Download Batteries and Bulbs

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