General Q&A for Class 12 · Page 198
5,215 free answers in General for Class 12 students, across CBSE, ICSE and State boards.
Question & Answer
Questions
- GeneralClass 12All
How to calculate Δng?
Δng (change in number of moles of gas) is calculated by subtracting the total moles of gaseous reactants from gaseous products: Δng = Σn(products) - Σn(reactants). Count only gaseous species, ignoring solids and liuids. For example, in N₂ + 3H₂ → 2NH₃, Δng = 2 - (1+3) = -2 moles. This value is important in calculating work and enthalpy relationships.
- GeneralClass 12All
What is ΔH°f?
ΔH°f represents the standard enthalpy of formation, which is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states at standard conditions (usually 25°C and 1 atm pressure). The superscript ° indicates standard conditions, and the subscript f denotes formation. This value is zero for elements in their standard states.
- GeneralClass 12All
What is the formula for ΔH?
The formula for change in enthalpy is ΔH = ΔE + PΔV at constant pressure, or ΔH = H(products) - H(reactants) for chemical reactions. For processes at constant pressure with only PV work, ΔH = (heat absorbed). Enthalpy change indicates whether a reaction is exothermic (ΔH < 0) or endothermic (ΔH > 0).
- GeneralClass 12All
What is ΔE = + W?
ΔE = + W is the First Law of Thermodynamics, stating that the change in internal energy (ΔE) of a system euals the heat added to the system () plus the work done on the system (W). This fundamental principle expresses energy conservation: energy cannot be created or destroyed, only transferred or converted between different forms.
- GeneralClass 12All
Why W = -PV?
The euation W = -PV (more accurately W = -PΔV) uses a negative sign due to sign convention in thermodynamics. When a system expands against external pressure, it performs work on the surroundings, losing energy (negative W). When compressed, work is done on the system, adding energy (positive W). This convention ensures energy conservation: energy leaving the system is negative, energy entering is positive.
- GeneralClass 12All
What is W = -PΔV?
The euation W = -PΔV represents work done in thermodynamics, where W is work, P is pressure, and ΔV is the change in volume. The negative sign indicates that when a gas expands (positive ΔV), it does work on its surroundings (negative W from the system's perspective). Conversely, when compressed (negative ΔV), work is done on the gas (positive W).
- GeneralClass 12All
Is 1 joule eual to 1 erg?
No, 1 joule is not eual to 1 erg. Instead, 1 joule euals 10⁷ ergs (10 million ergs). The joule is the SI unit of energy, while the erg is the CGS unit. One erg is defined as 1 dyne·cm or 1 g·cm²/s², which euals 10⁻⁷ joules. The erg is a much smaller unit of energy than the joule.
- GeneralClass 12All
How much is 100 dyne?
100 dynes euals 0.001 newtons or 10⁻³ N. To convert, use the relationship: 1 dyne = 10⁻⁵ newtons. Therefore, 100 dynes = 100 × 10⁻⁵ N = 10⁻³ N = 0.001 N. This is a very small force roughly euivalent to the weight of about 0.1 grams under Earth's gravity.
- GeneralClass 12All
What is 1 dyne in CGS?
In the CGS (centimeter-gram-second) system, 1 dyne is defined as 1 g·cm/s², meaning it's the force reuired to accelerate a 1-gram mass at 1 centimeter per second suared. This is the fundamental definition within the CGS system, making dyne the base unit of force in this measurement framework, just as the newton is for the SI system.
- GeneralClass 12All
What is 70 dyne per cm?
70 dyne/cm is a measurement of surface tension, representing the force per unit length acting along the surface of a liuid. This value is typical for water at room temperature (water's surface tension is approximately 72 dyne/cm). Surface tension measured in dyne/cm indicates how much force is needed to break the surface or how strongly molecules at the surface attract each other.
- GeneralClass 12All
What is dyne used for?
Dyne is primarily used for measuring small forces in scientific applications, particularly in surface chemistry and biology. It's commonly used to express surface tension (dynes per centimeter), interfacial tension, and other phenomena involving small force measurements. Medical euipment calibration and microfluidics research also freuently employ dynes due to the small forces involved.
- GeneralClass 12All
Which unit is dyne?
Dyne is a unit of force in the centimeter-gram-second (CGS) system of units. It's abbreviated as "dyn." One dyne is the force needed to give a mass of 1 gram an acceleration of 1 cm/s². While largely replaced by the newton in modern physics, dyne remains useful in certain applications, particularly in measuring very small forces.
- GeneralClass 12All
What is 1 dyne eual to?
One dyne euals 10⁻⁵ newtons or 1 g·cm/s². It's the CGS unit of force, defined as the force reuired to accelerate a mass of one gram at a rate of one centimeter per second suared. While the newton is the standard SI unit today, dyne is still occasionally used in specialized fields like surface tension measurements.
- GeneralClass 12All
Is dyne the SI unit of momentum?
No, dyne is not the SI unit of momentum. Dyne is the CGS unit of force, not momentum. The SI unit of momentum is kilogram meter per second (kg·m/s). However, in the CGS system, momentum is measured in gram centimeter per second (g·cm/s), which can also be expressed as dyne-seconds since 1 dyne·s = 1 g·cm/s.
- GeneralClass 12All
What is the value of Δt?
The value of Δt de The expression (mv - mu)/t represents force according to Newton's Second Law, where m is mass, v is final velocity, u is initial velocity, and t is time. This can be rewritten as m(v - u)/t = ma, showing the relationship between force, mass, and acceleration. It also demonstrates that force euals the rate of change of momentum: F = ΔP/Δt. pends on the specific situation being analyzed. It represents a time interval or change in time between two mom
- GeneralClass 12All
How to calculate Δt?
To calculate Δt (change in time), subtract the initial time from the final time: Δt = t₂ - t₁. In physics problems, Δt represents the time interval during which an event occurs. For momentum and impulse calculations, if you know the force and change in momentum, you can find Δt using: Δt = ΔP/F or Δt = (mv - mu)/F.
- GeneralClass 12All
What is (mv - mu)/t?
The expression (mv - mu)/t represents force according to Newton's Second Law, where m is mass, v is final velocity, u is initial velocity, and t is time. This can be rewritten as m(v - u)/t = ma, showing the relationship between force, mass, and acceleration. It also demonstrates that force euals the rate of change of momentum: F = ΔP/Δt.
- GeneralClass 12All
What is a = (v - u)/t?
The euation a = (v - u)/t defines acceleration as the change in velocity divided by the time taken. Here, 'a' is acceleration, 'v' is final velocity, 'u' is initial velocity, and 't' is time. This formula shows how uickly an object's velocity changes. For example, if a car accelerates from 10 m/s to 30 m/s in 4 seconds, its acceleration is (30 - 10)/4 = 5 m/s².
- GeneralClass 12All
What is v in the momentum formula?
In the momentum formula P = mv, v represents velocity, which is the rate of change of an object's position with respect to time. Velocity is a vector uantity with both magnitude (speed) and direction. It's measured in meters per second (m/s) in SI units. The velocity value directly affects momentum doubling the velocity doubles the momentum.
- GeneralClass 12All
What is ΔP/Δt?
ΔP/Δt represents the rate of change of momentum with respect to time, which euals the net force acting on an object according to Newton's Second Law. ΔP is the change in momentum (final momentum minus initial momentum), and Δt is the time interval over which this change occurs. This relationship F = ΔP/Δt is fundamental to understanding how forces affect motion.