SI Unit, Definition & Calculation of Voltage with Practical Examples
Voltage is one of the core concepts in electricity and circuits. It represents the electric potential difference between two points in a circuit. Voltage is what pushes electric charge through a conductor, enabling current to flow and allowing devices to operate. Understanding the unit and nature of voltage is vital for students studying Physics, as it lays the foundation for related topics like current, resistance, and electrical energy.
What is the Unit of Voltage?
The standard unit for measuring voltage is the volt, represented by the symbol V. A volt is a derived SI unit used for electric potential, potential difference, and electromotive force.
1 volt is defined as the potential difference between two points when one joule of work is required to move one coulomb of charge from one point to another.
Key Definitions and Formulas
Voltage can be understood and calculated using several closely linked formulas, depending on the context:
- Definition by Work and Charge:
V = W / Q
(V = voltage in volts, W = work done or energy in joules, Q = charge in coulombs) - Ohm's Law:
V = I × R
(I = current in amperes, R = resistance in ohms) - Electric Field Relation:
V = E × L
(E = electric field strength, L = length or distance between points)
Voltage can also be represented as ampere times ohm, or watt per ampere, showing voltage's role in relating current, resistance, and power.
Units and Symbols Table
| Electrical Parameter | SI Unit | Symbol |
|---|---|---|
| Voltage | Volt | V or E |
| Current | Ampere | A |
| Resistance | Ohm | Ω |
| Power | Watt | W |
| Capacitance | Farad | C |
Voltage in Terms of Fundamental SI Units
Since a volt is defined as a joule of energy per coulomb of charge, and a joule is kg·m²/s², the volt is expressed in fundamental SI units as:
- 1 V = 1 J / 1 C = 1 (kg·m²·s-3·A-1)
This expression shows the relationships among mass, length, time, and electric current in voltage.
Voltage Source and Potential Difference
A voltage source is any device that provides a fixed potential difference between its two terminals. Batteries are common voltage sources used in circuits. When connected properly, current flows through the circuit components like resistors and voltmeters, driven by the voltage source.
The actual voltage available at the terminals of a real battery (called terminal voltage) may be less than its electromotive force (emf), because of internal resistance. The formula reflecting this is:
- V = ε - Ir
(where V = terminal voltage, ε = emf, I = current, r = internal resistance)
How to Calculate Voltage: Step-by-Step
| Step | Action |
|---|---|
| 1 | Identify what is given (current and resistance, or work and charge). |
| 2 | Select the suitable formula (for example, V = I × R or V = W / Q). |
| 3 | Check all values are in correct SI units (A, Ω, J, or C). |
| 4 | Calculate and write your answer with the unit V. |
Illustrative Examples
| Scenario | Calculation | Answer |
|---|---|---|
| A 10Ω resistor with 2A current flows | V = I × R = 2 × 10 | 20 V |
| 5 J work moves 1 C charge | V = W / Q = 5 / 1 | 5 V |
Common Misconceptions
It is important to note that voltage does not "flow" through a circuit. Only current (the flow of charge) moves through the circuit. Voltage is the "push" or potential difference that causes current to flow, similar to water pressure in pipes.
Key Formulas for Voltage
| Formula | Description | Units |
|---|---|---|
| V = W / Q | Voltage as work per unit charge | Joule (J), Coulomb (C), Volt (V) |
| V = I × R | Ohm’s Law | Ampere (A), Ohm (Ω), Volt (V) |
| V = E × L | Potential difference in uniform electric field | V/m, m, V |
| V = ε - Ir | Terminal voltage of real battery | Volt (V), Ampere (A), Ohm (Ω) |
Why Units Matter in Physics
Always write the correct units while solving Physics problems. Using volts (V) for potential difference and amperes (A) for current helps you avoid calculation errors and obtain full marks in exams.
Related Vedantu Resources for Further Learning
- Working of Voltmeter
- Difference Between Voltage and Current
- Difference Between EMF and Voltage
- Unit of Electricity
Summary and Next Steps
- Voltage is always measured as the potential difference between two points in an electric circuit.
