistor), insert the element between a pair of springs to ensure contact. The circuit board has some mountings or elements already wired into place. In this experiment we will use: On the left side of the circuit board: two mountings for batteries with + (positive) and (negative) terminals indicated (see A in Figure 1). On the top left of the board: three 3 Volt bulbs with two springs on each side (see B in Figure 1). On the bottom of the board, two rows of four spring pairs with a banana jack at the end of the row on the right side (see C in Figure 1). Each pair of springs in a row is wired together. Experiment 1: Measuring Resistance with the Multimeter Measure the resistance of each of the three 33 Ω resistors by placing the multimeter clip leads on either side of the resistor. Figure 2: Multimeter
Physics 8.02T 3 Fall 2001 In order to use the multimeter (Figure 2 above) to measure resistance, insert the red clip lead into the + V-Ω-A pin jack and the black clip lead into the COM pin jack. Set the range selector switch to the RX1 Ohms scale. Zero the scale by connecting the ends of the clip leads together and adjusting the OHMS ADJUST dial until the needle points to zero on the green OHMS scale. Experiment 2: Measuring Resistance for Resistors in Parallel and Series: Measure the equivalent resistance of two 33 Ω resistors connected in parallel. Two resistors R 1 and R 2 in parallel give an equivalent resistance of R 1 R 2 /(R 1 +R 2 ). Measure the equivalent resistance of two parallel 33 Ω resistors connected in series to a third 33 Ω resistor. Two resistors R 3 and R 4 in series give an equivalent resistance of (R 3 +R 4 ). You should now connect two 33 Ω resistors in parallel, labeled R 1 and R 2, and the third 33 Ω resistor, labeled R 3, in series with the other two as shown in Figure 3. Figure 3: Resistors in parallel and series. WebAssign Question A: Go to WebAssign either now or at the end of this experiment and open the Resistors In Series And Parallel exercise. Calculate the equivalent resistance of a pair of 33 Ω resistors that are in parallel and enter it in WebAssign. Measure the equivalent resistance between the points A and B in Figure 3 using your multimeter (across the parallel combination R 1 and R 2 ) and record it here: ohms Does it agree with the value you calculated above? Measure the resistance between the points B and C in Figure 3 using your multimeter (across the resistor R 3 ). and record it here: ohms
Physics 8.02T 4 Fall 2001 WebAssign Question B: Calculate the total equivalent resistance of the three resistors and enter it in WebAssign. Measure the resistance across the entire combination of resistors between the points A and C in Figure 3 and record it here: ohms. Does it agree with the value you calculated above? Experiment 3: Measuring Current In A Closed Circuit: You will measure the current in each branch of a closed circuit containing a resistor network and batteries. Connecting the Batteries: You will now make a closed circuit consisting of two 1.5 V batteries in series with your combination of resistors (Figure 3). Figure 3: Closed circuit with batteries and resistors Insert the two 1.5 V batteries in the plastic battery mounts on the AC/DC Electronics Lab circuit board. Use the long white wire lead to connect the positive terminal of the battery on the upper left corner of the board to a component spring pair on the left side of the upper row. Use a short white wire lead to connect the negative terminal of the upper battery to the positive terminal of the lower battery. Use another white wire lead to connect the negative terminal of the lower battery to a component spring pair on the lower row. If you want to disconnect the battery, disconnect any of these connections. WebAssign Question C: When the batteries are connected, calculate the total current that flows in the circuit and enter it in WebAssign.
Physics 8.02T 5 Fall 2001 We now measure the current that flows in the branch of the circuit between the batteries and the resistors, as follows. Figure 4: Multimeter in series in the closed circuit Always set the multimeter to the proper scale (voltage, current, etc) before you connect it. In order to measure the current, rotate the range selector switch on the multimeter to the 250m DCA setting. Place the multimeter in series in the circuit (Figure 4 above). You can do this by removing the wire lead from the negative terminal of the lower battery. Then connect the black clip lead from the multimeter to the negative terminal and the red clip from the multimeter to the spring pair on the lower row that also connects the end of the resistor R 3 (this is the point C in Figure 4). With the batteries connected measure the current that flows in the circuit and record it here: amps. Does your result agree with what you calculate above? WebAssign Question D: Calculate the current that flows through each resistor in the parallel pair and enter it in WebAssign. With the batteries connected measure the current that flows through one of parallel pair of 33 Ω resistors and record it here: amps. Does it agree with what you calculated above? Experiment 4: Measuring Voltage Drops You will measure the voltage drop across different resistor combinations for a closed circuit containing batteries. Remove the multimeter from your circuit and close the circuit again. In order to measure the voltage across a circuit element with your multimeter, rotate the range selector switch on the multimeter to the 5 V DC setting. Use the red and black clips across any element to measure the voltage drop (this puts the multimeter is parallel with the circuit element Figure 6).
Physics 8.02T 6 Fall 2001 Figure 6: Multimeter in parallel with a circuit element to measure voltage drop Question: Calculate the voltage drop across the parallel resistors R 1 and R 2. Measure the voltage drop across the parallel resistors and record it here: volts. Question: Calculate the voltage drop across the resistor R 3. Measure the voltage drop across the series 33 Ω resistor and record it here: volts Question: (Discussion Question): If you now try to measure the resistance of any resistor, will you get the same resistance measurements as you did when the batteries were disconnected? Try this out and see what you get. Can you explain your result? What is the source of the current when the batteries are disconnected?