Lesson summary
7.1.4 Wave Intensity — AS PHYSICS 9702 notes: Define intensity and use its proportionality to amplitude squared, frequency squared and the inverse square law.Definition — the energy passing through a unit area per unit time, which is power per unit area.Equation — I = P/A.What th…
- What the symbols mean
- Comparing waves
- Spherical wave
- Distance doubled
- Rapid fall
- Wavelength
Section 1 of 7Intensity
Definition
- Definition the energy passing through a unit area per unit time, which is power per unit area.
Equation
- I = P/A.What the symbols mean: I = intensity (W m−2) · P = power (W) · A = area the power falls on (m2)
- I is intensity in W m−2.
- P is power in W.
- A is area in m2.
Area
- Area the area is perpendicular to the direction of the wave’s velocity.
Section 2 of 7Intensity, amplitude and frequency
Proportionality
- I is proportional to A2 and I is proportional to f2.
Doubling
- Doubling if the amplitude or frequency is doubled, the intensity increases by a factor of 4.
Comparing waves
- Comparing waves find the ratio of amplitudes squared and the ratio of frequencies squared, then multiply the factors.
Section 3 of 7Spherical waves
Spherical wave
- Spherical wave a wave from a point source that spreads out equally in all directions.
Area
- Area the area the wave passes through is the surface area of a sphere, 4πr2.
Spreading
- Spreading as the wave travels further, the energy passes through increasingly larger areas.
Section 4 of 7Inverse square law
Statement
- Statement intensity is proportional to 1/r2 if no wave energy is absorbed.
Distance doubled
- Distance doubled the intensity becomes 4 times smaller.
Rapid fall
- Rapid fall the relationship is inverse square, not linear, so energy decreases very rapidly with distance.
Section 5 of 7More from the specification
1
Wave intensity distribution in ripple tanks
- Wave intensity distribution in ripple tanks spreading wave pulses across shallow ripple tanks provides an experimental means to verify inverse square intensity.
2
Progressive Wave Parameters
- A progressive wave propagates energy through a medium;
- wave speed v is related to wavelength λ and oscillation frequency f by the fundamental wave equation v = f·λ,
- where frequency is inversely proportional to period (f = 1/T).
3
Wave Intensity and Amplitude
- Wave intensity I is defined as the power transmitted per unit area perpendicular to wave propagation (I = P/A);
- intensity is directly proportional to the square of the oscillation amplitude (I ∝ A²).
4
Cathode-Ray Oscilloscope Analysis
- Connecting a detector to a cathode-ray oscilloscope (CRO) displays particle displacement against time;
- by measuring the trace division spacing, one can determine the wave period, frequency,
- and phase difference between two signals.
5
Equilibrium
- State with zero net force.
6
Phase difference
- How far one oscillation is ahead of or behind another.
7
Progressive wave
- Wave that transfers energy without transferring matter.
8
Cathode-ray oscilloscope
- Instrument that displays voltage against time for electrical signals.
9
Equilibrium position
- Position where the net force is zero.
10
Longitudinal wave
- Wave whose vibrations are parallel to the direction of travel
11
Ripple tank
- Shallow tray of water used to show water waves
12
Wavelength
- Distance between a point on one wave and the same point on the next wave
13
Period
- Time for one revolution.
14
Oscilloscope
- Instrument that displays electrical signals against time
15
Displacement
- Change in position.
16
Vibration
- A repeated backwards and forwards movement.
17
Time-base
- Time represented by each division on the horizontal axis.
18
Oscillation
- Repeated back-and-forth motion.
19
Ripple
- The small fluctuation left in the smoothed output
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Intensity | Power per unit area perpendicular to the wave direction. |
| Spherical wave | Wave spreading equally in all directions from a point source. |
| Inverse square law | Intensity is proportional to 1/r². |
| Amplitude | Maximum displacement from equilibrium. |
| Power | Rate of energy transfer. |
Section 7 of 7Exam technique
1
Treating the intensity–distance relationship as linear — it is inverse square.
2
Forgetting that doubling amplitude quadruples intensity.
3
Using the wrong area — the area must be perpendicular to the wave velocity.
4
Combining amplitude and frequency effects by adding rather than multiplying the factors.