- The SI unit of voltage is volt (V), defined as 1 joule per coulomb.
- Practice calculations by using Ohm’s Law and the definition of voltage in real circuit problems.
- Explore related concepts such as electromotive force and unit of electric field for deeper understanding.
FAQs on What is the Unit of Voltage in Physics?
1. What is the SI unit of voltage, and how is 1 volt defined?
The SI unit of voltage is the volt (V). One volt is defined as the potential difference between two points when one joule of energy is used to move one coulomb of charge between them. In summary:
• 1 volt (V) = 1 joule (J) per coulomb (C), or 1 V = 1 J/C.
• This means if 1 J of work moves 1 C of charge between two points, the potential difference is 1 V.
2. How are voltage, current, and resistance related in a simple circuit?
Voltage (V), current (I), and resistance (R) are related by Ohm’s Law:
• Formula: V = I × R
• Voltage is the product of electric current and resistance.
• This relationship helps calculate any one quantity if the other two are known.
3. What are the standard SI units for voltage, current, and resistance?
The standard SI units used in Physics for these electrical quantities are:
• Voltage: Volt (V)
• Current: Ampere (A)
• Resistance: Ohm (Ω)
4. What is the difference between electromotive force (emf) and terminal voltage?
Electromotive force (emf) is the maximum potential difference a power source can provide when no current flows. Terminal voltage is the actual voltage available to the external circuit when current is flowing.
• EMF: Maximum possible voltage, measured with open circuit (no current).
• Terminal Voltage: Slightly less than emf due to energy lost in the internal resistance when the circuit is closed.
5. Why is voltage always measured between two points?
Voltage is always a measure of potential difference between two points.
• It compares the electrical potential energy of a unit charge at two different positions.
• Like measuring height with reference to the ground, voltage requires two reference points.
• A voltmeter must be connected across two points to measure voltage.
6. What is a common misconception about the relationship between voltage and current?
A common misconception is that voltage 'flows' through a circuit.
• In reality, current (movement of charge) flows; voltage is the force or push causing this flow.
• Think of voltage as water pressure and current as the water flow inside a pipe.
• Correct: Voltage is the cause, current is the effect.
7. How is voltage related to the electric field?
In a uniform electric field (E), the voltage (V) or potential difference between two points is calculated as:
• Formula: V = E × d
• V is voltage, E is electric field strength, and d is the distance between points.
• This shows a stronger electric field produces a larger voltage across the same distance.
8. How is the unit 'volt' expressed in terms of fundamental SI units?
The volt (V) is a derived SI unit that can be broken down into fundamental units:
• Volt = Joule per Coulomb (V = J/C)
• 1 Joule = 1 kg·m²/s² and 1 Coulomb = 1 A·s
• So, 1 Volt = 1 kg·m²·s−3·A−1
• This expresses voltage using mass (kg), length (m), time (s), and current (A).
9. Can voltage exist at a single point?
No, voltage cannot exist at a single point. It is always defined between two points as a potential difference.
• Like height is measured relative to sea level, voltage needs two reference locations.
• This is why we say 'potential difference' and use voltmeters across two terminals.
10. What are some real-world sources of voltage in an electrical circuit?
Common sources of voltage (potential difference) in circuits include:
• Batteries (chemical cells)
• Generators or alternators
• Power supplies and adapters
• Solar cells (photovoltaic)
• These devices maintain a fixed (or variable) potential difference to drive current through circuits.
11. What is the CGS unit of voltage, and how does it relate to the SI unit?
The CGS unit of voltage is the statvolt.
• 1 statvolt = 299.79 volts (V) in the SI system.
• The statvolt is rarely used in modern Physics as SI units are internationally accepted.
12. Why is understanding correct units important in Physics exams?
Using correct SI units is essential for full marks in Physics calculations and answers.
• Marks are deducted for missing or incorrect units.
• Units help confirm calculations and avoid confusion between similar concepts.
• Regular practice of unit conversions and consistent notation strengthens problem-solving skills for competitive exams.
