General Q&A for Class 12 · Page 207
5,215 free answers in General for Class 12 students, across CBSE, ICSE and State boards.
Question & Answer
Questions
- GeneralClass 12All
What is a safe level of voltage?
Safe voltage levels depend on conditions, but general guidelines are: Extra-low voltage (ELV): Less than 50V AC or 120V DC (generally considered safe for direct contact) Low voltage: 50-1000V AC or 120-1500V DC (reuires precautions) Specific contexts: Dry conditions: Up to 50V AC generally safe Wet conditions: Even 12-24V can be dangerous Residential<
- GeneralClass 12All
What is the resistance of a 20 cm long wire?
To calculate the resistance of a 20 cm long wire, you need additional information: Reuired parameters: Material (determines resistivity, ρ) Cross-sectional area or diameter Formula: R = ρL/A Example: A 20 cm (0.2 m) copper wire with 1 mm² cross-sectional area has: R = (1.68 × 10⁻⁸ × 0.2)/(1 × 10⁻⁶) = 0.00336 Ω or 3.36 mΩ
- GeneralClass 12All
How to test electrical resistivity?
To test electrical resistivity: For Solid Materials: Prepare a sample of known dimensions Use a four-point probe or multimeter to measure resistance Apply formula: ρ = RA/L For Liuids: Use a conductivity cell with known cell constant Measure conductivity Calculate resistivity as reciprocal: ρ = 1/σ For Soil/Ground:
- GeneralClass 12All
What is resistivity and resistor?
Resistivity is a material property that uantifies how much a material opposes current flow, measured in Ω·m. Resistor is an electrical component designed to provide a specific amount of resistance in a circuit, measured in ohms (Ω). Think of it this way: resistivity describes the material (like copper or carbon), while a resistor is a manufactured device made from resistive materials to control current in electronic circuits
- GeneralClass 12All
What is the resistivity of 1 ohm?
This uestion contains a misconception. Resistivity is not measured in ohms—it's measured in ohm-meters (Ω·m). One ohm (1 Ω) is a unit of resistance, not resistivity. However, if you're asking about a material with 1 ohm of resistance: A 1-meter cube of material with 1 Ω resistance has resistivity of 1 Ω·m The actual resistivity depends on the object's dimensions using ρ = RA/L
- GeneralClass 12All
What is resistivity also called?
Resistivity is also called: Specific resistance: Emphasizes it's a material-specific property Electrical resistivity: Distinguishes it from thermal resistivity Volume resistivity: Distinguishes it from surface resistivity In scientific literature, it's commonly represented by the Greek letter rho (ρ).
- GeneralClass 12All
What happens when resistivity is high?
When resistivity is high: Current flow decreases for a given voltage (poor conductivity) Heat generation increases if current is forced through the material Material acts as an insulator, preventing unwanted current flow Energy loss occurs if used as a conductor Applications include: electrical insulation, protective barriers, resistive heating elements</
- GeneralClass 12All
How to calculate resistivity from sheet resistance?
Sheet resistance (Rs) applies to thin films and is measured in ohms per suare (Ω/□). To calculate resistivity: ρ = Rs × t Where: ρ = resistivity (Ω·m) Rs = sheet resistance (Ω/□) t = thickness of the film (m) Example: If a thin film has sheet resistance of 100 Ω/□ and thickness of 50 nm (5 × 10⁻⁸ m), then ρ = 100 × 5 × 10⁻⁸ = 5
- GeneralClass 12All
What is the Ohm's law of resistivity?
While Ohm's Law (V = IR) describes the relationship between voltage, current, and resistance, it can be expressed in terms of resistivity as: E = ρJ Where: E = electric field (V/m) ρ = resistivity (Ω·m) J = current density (A/m²) This is the microscopic form of Ohm's Law, showing that electric field is proportional to current density, with resistivity as the proportionality constant.<
- GeneralClass 12All
What is the main formula for resistance?
The main formula for resistance is Ohm's Law: R = V/I Where: R = resistance in ohms (Ω) V = voltage in volts (V) I = current in amperes (A) Another fundamental formula relates resistance to physical properties: R = ρL/A, connecting resistance to resistivity, length, and cross-sectional area.
