Lesson 1 of 6
Propagation and particle motion
I can distinguish oscillating particles in a material medium from oscillating electromagnetic fields in space and time.Syllabus
Syllabus 9478, 10(a). Distinguish oscillating particles in a material medium from oscillating electromagnetic fields in space and time.
I can explain energy transfer by a progressive wave without net transport of the medium.Syllabus
Syllabus 9478, 10(f). Explain energy transfer by a progressive wave without net transport of the medium.
Make a guess. You are not marked.
A transverse wave travels to the right along a string. One point of the string is at a crest at this instant. What is that point's velocity?
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Zero, because a crest is a point of maximum displacement.
Each point oscillates up and down. At the top of its oscillation it momentarily stops before moving back down, so its velocity is zero at the crest.
A progressive wave carries a disturbance through space. Identify what oscillates locally and distinguish that oscillation from the direction in which the pattern and energy travel.
What oscillates?
A mechanical wave involves particles of a material medium, such as a string or a fluid. The particles oscillate about equilibrium positions. Neighbouring parts exert forces on one another; as they move, work transfers energy through the medium.
An electromagnetic wave involves oscillating electric and magnetic fields in space and time. It can propagate through a vacuum, without a material medium. A curve representing a field does not show particles following that curve.
Classify the wave by comparing oscillation with propagation:
- Transverse: the oscillation is perpendicular to propagation, as for the displacement of an ideal transverse string wave.
- Longitudinal: the particle oscillation is parallel to propagation, as for sound in a fluid.
A sinusoidal graph alone does not tell you which type it represents. First read which quantity is plotted and how its physical direction relates to propagation.
Track a particle separately from a crest
In the ideal progressive mechanical-wave model, particles oscillate locally while energy is transferred through the medium. The medium does not advance with each crest. Other bulk flows can occur in real fluids or surface waves; they are not the oscillatory particle motion represented by this model.
Consider a transverse wave travelling right, with amplitude 4.0 mm, wavelength 0.80 m and period 0.20 s. A crest is the position of maximum positive displacement at an instant, not the identity of one particle.
The pattern moves right; the tagged particle stays at x = 0.10 m
The filled circle marks the same particle in all three frames. The hollow diamond tracks the same crest. The small downward arrow gives the tagged particle's instantaneous direction of motion.
t = 0.000 s; tagged displacement +2.83 mm
t = 0.025 s; tagged displacement 0.00 mm
t = 0.050 s; tagged displacement -2.83 mm
The horizontal and vertical display scales differ. Across these 0.050 s, the crest moves 0.20 m right; the tag moves transversely through equilibrium. A crest is a travelling pattern, not a particle carried along the string.
At times 0, 0.025 and 0.050 s, the tagged particle at equilibrium position x = 0.10 m has displacements about +2.83, 0 and -2.83 mm. It moves down in all three shown states. Meanwhile, the corresponding crest advances from x = 0.20 to 0.30 to 0.40 m.
Positive displacement means above equilibrium, not necessarily upward velocity. At the first instant, the particle is above equilibrium but moving down. Comparing the successive frames reveals its motion.
Check your understanding
A transverse-wave profile moves right. At the marked particle, the profile rises as you look from left to right, and the particle is above equilibrium. What is happening there in the ideal string-wave model?
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The particle moves downward while the pattern and energy propagate right.
At a fixed horizontal position, a right-moving rising profile is replaced by the smaller displacement that was just to its left. The tagged particle moves down, while propagation remains rightward.
Electromagnetic fields and propagation
In the plane electromagnetic-wave model, the electric and magnetic fields oscillate perpendicular to one another and to propagation. The electric-field direction is the direction used to describe the wave's polarisation.
Electromagnetic waves involve fields, not a material particle path
The arrows show directions at one instant. The electric and magnetic fields oscillate; an electromagnetic wave can propagate without a material medium. The circled dot here denotes magnetic field out of the page.
The fields reverse during the cycle, while the progressive wave continues in its propagation direction. These field arrows describe local oscillating quantities; they are not paths followed by matter through the diagram.