Batteries and Bulbs (A study of simple direct-current circuits) Goals: To learn to use a digital multimeter as an ammeter and a voltmeter To demonstrate the “loop” rule for potential differences To understand the role of batteries, connecting wires, and resistors in circuits To compare physical circuit layouts and circuit diagrams Equipment: LabKit Module (7) banana-plug hookup wires Digital multimeter (DMM) Introduction: The same principles which govern the operation of a simple flashlight circuit consisting of a battery, connecting wires, and a light bulb also govern the operation of every electrical circuit from the most sophisticated integrated circuit to the wiring in our homes and apartments. Today, we will be studying the operation of this simple circuit, and a subtle variation of it, in order to better understand the concepts of current, potential difference, resistance, and power. We‟ll start by looking at the components involved. Our “power supply” for today will be an ordinary D-cell battery as shown in Figure . (Technically, a single object like that shown in Figure is a “cell”. When used in tandem, or when multiple cells are housed together in one casing, the device is known as a “battery”. We hardly ever worry about this distinction anymore.) r Figure 1 A common household D-cell. The cut-away drawing represents a simplified model of a battery as consisting of an emf (ε) and an internal resistance (r). For most situations, the internal resistance is negligible, but as we will see later, it can become important in circuits with larger currents. Regardless of the current, the potential difference between the bottom (negative) terminal of the battery and the top (positive) terminal is known as the terminal voltage. A battery uses a chemical reaction to produce a potential difference, which we frequently will call its electromotive force or emf (although the word „force‟ is misleading). Always keep in mind that an emf refers to a potential difference, and could thus be measured in volts. The Batteries and Bulbs potential difference between the two terminals of an ideal battery would be equal to the emf ε. All real batteries and power supplies, however, will have some amount of internal resistance which will dissipate some of the battery‟s energy whenever current flows through it. So, in practice, the potential difference measured across the terminals of the battery (which we call the terminal voltage) will be less than the battery‟s potential difference when not connected to a circuit. When the battery is connected to a circuit, though, the potential difference causes a current to flow throughout the circuit. Physically, we observe that electrons slowly drift from the negative terminal of the battery into the conductive wiring, and electrons at the other end of the circuit slowly drift into the positive terminal. The electrons themselves move very slowly through a circuit, but the effect of one electron moving is felt throughout the circuit almost instantaneously. When we use the word „current‟, we are always referring to the conventional current, that is, the direction in which positive test charges would move. By this definition, the conventional current always flows from the positive terminal of the battery, around the circuit, and into the negative terminal. Tungsten Filament Insulation Screw Thread Contact Foot Contact Figure 2 Cutaway diagram of a common incandescent light bulb. Current flows through the light bulb, making contact with an external socket at two points – the “foot” contact at the bottom of the bulb, and the screw-thread contact at the side. These two contacts are insulated from each other, so current may only flow through the thin wires leading to the tungsten filament, which heats to the point of glowing when enough current flows through it. The common incandescent light bulb, shown in Figure 2, works by resistive heating. As a large number of electrons flow through a conducting material, we can imagine that those electrons frequently collide with the atoms of the material, causing the material to heat up. Every light bulb has a tiny wire inside the bulb known as a filament, which gets quite hot when a large enough current passes through it. As we will investigate later in the semester, when objects become very hot, they can emit light in the visible spectrum. The tiny tungsten filament in the light bulb is connected to the exterior of the bulb by two connecting wires, one connected to the screw-thread contacts at the base of the bulb and one connected to the small metallic “foot” contact at the very bottom of the bulb. In order to prolong the life of the fragile filament, which would quickly burn away in the presence of oxygen, the bulb is evacuated of all air, and an inert gas such as argon is used to fill the volume of the bulb. Look closely at the bulbs in today‟s experiment to see if you can recognize the filament. Batteries and Bulbs Figure 3 A digital multimeter (DMM). Most DMMs have the ability to measure current, resistance, or potential difference, with a digital display for easy readout. Pay attention to the settings, as you may need to connect the probes differently when measuring current and potential differences, and watch for a switch or button to toggle between measurements of direct-current (DC) and alternating-current (AC) circuits. An example of a digital multimeter (DMM) is shown in Figure 3. The probes of the multimeter may be permanently attached to the DMM or may be able to be plugged into different portions to enable different features of the DMM. The most common use of any DMM is undoubtedly to function as a voltmeter, a device which measures the potential difference between two points in a circuit. Most multimeters also can function as an ammeter, a device which measures the electric current flowing through it. In order to measure the current flowing between two points in a circuit (say point 1 and 2), an ammeter must be inserted in between these points such that all the current must flow from point 1 into the ammeter, and out of the ammeter into point 2. This means that the connection