Atomic structure refresher…..

An atom contains: protons, neutrons and what other particle?....

A proton has a positive charge? (true or false)

In the nucleus are protons and which other particle?....

Which particle is not in the nucleus?....

The ATOMIC number means the number of what?....

The MASS number means the number of what?.... Plus what?....

An isotope of an atom has the same number of protons, but different number of what?....

An ion (charged atom) is formed when by the addition or removal of what?....

What particle is modelled to exist only at fixed distances from the nucleus according to the corresponding energy level?...

Physics of RadiographyElectricity, EM and waveparticle duality

By the end of the session you should be able to:

1. Explain the difference in how energy is transferred with transverse and longitudinal waves

2. Give one example of each type of wave3. Point out on a wave diagram what the

amplitude/wavelength/crest/trough are4. Show how a wave would look if it’s frequency or amplitude

were changed5. Describe wave particle duality6. Understand how electricity is generated

How does energy change across the electromagnetic spectrum? Which is more energetic microwave or x-rays?

• Waves transfer energy & information but without transferring matter

• The energy is transferred by oscillations in the material which the wave is travelling though

1) Mechanical WavesWaves that pass though a material are vibrations of

that material eg. Sound waves, seismic waves, strings

2) Electromagnetic WavesVibrating electrical or magnetic fields through

space (no material needed) eg electromagnetic spectrum

1. Longitudinal Waves

• Oscillations occur parallel to direction of travel

• Sound waves – a vibrating surface in contact with air.

• The surface pushes air molecules away which push adjacent air molecules

Compression (C)

Expansion (E)

E E EC C CDirection

of Travel

which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules…….

2. Transverse Wave

Direction of

Travel

• Oscillations at right angles (90o) perpendicular to direction of travel

• Electromagnetic waves – radiowaves, X-rays, microwaves, visible light etc

• Waves on a string or wire – plucking a guitar string

A

Parts of a Wave

x

Wavelength

Peak

Amplitude

Trough

1 complete wave

Key Terms• Displacement (s) – the distance from the equilibrium position

• Wavelength (l) – the distance between identical points in a wave train

• Amplitude (A) – maximum displacement of a particle (peak or trough max.)

• Period (T) – the time taken for 1 complete wave to pass a point

• Frequency (f) – number of cycles per second. Measured in Hertz (Hz).

fT

1

Amplitude and FrequencyLow amplitude, low frequency:

Low amplitude, high frequency:

High amplitude, low frequency:

High amplitude, high frequency:

WAVE SPEED• We know the relationship between

distance, speed and time.• Speed = Distance / Time• Therefore…

D

TS

Wave Speed = Wavelength / Period

V = λ / T

But we know that T = 1/f

So we can substitute….

The Wave Equationrelates the speed of the wave to its frequency and wavelength

Wave speed (v) = frequency (f) x wavelength ()

m/s Hz m

V

f

Speed (m/s) = wavelength (m) x frequency (Hz)Let’s try a few…

An ocean wave has a wavelength of 1.5m. There are 2 of these waves hitting the shore each second. What speed is the wave travelling at?

Wavelength = 1.5m Frequency = 2HzSpeed = 1.5m x 2Hz = 3m/s

What speed would a wave be travelling if it had a wavelength of 4m and a frequency of 3Hz?

Wavelength = 4m Frequency = 3HzSpeed = 4m x 3Hz = 12m/s

1. Reflection

Waves will bounce off a surface under certain conditions

eg the surface must be shiny for electromagnetic waves

Reflective surface

Incident Ray

Reflected Ray

Normal

Angle of Incidence

Angle of Reflection

Angle of Incidence = Angle of Reflection

2. Refraction

Glass Block

Air

Waves cross a boundary causing a change in speed and consequently wavelength

Depends on the refractive index of different substances Normal

Normal

3. Diffraction• Occurs when waves pass through a gap or around an object of roughly the same size or smaller than their wavelength.

•Large gap - the middle parts of the waves go straight through the gap, with a slight curving at the edges of the waves.

•Small gap - if the gap is smaller than the wavelength of the waves, the waves fan out in circles.

1. Constructive Interference - when the crests (or troughs) of two waves coincide, they combine to create an amplified wave.

The two waves are in phase with each other – there is zero phase difference between them.

2. Destructive Interference - where the crests of one wave are aligned with the troughs of another, they cancel each other out.

The waves are out of phase (or in antiphase) with each other – they are half a cycle different from each other.

The electromagnetic spectrum

s

To observe the photoelectric effect, you create a vacuum chamber with the photoconductive metal at one end and a collector at the other. When a light shines on the metal, the electrons are released and move through the vacuum toward the collector. This creates a current in the wires connecting the two ends, which can be measured with an ammeter.

When electromagnetic radiation hits a metallic surface, the surface can emit electrons (called photoelectrons)

An electric current is a flow of electric charge around a circuit.

Definitions:

Photon: a quantum of electromagnetic radiation, usually considered as an elementary particle that has zero mass and charge

Electromagnetic radiation:Energy in the form of transverse magneticand electric waves. In a vacuum, these waves travel at the speed of light.

Wave-Particle duality: Electromagnetic radiation exhibit wave like and particle like properties

How are energies and frequency related?

Using the formula

(h always stays the same it is constant)

E=hfWhat is the relationship between energy and frequency?i.e. if the frequency goes up, what happens to the energy?

In excited atoms an electron from a lower energy level is given energy to move to an outer shell.

Recall

Binding energy – energy required to free electrons from their atomic orbit

Half filled electron shells more stable so higher binding energy

Electrons in outer orbits are shielded from the pull of the nucleus so lower binding energy

K

MN

OP

L

Energy level number

Orbital letter

1 K

2 L

3 M

4 N

5 O

6 P

Maximum electrons

2

8

18

32

50

…..