1 Voltage Measurement and Common
Friday, May 25, 2018
5:37 AM
Energy is required to move a charge from a point of lower potential to one of higher potential. Voltage is a measure of this energy per charge. Energy is given up when a charge moves from a point of higher potential to one of lower potential.
Voltage is always measured with respect to some point in the circuit. For this reason, only potential differences have meaning. We can refer to the voltage across a component, in which case the reference is one side of the component. Alternatively, we can refer to the voltage at some point in the circuit. In this case the reference point is assumed to be
"0 V" or "common." The term common is used to differentiate it from the potential of the earth, which is called Earth. Common and Earth symbols are shown below.
"0 V reference" or "common"
"Earth"
Figure 3-1 illustrates the same circuit with two different common points.
Figure 3-1 (a)
Figure 3-1 (b)
For both circuits: 𝑉𝐴𝐵=4 V, 𝑉𝐵𝐶=8 V, 𝑉𝐴𝐶=12 V
The circuit in Figure 3-1 (a) has as its reference point (or "common") the node labelled B. Positive and negative voltages are shown. If the common is moved to point C, the circuit voltages are all positive, as shown in Figure 3-1(b). Voltage is always measured between two points. To define the two points, subscripts are used. The voltage difference (or simply voltage) between points A and B is written as 𝑉𝐴𝐵 where the second letter in the subscript identifies the reference point. If a single subscripted letter is shown, the voltage is defined between the lettered point and the circuit's common.
Voltage Measurement
From your kit, retrieve the resistors whose value is given in Table 3-1 below. Measure and record the values in Table 3-1. You should always use the measured value in experimental work.
Table 3-1
Component
Listed
Value
Measured
Value
R1
390 Ω
R2
560 Ω
R3
1.0 kΩ
Construct the circuit shown in Figure 3-2.
Figure 3-2
Set the power supply to 𝑉𝑆=+10 V.
Measure the voltage across each resistor in the circuit. Enter the measured values in Table 3-2.
Table 3-2
Measured
Voltage
VS
VAB
VBC
VCD
Assign point D as the reference ground. Measure the voltage at points A, B, and C with respect to point D. The voltage readings are made with the reference (common) connected to point D. Enter the measured values in Table 3-3. Then use the measured voltages to compute the voltage differences 𝑉𝐴𝐵, 𝑉𝐵𝐶 and 𝑉𝐶𝐷.
Table 3-3
Measured
Voltage
VA
VB
VC
VD
0.0 V (ref)
Now measure the voltages in the circuit with respect to point C. The circuit is not changed. Only the reference point changes. Move the reference point (common) of the voltmeter to point C. This point will now represent 0 V. The voltage at point D now has a negative value. Enter the measured voltages in Table 3-4. Compute the voltage differences as before and enter them in Table 3-4.
Table 3-4
Measured
Voltage
VA
VB
VC
0.0 V (ref)
VD
Move the circuit reference point to point B. Again, there is no change to the circuit other than the reference point (common). Repeat the measurements of the voltages with respect to common. Compute the voltage differences and enter the data in Table 3-5.
Table 3-5
Measured
Voltage
VA
VB
0.0 V (ref)
VC
VD
Now make point A the reference point and repeat the measurements. Enter the data in Table 3-6.
Table 3-6
Measured
Voltage
VA
0.0 V (ref)
VB
VC
VD
Voltage Difference Calculation
VAB = VA - VB = 
VBC = VB - VC = 
VCD = VC - VD = 
Voltage Difference Calculation
VAB = VA - VB = 
VBC = VB - VC = 
VCD = VC - VD = 
Voltage Difference Calculation
VAB = VA - VB = 
VBC = VB - VC = 
VCD = VC - VD = 
Voltage Difference Calculation
VAB = VA - VB = 
VBC = VB - VC = 
VCD = VC - VD = 