between points 1 and 2 must be broken (or opened) to allow the probe to be connected between both points. We say that the ammeter has been placed in series with the circuit elements between points 1 and 2. All multimeters function through the use of two conducting probes, with one typically colored red and the other black. For direct-current circuits like the ones we will be studying today, the display will register a positive current when the conventional current is flowing into the red probe, though the ammeter, and out of the black (common) probe. The display will show a minus sign when the current flows the other direction: into the black probe, through the ammeter, and out of the red probe. A voltmeter operates very differently; it is designed to measure the potential difference across an electrical component, or between any two points in the circuit. For instance, if the probes are placed on opposite ends of an electrical component in a circuit, the display will show the potential difference across that component. The circuit need never be broken or interrupted to use a voltmeter. We say that the voltmeter is placed in parallel with the circuit elements. The DMM will display the potential difference ∆V = Vred − Vblack. If the red probe is in contact with a point at a higher potential than the point in contact with the black probe, the display will Batteries and Bulbs register a positive potential difference. If the probes are reversed, and the black probe is at a higher potential than the red probe, the display will show a negative potential difference. When we work with circuits, we will typically refer to a circuit diagram, in which the actual wiring of the circuit is conveyed via a schematic picture. Particular circuit elements such as batteries, resistors, lights, switches, etc. will be represented by particular symbols. Below we illustrate some of the more common circuit elements that you‟re likely to encounter. + − power supply/ battery capacitor resistor V A voltmeter ammeter Batteries and Bulbs diode inductor light bulb switch Name: ____________________________ Sect.: _______ Name: ____________________________ Name: ____________________________ Directions: In today‟s lab, you will be using the different functions of a digital multimeter (DMM) to learn about the electric currents and voltages (potential differences) common in everyday flashlightlike circuits. It is very important that you follow all directions regarding the use of the DMM exactly. Failure to follow the directions could result in damage to the DMM. 1 4 5 A 3 B 2 C 6 2 + − 3 1 6 4 5 − + Figure 4 Physical connection of simple “flashlight” circuit involving a single battery, switch, and flashlight bulb. Above, we see the actual wiring of the circuit using three connecting wires to connect all the components. At left, the circuit diagram corresponding to this circuit is shown. When the switch is in the middle (B) position, the circuit is open. When the switch is moved to the bottom (C) position, the circuit will be closed. Activity 1: Measuring the current in a single-bulb circuit 1. Connect the circuit as depicted in Figure 4, using the “round” light bulb in the small bulb socket. If you‟ve completed the circuit correctly, the bulb should light up when you move the switch to position C. Do not leave the circuit connected for long, as this will drain the batteries. Q1. In what direction is the conventional current flowing? 2. Set the digital multimeter (DMM) to measure direct current on the 2-A scale. On the DMM itself, you should move the red probe into the white hole marked ‘A’, and the black probe Batteries and Bulbs should be plugged into the black hole marked ‘COM’. Turn the dial on the DMM to the „2-A‟ mark. The DMM is now configured to function as an ammeter. 3. As was discussed in the introduction, an ammeter must be inserted between electronic components in order to A measure the current flowing between them. So, to measure the current between any two points, you must + break the connection between them, and connect the − ammeter in series. For example, to measure the current between the positive terminal of the battery and the bottom terminal of the switch, disconnect the wire between points 1 and 2. Then the black (COM) probe of the DMM should be connected to the switch, and the red probe should be connected to the battery. With the circuit thus reconfigured, measure the current between the positive terminal of the battery and the switch (with the switch closed in position C): Current between positive terminal of battery and switch: _____________________ Q2. What do you predict will be the current measured between the switch and the light bulb? Explain your reasoning. 4. Now, connect the DMM between the switch and the light bulb to read the current between these points. You‟ll need to restore your original connection between the battery and the switch. (Note that the black probe of the DMM should be connected to the light bulb and the red probe should be connected to the middle terminal of the switch.) Measure the current between these two components (with the switch closed): A + − Current between switch and round bulb: _____________________ Q3. Based on these two measurements thus far, what do you predict will be the current measured between the light bulb and the negative terminal of the battery? Why? Batteries and Bulbs 5. Connect the DMM in series between the light bulb and the battery. You‟ll need to reconnect the wire between the switch and the bulb. Again, the red probe of the DMM should be connected to the light bulb, and the black probe should be connected to the negative terminal of the battery. Measure the current between these two components (with the switch closed): + − A Current between round bulb and negative terminal of battery: _____________________ 6. Now, instead of using the round bulb, create a circuit consisting of three wires, one battery, the switch, and the “long” light bulb. (Basically, re-create the circuit depicted in Figure 4, using the “long” bulb at the bottom of the LabKit module in place of the “round” bulb.) Q4. Which bulb is brighter when connected to a single D-cell battery? 