Unit 5: Work, Energy and Power
Cards (12)
- Principle of conservation of energy:
- Energy can neither be created nor destroyed, only transferred into alternative forms of energy
- Total energy in a closed system will always remain the same
- Definition of work done:
- Work done is equal to the energy transferred usefully
- It is the product of the force applied and the displacement moved in the direction of the force
- Definition of kinetic energy:
- The energy associated with the motion of an object with mass
- SI base unit is J and its SI base unit is kgm^2s^-2
- Formula: Ek = 1/2 mv^2
- Definition of gravitational potential energy:
- The energy stored by an object at a point in a gravitational field
- Formula: GPE = mgΔh
- Definition of elastic potential energy:
- The energy stored by an object as a result of a reversible change in an object’s shape
- Formula: EPE = 1/2kx^2 where k is spring constant N/m and x is the extension in m
- Formula for the velocity of an object in a gravitational field:
- All initial GPE is converted to KE as the object falls, and this KE is converted back to GPE as it rises
- Equation: mgh = 1/2 mv^2
- Rearranging gives v^2 = 2gh, so v = 2gh^(1/2)
- The velocity of the object is independent of mass
- Rate of work done is equal to:
- Power
- Definition of efficiency:
- Efficiency (%) = (The useful output energy / total input energy) x 100 = (The useful output power / total input power) x 100
- How to maximize efficiency:
- By minimizing energy losses into non-useful forms
- Reduce friction, air resistance, noise generation, heat production, etc., in a system designed to do work
- Calculation of work done by an expanding gas:
- Work done = pΔV where p is pressure (constant) and ΔV is the change in the gas’s volume
- Increasing efficiency of a system losing energy due to friction:
- Lubricate parts of the system and reduce the number of parts that touch
- Derivation of power as the product of force and velocity:
- Power = force x displacement / time
- Power = force x velocity