Amplitude is the maximum displacement of a point on a wave from its equilibrium position, measured from the rest position to a crest or trough. In AP Physics 2 (Topics 6.2 and 6.3), amplitude determines the energy a wave carries, with intensity proportional to amplitude squared.
Amplitude is the maximum distance a point on a wave moves away from its equilibrium (rest) position. On a transverse wave, that's the height from the centerline up to a crest, or down to a trough. Important detail people get wrong: amplitude is measured from equilibrium to the peak, not from crest to trough. The crest-to-trough distance is twice the amplitude.
Why does AP Physics 2 care? Because amplitude is the wave property tied to energy. A wave with bigger amplitude carries more energy, and the relationship isn't linear. Energy and intensity scale with the square of the amplitude, so doubling the amplitude quadruples the energy delivered. This is true for mechanical waves like sound (where amplitude shows up as loudness) and for classical electromagnetic waves (where the amplitude of the oscillating electric and magnetic fields sets the wave's intensity). Crucially, amplitude is independent of frequency and wavelength. You can have a high-frequency, low-amplitude wave or a low-frequency, high-amplitude wave.
Amplitude lives in Unit 6, specifically Topic 6.2 (Electromagnetic Waves) and Topic 6.3 (Periodic Waves). It's one of the core descriptors of any periodic wave, alongside frequency, wavelength, and speed, and it's the one that controls energy. When you sketch or read a wave graph on the exam, amplitude is the vertical measurement; wavelength and period are the horizontal ones. Mixing those axes up is one of the most common wave-graph errors.
Amplitude also matters beyond Unit 6 because it's the star of a classic conceptual trap. Classically, more amplitude means more energy, so you'd expect brighter light to knock electrons out of a metal with more kinetic energy. The photoelectric effect says no. The energy of individual photons depends on frequency, not amplitude. Knowing exactly what amplitude does (and doesn't) control is how you avoid that trap in the modern physics unit.
Keep studying AP Physics 2 Unit 6
Intensity (Unit 6)
Intensity is energy delivered per area per time, and it scales with amplitude squared. Double a wave's amplitude and its intensity goes up by a factor of 4. If an exam question gives you an amplitude change and asks about energy or intensity, square the ratio.
Crest and Trough (Unit 6)
Amplitude is the distance from equilibrium to a crest (or to a trough). The full crest-to-trough distance is 2A, which is exactly the detail a multiple-choice distractor will test when you read a wave graph.
Loudness (Unit 6)
Loudness is what amplitude sounds like. For a sound wave, a bigger amplitude means larger pressure variations and a louder sound, while pitch is controlled by frequency. Same wave, two independent knobs.
Frequency (Units 6-7)
Amplitude and frequency are completely independent properties of a wave. This independence becomes the whole point in the photoelectric effect, where the maximum kinetic energy of ejected electrons depends on the light's frequency, not its amplitude or brightness.
Transverse wave (Unit 6)
On a transverse wave, particles oscillate perpendicular to the wave's direction of travel, and amplitude is the maximum size of that perpendicular displacement. EM waves work the same way, except it's the electric and magnetic fields oscillating instead of particles.
Multiple-choice questions love amplitude in two flavors. First, graph reading: you're shown a wave and asked for its amplitude, where the trap answer is the crest-to-trough distance (2A) instead of A. Second, scaling: 'if the amplitude doubles, what happens to the intensity?' The answer uses intensity ∝ A², so it quadruples. You should also be ready to state that changing amplitude does not change a wave's frequency, wavelength, or speed.
On the free-response side, amplitude shows up most powerfully as a foil in modern physics. The 2018 exam included an FRQ on the photoelectric effect, where light shines on a metal and the maximum kinetic energy of emitted electrons is analyzed as frequency varies. The classical prediction (bigger amplitude, more electron energy) fails, and explaining why frequency matters instead of amplitude is exactly the kind of paragraph-length reasoning those questions reward.
Amplitude is how far the wave displaces from equilibrium (the vertical size of the oscillation); frequency is how many oscillations happen per second. They're independent. For sound, amplitude controls loudness and frequency controls pitch. For light, amplitude (via intensity) controls brightness, while frequency controls color and, in quantum contexts, the energy of each photon. The exam tests this independence directly: in the photoelectric effect, cranking up amplitude ejects more electrons but does not increase any single electron's maximum kinetic energy. Only a higher frequency does that.
Amplitude is the maximum displacement from equilibrium, measured from the rest position to a crest or trough, not from crest to trough.
Wave energy and intensity scale with amplitude squared, so doubling the amplitude quadruples the intensity.
Amplitude is independent of frequency, wavelength, and wave speed; changing one does not change the others.
For sound, amplitude corresponds to loudness; for light, amplitude corresponds to brightness through intensity.
In the photoelectric effect, increasing the light's amplitude ejects more electrons but does not raise their maximum kinetic energy. Only increasing the frequency does that.
On a wave graph, amplitude is read off the vertical axis, while wavelength and period are read off the horizontal axis.
Amplitude is the maximum displacement of a point on a wave from its equilibrium position. It appears in Topics 6.2 and 6.3 and determines the energy a wave carries, with intensity proportional to the square of the amplitude.
No. Amplitude is measured from the equilibrium line to a crest or trough. The crest-to-trough distance equals twice the amplitude, and confusing the two is a classic wave-graph trap on the exam.
No. Amplitude and frequency are independent properties. You can make a wave bigger (more amplitude) without making it oscillate faster (frequency), which is why turning up the volume on a speaker doesn't change the pitch.
Amplitude is the maximum displacement of the wave itself, while intensity is the energy delivered per unit area per unit time. They're linked by intensity ∝ amplitude², so a 2x amplitude increase means a 4x intensity increase.
Not the way classical physics predicts. Increasing a light wave's amplitude (brightness) increases the number of electrons ejected but not their maximum kinetic energy, which depends only on the light's frequency. The 2018 free-response exam tested exactly this idea.