7. Much as you did in steps 3 – 5, measure the current between the battery and switch, the switch and the bulb, and the bulb and the battery. Record your findings below. Current between positive terminal of battery and switch: _____________________ Current between switch and long bulb: _____________________ Current between long bulb and negative terminal of battery: _____________________ Q5. What can you conclude about the current flowing in a single-loop circuit such as the ones you‟ve studied? Activity 2: Measuring the potential differences in a single-bulb circuit 8. Configure the DMM to measure DC potential differences (voltages) on the 2-V scale. To do this, the red probe (on the DMM) should be plugged into the red hole on the DMM marked ‘V·Ω’. The black probe should remain plugged into the black hole marked ‘COM’. Turn the dial on the DMM to the „2-V‟ scale. The DMM is now configured to function as a voltmeter. Batteries and Bulbs to DMM Figure 5 Measuring the potential difference (terminal voltage) across a single D-cell. 9. To measure potential differences, you do not need to open the circuit. You simply place your two probes in contact with the points between which you wish to know the potential difference. Measure the potential difference across the terminals of a single D-cell as shown in Figure 5 and record it below. Potential difference across terminals of single D-cell: _____________________ 10. Reconnect the original circuit shown in Figure 4 with the round bulb in the bulb socket. Again, only keep the circuit closed for short times while making measurements. 11. With the circuit closed, measure the potential differences between various points indicated in the circuit diagram of Figure 4. Record your measurements below. Important: In each instance, when asked for the potential difference between points A and B (i.e., VB − VA), you should place the black probe at point A and the red probe at point B. You must remain consistent with the placement of your probes in order to measure the sign of the potential difference correctly. V2 − V1 V3 − V2 V4 − V3 V5 − V4 V6 − V5 2 + − 1 6 3 4 5 V1 − V6 Table 1 Potential differences between various points in single bulb circuit (using round bulb) Q6. Which item in the table corresponds to the potential difference across the D-cell in this circuit? Offer an explanation as to why this potential difference across the D-cell is different than when the measurement you took when the cell was not connected to a circuit. Batteries and Bulbs Q7. If you add up all the potential differences in Table 1, going around the full loop, what is the total potential difference? 12. Now, re-wire the circuit using the long bulb instead of the round bulb. Again measure the potential differences between the various points in the circuit and record them below. V2 − V1 V3 − V2 V4 − V3 V5 − V4 V6 − V5 2 + − 1 6 3 4 5 V1 − V6 Table 2 Potential differences between various points in single bulb circuit (using long bulb) Q8. Is there a significant potential difference across any of the connecting wires? Is it safe to say that, for all practical purposes, the connecting wires have negligible resistance? Activity 3: Measuring potential differences in two-bulb circuit Examine the circuit shown in Figure 6. Q9. Without connecting the circuit yet, what do you expect will happen when the switch is closed? (Do you expect both bulbs to shine equally brightly? If so, why? If not, which bulb will shine brighter?) Q10. Now perform the experiment. What do you observe when the switch is closed? Can you explain why the circuit behaves as it does? Batteries and Bulbs 4 9 10 5 A 1 3 B 2 C 7 2 + 1 − 10 + 9 − 8 − 8 3 4 5 7 6 + 6 Figure 6 Two-bulb series circuit using two D-cell batteries, and both the round and long bulb. Above, the actual wiring is depicted. To the left, the circuit diagram corresponding to this circuit is displayed. Note that this is still a single-loop circuit (i.e., all components are connected in series. Be sure to verify the polarity of your batteries in their holder, and adjust your circuit accordingly: the positive terminal of one battery should be connected to the negative terminal of the other. 13. Set the DMM to the 20-V scale. Now connect the circuit shown in Figure 6. Refer to the circuit diagram, and with the switch closed (in position C), measure the potential differences between the various labeled points. Record your measurements below. V2 − V1 V3 − V2 V4 − V3 V5 − V4 V6 − V5 V7 − V6 V8 − V7 V9 − V8 V10 − V9 V1 − V10 Table 3 Potential differences between various points in dual-bulb circuit Q11. If you add up all the potential differences in Table 3, going around the full loop, what is the total potential difference? Batteries and Bulbs Q12. While the circuit is closed, what do you suppose would happen if you were to unscrew one of the bulbs? (Record your prediction below and then test it to see what occurs.) Q13. If you unscrewed one of the bulbs as suggested above, what do you think the potential difference across the empty socket would be? Explain your prediction. 14. Unscrew one of the bulbs, leaving the rest of the circuit intact (and closed), and measure the potential difference across the empty socket as well as the two batteries. ΔVbattery1 ΔVbattery2 ΔVbulb socket Q14. When you unscrew a light bulb with the switch still closed, does the potential difference across the terminals of the socket go to zero? Explain your observation in terms of your earlier “total potential difference around a loop” observations. 15. Be sure to screw the bulbs back into their sockets, and completely disassemble your circuit. Batteries and Bulbs Analysis Q15. Ideally (that is, considering internal resistance to be negligible) does a battery always provide the same potential difference (voltage) or the same current (amperage), independent of the circuit connected to the battery? Q16. Which light bulb had the greater resistance? Determine the resistance of each bulb (based on your measurements collected in this lab) and explain how you determined these values. Batteries and Bulbs