Energy, Waves, Light and Sound
Energy and efficiency
Energy is conserved: it is transferred between stores but not created or destroyed. Work done = force × distance, and power = energy transferred ÷ time. Efficiency = useful energy output ÷ total energy input × 100%. Heat transfers by conduction (solids), convection (fluids) and radiation (infrared, no medium needed).
Waves
Wave speed = frequency × wavelength. Transverse waves vibrate at right angles to travel, such as light; longitudinal waves vibrate along the direction of travel, such as sound. Waves reflect, refract and diffract. Refraction occurs because speed changes at a boundary; light bending into a denser medium slows down and bends towards the normal.
Light and sound
Ray diagrams show reflection and refraction. A convex lens converges light and can form real or virtual images. Sound needs a medium, travels fastest in solids, and is reflected as echoes. Pitch depends on frequency and loudness on amplitude.
Worked example
A wave has frequency 50 Hz and wavelength 4 m. Wave speed = 50 × 4 = 200 m/s. If a machine takes 500 J of energy and transfers 200 J usefully, efficiency = 200 ÷ 500 × 100 = 40%.
Traps that cost marks
Frequency and amplitude are not the same: frequency affects pitch, amplitude affects loudness. Real images can be projected on a screen; virtual images cannot. Refraction is caused by a change in speed, not by a change in frequency. Radiation does not need particles.