IBDP Physics 2025 SL&HL: 3.3 Power and efficiency Study Notes

Learning Objectives

Students should understand:
• the principle of the conservation of energy
• that work done by a force is equivalent to a transfer of energy
• that energy transfers can be represented on a Sankey diagram
• that work W done on a body by a constant force depends on the component of the force along the line of displacement s is  given by W = Fs cos θ
• that work done by the resultant force on a system is equal to the change in the energy of the system
• that mechanical energy is the sum of kinetic energy, gravitational potential energy and elastic potential energy
• that in the absence of frictional, resistive forces, the total mechanical energy of a system is conserved
• that if mechanical energy is conserved, work is the amount of energy transformed between different forms of mechanical energy in a system, such as:
◦  the kinetic energy of translational motion as given by \(E_k=\frac{1}{2}mv^2 =\frac{p^2}{2m}\)
◦ the gravitational potential energy, when close to the surface of the Earth as given by ΔE_p = mgΔh
◦ the elastic potential energy as given by \(E_H=\frac{1}{2}k(\Delta x)^2\)
• that power developed P is the rate of work done, or the rate of energy transfer, as given by \(p=\frac{\Delta W}{\Delta t}=Fv\)
• efficiency η in terms of energy transfer or power as given by \(\eta=\frac{E_{output}}{E_{input}}=\frac{P_{output}}{P_{input}}\)
• energy density of the fuel sources.