What is Voltage Measured In (Voltage Units)? SI Unit of Voltage
What is Voltage Measured In (Voltage Units)?
SI Unit of Voltage
The SI unit for voltage is volts. This is represented by a V. Volt is a derived SI unit of voltage. The Italian physicist Alessandro Volta (1745-1827), who invented the voltaic pile, which was the first electrical battery hence, the unit volt is named in honor of them.
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Volt in SI Base Units
Volt can be defined as the electric potential difference between two points in an electric circuit that dissipates one joule of energy per coulomb of charge that passes through the electric circuit. Mathematically, it can be expressed as,
\begin{align*} 1\,\,Volt = \frac{potential \ energy} {chrage} = \frac{1\,\, joule}{1\,\,coulomb} = \frac{kg\,\, m^2}{A\,\,s^3} \end{align*}
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Hence, volt can be expressed in terms of SI base units as \frac{kg\,\,m^2}{A\,\,s^3} or kg\,\,m^2\,\,s^-^3\,\,A^-^1.
It can also be measured in watts per ampere or ampere times ohms.
Voltage Formula
The basic formula for the voltage is shown in the below image.
voltage formula triangle
Voltage Formula Triangle
Voltage Formula 1 (Ohm’s Law)
According to Ohm’s law, voltage can be expressed as,
\begin{align*} Voltage = Current * Resistance \end{align*}
\begin{align*} V = I * R \end{align*}
Example 1
As shown in the below circuit a current of 4 A is flowing through the resistance of 15 Ω. Determine the voltage drop across the circuit.
example 1
Solution:
Given Data: I = 4\,\,A , R=15\,\,\Omega
According to Ohm’s law,
\begin{align*} & V = I * R \\ & = 4 * 15 \\ & V = 60\,\,Volts \end{align*}
Thus, by using the equation we get a voltage drop across the circuit of 60 volts.
Voltage Formula 2 (Power And Current)
The power transferred is the product of supply voltage and electric current.
\begin{align*} P = V * I \end{align*}
Now, put I=\frac{V}{R} in the above equation we get,
(1) \begin{equation*} P = V * I = \frac{V^2}{R} \end{equation*}
Thus, we get voltage equals the power divided by current. Mathematically,
\begin{align*} V = \frac{P}{I} \,\,Volts \end{align*}
Example 2
As shown in the below circuit a current of 2 A is flowing through a lamp of 48 W. Determine the supply voltage.
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example 2
Solution:
Given Data: I = 2\,\,A , P = 48 \,\,W
According to the formula between voltage, power, and current mentioned above,
\begin{align*} & V = \frac{P}{I} \\ & = \frac{48}{2} \\ & V = 24 \,\,Volts \end{align*}
Thus, by using the equation we get a supply voltage of 24 volts.
Voltage Formula 3 (Power And Resistance)
According to equation (1), Voltage is the square root of the product of power and resistance. Mathematically,
\begin{align*} V = \sqrt{P*R} \end{align*}
Example 3
As shown in the below circuit determine the necessary voltage to glow a 5 W lamp with a resistance of current of 2 Ω.
example 3
Solution:
Given Data: P = 5 \,\, W , R = 2 \,\, \Omega
According to the formula mentioned above,
\begin{align*} & V = \sqrt{P*R} \\ & = \sqrt{5*2} \\ & = \sqrt{10} \\ & V = 3.16 \,\,Volts \end{align*}
Thus, by using the equation we get the necessary voltage to glow 5 W, 2\Omega lamp 3.16 Volts.
Voltage Circuit Symbol (AC And DC)
AC Voltage Symbol
The symbol for AC (alternating current) voltage is shown below:
AC Voltage Symbol
AC Voltage Symbol
DC Voltage Symbol
The symbol for DC (direct current) voltage is shown below:
DC Voltage Symbol
DC Voltage Symbol
Dimensions of Voltage
Voltage (V) is a representation of the electric potential energy per unit charge.
The dimensions of voltage can be expressed in terms of mass (M), length (L), time (T), and ampere (A) as given by M L^2 T^-^3 A^-^1.
\begin{align*} V = \frac{W}{Q} = \frac{M L^2 T^-^2}{A T} = M L^2 T^-^3 A^-^1 \end{align*}
Note that some also use I in place of A to represent current. In this case, the dimension of voltage can instead be represented as M L^2 T^-^3 I^-^1.
How to Measure Voltage
In an electrical and electronic circuit, a voltage measurement is an essential parameter that needs to be measured. We can measure the voltage between a particular point and the ground or zero-volt line on a circuit.
In a 3-phase circuit, if we measure the voltage between any of one phase from the 3-phase and neutral point then it is known as line-to-ground voltage.
Similarly, if we measure the voltage between any two phases from 3-phase then it is known as line-to-line voltage.
There are various instruments used to measure the voltage. Let’s discuss each method.
Voltmeter Method
The voltage between two points in a system can be measured by using a voltmeter. To measure a voltage, a voltmeter must be connected in parallel with the component whose voltage is to be measured.
One lead of the voltmeter must be connected to the first point and one to the second point. Note that the voltmeter should never be connected in series.
Voltmeter can also be used to measure the voltage drop across any component or the sum of the voltage drop across two or more components within a circuit. The below image shows the voltmeter connection for the measurement of voltage across the resistor.
voltmeter connection for measurement of voltage across resistor
Voltage Connection for Measurement of Voltage Across Resistor
An analog voltmeter works by measuring the current through a fixed resistor. Now, according to Ohm’s law, the current through the resistor is directly proportional to the voltage or potential difference across the fixed resistor. Thus, we can determine the unknown voltage.
Another example of a voltmeter connection for the measurement of voltage across a 9 V battery is shown in the figure below:
voltmeter connection for measurement of battery voltage
Voltmeter Connection for Measurement of Battery Voltage
Multimeter Method
In present days, one of the most common methods to measure the voltage is by using a multimeter. The multimeter can be either analog or digital but digital multimeters are most commonly used because of higher accuracy and low cost.
The voltage or potential difference across any equipment can simply be measured by connecting probes of a multimeter across the two points where the voltage is to be measured. The measurement of battery voltage by using a multimeter is shown in the below image.
Multimeter for Voltage Measurement
Multimeter Connection for Measurement of Battery Voltage
Potentiometer Method
The potentiometer works on the principle of the null balance technique. It measures the voltage by comparison of an unknown voltage with the known reference voltage. The potentiometer circuit for the measurement of voltage is shown in the below figure.
potentiometer circuit for measurement of voltage
Potentiometer Circuit for Measurement of Unknown Voltage
Other instruments like an oscilloscope, an electrostatic voltmeter can also be used to measure the voltage.
