Waves

TEACHING PRESENTATION

WHAT ARE WAVES ?

• Of course, everybody knows sea waves.• Physics waves can be assimilated to those waves. • They are deformations of the system crossed by the waves. They

bring informations.

WHERE ARE THE WAVES ?

• They are used everyday by nearly everybody : the sound is spreaded by waves from mouth to hears.

• Other usefull :

• Microwave oven (microwaves)

• Radio transmission, radiography (electromagnetic waves)

• XV-XVIe century : Marin Mersenne : sound celerity

• About 1850 : Maxwell (1860) and Hertz (1881) : electromagnetic waves

• 1895 : First radiography (by Wilhelm Konrad Röntgen)

• 1958 : First echography by Ian Donald

WHO HAVE DISCOVERED THE WAVES ?WHEN HAVE THEY BEEN DISCOVERED ?

SOME DEFINITIONS

• Periodicity : when we have the repetition of a motif during the whole signal.

• Period : it’s the difference of time between two same points in two motives (for examples, two maximas or two minimas).

• Frequency : it’s the number of signal, during a second.

• Amplitude : it’s the difference between a maxima and a minima

Period

Maxima

Minima

Amplitude

SOME FORMULAS

• Some facts you can have by measuring :

• Period : T

• Maxima : Ma (between the highest point and the abscises)

• Minima : Mi (between the abscises and the lowest point)

• Amplitude : A (Ma + Mi)

• Some facts you can only have by calculating :

• Frequency : f = 1/T

EXERCICES

Ex 1 : Here are some radiographies.

1) Say whose are periodic signals

2) For each of them, determinate what is the period, the frequency, the maxima, the minima and the amplitude.

ab

c

Ex 2 : Attribute the facts to the right radiographies

1) f = 500 Hz , A = 2

2) T = 2 s , A = 0,54

3) T = 0,5 ms , Ma = 1,1, Mi = -1,1