Amplitude (Grade 10 NSC Matric Physical Sciences): Revision Notes
Amplitude
What is amplitude?
Amplitude is the greatest distance a wave moves away from its middle resting position. Think of it as how far the wave "reaches" above or below its normal, undisturbed level.
Definition: The amplitude of a wave is the maximum disturbance or displacement of the medium from the equilibrium (rest) position.
- Symbol:
- Unit: metre (m)
- Quantity: Amplitude ()
The equilibrium position is like the calm, undisturbed middle line of the wave. When the wave moves, it creates crests (high points) and troughs (low points). The amplitude measures how far these extreme points are from the equilibrium position.
Understanding amplitude through measurement
To properly understand amplitude, you need to know that:
- The distance from equilibrium to the crest equals the amplitude
- The distance from equilibrium to the trough also equals the amplitude
- Both measurements give you the same value - this is a key characteristic of waves
When measuring amplitude in any wave:
- Find the equilibrium (middle resting) line
- Measure the distance from this line to the highest crest
- Measure the distance from this line to the lowest trough
- Both distances should be equal - this is your amplitude value
Key Characteristic: Both the distance from equilibrium to crest AND the distance from equilibrium to trough give you the same amplitude value. This is fundamental to understanding wave behavior.
Worked example: Sea waves
Worked Example: Calculating Wave Amplitude
Question: If the crest of a wave measures 2 m above the still water mark in the harbour, what is the amplitude of the wave?
Solution:
Step 1: Analyse the information provided
- The harbour has a still water mark (this represents the equilibrium position)
- The crest is 2 m above this equilibrium position
Step 2: Determine the amplitude
- Amplitude is defined as the height of a crest above the equilibrium position
- The still water mark represents the equilibrium level
- Since the crest is 2 m above the still water mark, the amplitude is 2 m
Answer: The amplitude of the wave is 2 m.
Key points about amplitude
Amplitude tells us about wave energy: Larger amplitude means the wave carries more energy. This is why tsunami waves, which can have very large amplitudes, are so destructive.
Amplitude remains constant: In ideal conditions, a wave's amplitude stays the same as it travels, unless energy is lost through friction or other factors.
Measurement reference: Always measure amplitude from the equilibrium position, never from crest to trough. The total distance from crest to trough is actually .
Exam tips
Essential Exam Points:
- Remember that amplitude is always measured from the equilibrium position to either a crest or trough
- The symbol is always used for amplitude in calculations
- Units must be in metres for NSC Physics
- Don't confuse amplitude with wavelength - they measure completely different wave properties
Remember!
Key Points to Remember:
- Amplitude () is the maximum displacement from the equilibrium position
- Unit: metre (m)
- Symbol:
- The distance from equilibrium to crest equals the distance from equilibrium to trough
- Amplitude tells us about the energy carried by the wave - bigger amplitude means more energy