How to Use a Voltmeter in Physics Circuits (With Examples)
A voltmeter is an electrical instrument designed to measure the voltage, or electrical potential difference, between two points in an electric circuit. It plays a central role in analyzing both direct current (DC) and alternating current (AC) electric circuits. Voltmeters are available in analog and digital forms, with readings commonly given in volts (V), millivolts (mV), or kilovolts (kV). In modern practice, digital voltmeters are widely used due to their higher accuracy and direct numerical displays, while analog voltmeters involve a pointer moving over a scale.
Working Principle of a Voltmeter
The fundamental principle behind the voltmeter is that it is connected in parallel to the section of the circuit where the voltage is to be measured. This ensures that the instrument experiences the same potential difference as the component under test.
A voltmeter is constructed to have high resistance so that it draws minimal current from the circuit. This design reduces the effect of the measurement on the actual circuit conditions and avoids altering the circuit's current.
Types of Voltmeters
There are several types of voltmeters, each built for specific functions and accuracy levels:
- Analog Voltmeter: Uses mechanical movement (like a pointer and coil) to display voltage.
- Digital Voltmeter: Shows direct numeric values on a digital display.
- Electrostatic Voltmeter: Utilizes electrostatic forces and can directly measure voltage without relying on circuit current.
- Electronic Voltmeter: Employs amplification or rectification and can measure very low or high voltages with minimal circuit loading.
- Multimeter (Volt-Ohm-Milliammeter): Measures voltage, current, and resistance.
Voltmeter Connection and Usage
For accurate results, a voltmeter must always be connected in parallel to the component or section of the circuit under observation. If the voltmeter is incorrectly connected in series, its high resistance can block the current flow, significantly affecting circuit performance and leading to incorrect readings.
Because voltmeters are marked by high resistance, they minimize current draw and circuit disturbance. This feature is crucial for maintaining circuit integrity during measurements.
Key Formulas and Calculations
The most basic relationship used in voltmeter measurements is Ohm’s Law, which links the voltage, current, and resistance in a circuit:
Where V is the voltage (in volts), I is the current (in amperes), and R is the resistance (in ohms).
| Parameter | Formula | Unit |
|---|---|---|
| Potential Difference | V = I × R | Volt (V) |
| Resistance of Ideal Voltmeter | Approaches Infinity | Ohm (Ω) |
Step-by-Step: How to Use a Voltmeter
- Ensure the circuit is powered off before making any connections.
- Connect the voltmeter’s positive terminal to the higher potential point and the negative terminal to the lower potential point, across the component to be measured.
- Switch on the circuit and observe the voltage reading on the scale or digital display.
- After measurement, power off the circuit and disconnect the voltmeter.
Comparison: Voltmeter, Ammeter, and Multimeter
| Instrument | Measured Quantity | Connection in Circuit | Internal Resistance | Common Unit |
|---|---|---|---|---|
| Voltmeter | Voltage (Potential Difference) | Parallel | High | Volt (V) |
| Ammeter | Current | Series | Low | Ampere (A) |
| Multimeter | Voltage, Current, Resistance | Series/Parallel | Variable | V, A, Ω |
Solved Example
Suppose a voltmeter is connected across a resistor of 5 Ω, and the current through the resistor is 0.4 A.
Applying Ohm’s Law:
Therefore, the voltmeter reads 2 volts across the resistor.
Practice Questions
- Why must a voltmeter have high resistance? What happens if it is connected in series?
- If a voltmeter measures 15 V across a 3 Ω resistor, what is the current through the resistor?
- List two differences between a voltmeter and a multimeter.
Explore More Resources
A voltmeter is an essential instrument for everyone studying or working with electricity. Mastering its working, types, and proper usage builds a strong foundation in the analysis and troubleshooting of electrical and electronic circuits.
FAQs on Voltmeter Explained: Working, Types, and Important Formulas
1. What is a voltmeter and what does it measure?
A voltmeter is an instrument used to measure the voltage (potential difference) between two points in an electric circuit. It accurately shows how much electrical energy exists between those points.
Key points:
• Measures in volts (V)
• Connects across (in parallel with) components
• Helps check the health of circuits and devices
• Used for both DC and AC voltage measurement, depending on type
2. How does a voltmeter work in a circuit?
A voltmeter measures the potential difference by comparing the energy level between two points in a circuit.
Working:
• Connected in parallel with the component
• Draws minimal current due to high resistance
• Reads the voltage directly without disturbing circuit flow
• Digital voltmeters use internal electronic circuits for higher accuracy
3. Why must a voltmeter have high resistance?
A voltmeter must have high resistance to avoid altering the circuit’s current.
Details:
• High resistance ensures almost no current flows through the voltmeter
• Prevents circuit loading, so voltage measured is accurate
• Low resistance would divert current, causing error in voltage reading
4. What is the symbol of voltmeter in a circuit diagram?
The voltmeter symbol is a circle with the letter ‘V’ inside, connected across circuit points.
5. How is a voltmeter different from an ammeter?
Voltmeter and ammeter differ in what they measure and how they are connected in circuits.
Comparison:
• Voltmeter: Measures voltage, connected in parallel, high resistance
• Ammeter: Measures current, connected in series, low resistance
6. What is the basic formula related to a voltmeter?
The key formula is Ohm’s Law: V = IR.
• V = Voltage (Volts)
• I = Current (Amperes)
• R = Resistance (Ohms)
This formula helps calculate voltage when current and resistance are known.
7. Can a voltmeter measure both AC and DC voltages?
Yes, special types of voltmeters are designed for either AC, DC, or both.
• DC voltmeter: Measures direct current voltage
• AC voltmeter: Measures alternating current voltage
• Multimeters and electronic voltmeters often have settings for both AC and DC
8. How do you correctly connect a voltmeter in a circuit?
A voltmeter must always be connected in parallel across the component whose voltage you want to measure.
Steps:
1. Power off the circuit
2. Attach the positive lead to the higher potential point and the negative lead to the lower
3. Ensure both probes connect across (not along) the component
4. Power on and take the reading
9. What happens if a voltmeter is connected in series by mistake?
If a voltmeter is connected in series, it can block the current due to its high resistance, making the circuit non-functional and the reading inaccurate or zero. Always connect a voltmeter in parallel for correct measurement.
10. What are the main types of voltmeters?
There are several types of voltmeters based on technology and use:
• Analog voltmeter: Uses a needle and scale
• Digital voltmeter: Gives numeric digital display
• Electrostatic voltmeter: Measures high voltages directly
• Electronic voltmeter: Uses amplification for AC/DC
• Multimeter: Combines voltmeter, ammeter, and ohmmeter functions
11. What is a digital voltmeter and how is it different from an analog voltmeter?
A digital voltmeter displays voltage readings as numbers on a screen, offering greater accuracy than analog types.
• Analog voltmeter: Pointer moves on a scale
• Digital voltmeter: Numeric display, easy to read
• Digital versions are less prone to human error and usually more precise for modern labs
12. What safety tips should you follow when using a voltmeter?
Follow these safety tips for accurate and safe measurement:
• Turn off circuit before connecting or disconnecting the voltmeter
• Set the correct voltage range to avoid instrument damage
• Ensure hands are dry and leads are not damaged
• Connect in parallel, not in series
• Check polarity for DC voltmeters
