Constantan wire is used for making standard resistance, because it has
- Negligible temperature coefficient of resistance
- Low specific resistance
- High specific resistance
- High melting point
The Correct Option is A
Solution and Explanation
To understand why constantan wire is used for making standard resistances, we need to examine its key electrical properties.
1. Understanding Standard Resistance Requirements:
Standard resistors need to maintain a consistent resistance value over time and under varying conditions, particularly temperature changes.
2. Properties of Constantan:
Constantan (a copper-nickel alloy) has several important characteristics:
- Negligible temperature coefficient of resistance (α ≈ ±0.00002/°C)
- Moderate specific resistance (about 49 µΩ·cm)
- Good corrosion resistance
- Reasonable melting point (~1210°C)
3. Why Temperature Coefficient Matters Most:
For standard resistances, the most critical property is:
- Minimal change in resistance with temperature (low α)
This ensures the resistance value remains stable during use.
4. Why Other Options Are Less Important:
- Low specific resistance would require longer wires
- High specific resistance isn't necessary for standard resistors
- Melting point is secondary to resistance stability
5. Comparison with Other Materials:
Unlike copper (which has α ≈ 0.004/°C) or other alloys, constantan's near-zero temperature coefficient makes it ideal for precision applications.
Final Answer:
Constantan wire is used for making standard resistances because it has a negligible temperature coefficient of resistance.
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Concepts Used:
Current Electricity
Current electricity is defined as the flow of electrons from one section of the circuit to another.
Types of Current Electricity
There are two types of current electricity as follows:
Direct Current
The current electricity whose direction remains the same is known as direct current. Direct current is defined by the constant flow of electrons from a region of high electron density to a region of low electron density. DC is used in many household appliances and applications that involve a battery.
Alternating Current
The current electricity that is bidirectional and keeps changing the direction of the charge flow is known as alternating current. The bi-directionality is caused by a sinusoidally varying current and voltage that reverses directions, creating a periodic back-and-forth motion for the current. The electrical outlets at our homes and industries are supplied with alternating current.