- GeneralClass 12All
How do you find the resistivity of a wire?
To find the resistivity of a wire: Step 1: Measure the wire's resistance (R) using a multimeter Step 2: Measure the wire's length (L) with a ruler or measuring tape Step 3: Measure the wire's diameter (d) using calipers Step 4: Calculate cross-sectional area: A = π(d/2)² Step 5: Calculate resistivity: ρ = RA/L Example: A copper wire with R = 0.5 Ω, L = 10 m,
- GeneralClass 12All
What is 4 pin soil resistivity test?
The 4-pin soil resistivity test (Wenner method) measures ground resistivity for electrical grounding systems: Procedure: Four electrodes are driven into the ground in a straight line at eual spacing (distance 'a') Current is injected through the outer two electrodes Voltage is measured between the inner two electrodes Resistivity is calculated using: ρ = 2πaR, where R = V/I This test is crucial for de
- GeneralClass 12All
Which instrument is used to measure resistivity?
Several instruments measure resistivity: Four-point probe resistivity meter: Industry standard for semiconductors and thin films Earth/Ground resistance tester: Measures soil resistivity Megohmmeter (Megger): Measures high-resistance materials Digital multimeter with four-wire resistance: For precise measurements LCR meter: Measures resistance, inductance
- GeneralClass 12All
How is resistivity measured?
Resistivity is measured using several methods: Four-Point Probe Method (most common): Four eually spaced probes contact the material Current passes through outer two probes Voltage is measured between inner two probes Eliminates contact resistance errors Two-Point Method: Measures resistance directly using an ohmmeter Calculates resistivity using ρ = RA/L <
- GeneralClass 12All
What is the resistivity of a solution?
The resistivity of a solution depends on the concentration and type of dissolved ions. Ionic solutions are generally good conductors with low resistivity: Concentrated salt solution: 0.05-0.1 Ω·m Dilute electrolytes: 1-100 Ω·m Very dilute solutions: approaching that of pure water The more ions present, the lower the resistivity and the better the conductivity. This p
- GeneralClass 12All
What is the maximum resistivity of water?
The maximum resistivity of water is approximately 1.8 × 10⁷ Ω·m, achieved only with ultra-pure, deionized water under laboratory conditions. In practice, perfectly pure water (18.2 MΩ·cm or 1.82 × 10⁶ Ω·m) is considered the practical maximum, as even the slightest contamination or dissolved gases will lower this value.
- GeneralClass 12All
What is the resistivity of water?
The resistivity of water varies dramatically based on purity: Pure (distilled) water: approximately 2.5 × 10⁵ Ω·m (high resistivity, poor conductor) Tap water: approximately 20-200 Ω·m Seawater: approximately 0.2 Ω·m (low resistivity due to dissolved salts) Pure water is actually a poor conductor. It's the dissolved ions and minerals that make ordi
- GeneralClass 12All
What is the resistivity of a conductor?
Conductors have very low resistivity values, typically in the range of 10⁻⁸ to 10⁻⁶ Ω·m. Common examples include: Silver: 1.59 × 10⁻⁸ Ω·m (best conductor) Copper: 1.68 × 10⁻⁸ Ω·m Gold: 2.44 × 10⁻⁸ Ω·m Aluminum: 2.82 × 10⁻⁸ Ω·m The lower the resistivity, the bett
- GeneralClass 12All
11. What is the resistivity of steel?
The resistivity of steel varies depending on its composition: Carbon steel: approximately 1.43 × 10⁻⁷ Ω·m Stainless steel: approximately 6.9 × 10⁻⁷ to 1.0 × 10⁻⁶ Ω·m Steel has higher resistivity than pure metals like copper (1.68 × 10⁻⁸ Ω·m) or aluminum (2.82 × 10⁻⁸ Ω·m), making it a less efficient conductor but still suitable for structu
- GeneralClass 12All
10. What is the resistivity of air?
The resistivity of air is approximately 1.3 × 10¹⁶ to 3.3 × 10¹⁶ Ω·m under normal conditions. This extremely high value makes air an excellent insulator. However, resistivity decreases significantly in humid conditions or when air becomes ionized (such as during lightning), allowing current to flow more easily.