Current Electricity

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.

We need to explain why the Wheatstone bridge method is considered better than other methods for measuring resistances.

1. Understanding the Wheatstone Bridge:
The Wheatstone bridge is a circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit. It consists of four resistors, a voltage source, and a galvanometer. The unknown resistance is determined when the bridge is balanced (no current flows through the galvanometer).

2. High Accuracy and Precision:
The Wheatstone bridge offers high accuracy because it operates on a null method, meaning it measures the unknown resistance by achieving a balance condition where no current flows through the galvanometer. This eliminates errors due to the galvanometer's sensitivity or calibration, making the measurement independent of the voltage source's exact value.

3. Comparison with Other Methods:
- Voltmeter-Ammeter Method: This method measures voltage and current across the resistor to calculate resistance using Ohm’s law ( ). However, the voltmeter and ammeter have internal resistances, introducing errors, especially for very low or very high resistances.
- Ohmmeter Method: An ohmmeter directly measures resistance but is less accurate for precise measurements due to calibration issues and the influence of the meter’s internal resistance.
The Wheatstone bridge avoids these issues by not relying on direct current or voltage measurements across the unknown resistor.

4. Insensitivity to External Factors:
The Wheatstone bridge is less affected by variations in the power supply voltage or temperature changes, as the balance condition depends only on the ratio of resistances in the bridge arms, not their absolute values.

5. Wide Range and Versatility:
The Wheatstone bridge can measure a wide range of resistances (from very low to very high) with high precision by adjusting the known resistors in the bridge, making it more versatile than methods like the voltmeter-ammeter method, which struggle with extreme resistance values.

Final Answer:
The Wheatstone bridge is better than other methods of measuring resistances because it provides higher accuracy through its null measurement technique, is less affected by external factors like power supply variations, and can measure a wide range of resistances with precision.

We need to identify the term for the current assumed to flow in a circuit from the positive terminal to the negative terminal.

1. Understanding Electric Current:
Electric current is the flow of electric charge in a circuit. Historically, before the discovery of electrons, it was assumed that positive charges moved from the positive terminal to the negative terminal of a power source (like a battery).

2. Defining the Term:
This assumed direction of current flow, from the positive terminal to the negative terminal, is known as conventional current. In reality, electrons (which are negatively charged) move in the opposite direction, but circuit analysis uses the conventional current direction for consistency.

Final Answer:
The current assumed to be flowing in a circuit from the positive terminal to the negative is called conventional current.