Energy, Heat and Waves flashcards

$KE=\frac{1}{2}m{v}^2$ and $PE=mgh$, with m in kg, v in m/s, g about $9.8{\text{m/s}}^2$ and h in m; both in joules. Doubling speed makes KE four times larger; doubling height doubles PE.

$P=\frac{E}{t}$: power in watts (1 W = 1 J/s) equals energy in joules over time in seconds. Energy = power x time; 1 kWh = 3,600,000 J. A 1,500 W heater for 2 h uses 3 kWh.

Efficiency = useful output energy / total input energy x 100%. A pump using 5,000 J to give 3,500 J useful is 70% efficient. It is always below 100%; the rest becomes waste heat or sound, not destroyed.

$K={}^{\circ }C+273.15$ and ${}^{\circ }F=\frac{9}{5}{}^{\circ }C+32$. So 37 °C is 98.6 °F and 310.15 K. Kelvin has no degree sign and is never negative.

Conduction: direct contact, mostly in solids (hot spoon handle). Convection: bulk flow of a liquid or gas as warm fluid rises (boiling pot, sea breeze). Radiation: electromagnetic waves, crosses a vacuum (sunlight, campfire warmth).

$v=f\lambda$: speed (m/s) equals frequency (Hz) times wavelength (m). Period $T=\frac{1}{f}$. A 170 Hz sound at 340 m/s has a 2 m wavelength.

The echo travels out and back, so halve the round trip: $d=\frac{vt}{2}$. Sonar echo after 0.8 s in seawater at 1,500 m/s: 1,200 m round trip, so the floor is 600 m down.

Amplitude (displacement from rest) sets energy, loudness and brightness. Frequency (cycles per second) sets pitch and light color. Frequency stays the same when a wave enters a new medium; speed and wavelength change.