Atomic structure refresher…..

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Presentation transcript:

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 Radiography
Electricity, EM and waveparticle duality

By the end of the session you should be able to:
Explain the difference in how energy is transferred with transverse and longitudinal waves Give one example of each type of wave Point out on a wave diagram what the amplitude/wavelength/crest/trough are Show how a wave would look if it’s frequency or amplitude were changed Describe wave particle duality Understand how electricity is generated

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

WAVES Waves transfer energy & information but without transferring matter The energy is transferred by oscillations in the material which the wave is travelling though A wave transfers energy through a medium without the medium itself having to move. The energy is transferred by disturbances (oscillations) in the medium (material) through which the transfer is occurring. There are many different kinds of waves Electromagnetic waves pass through a vacuum & do not need to be carried by a substance. Light, infra red & microwaves all make things warm up. X-rays & gamma rays can cause ionisation and damage to cells, which also show that they carry energy. Loud sounds make things vibrate or move. Even the quietest sound move your eardrum Waves on the sea can toss big boats around and can generate electricity. Waves also transfer information, as well as energy. E.g. TV, radio, speech, fibre optics etc 5

CLASSIFYING WAVES Mechanical Waves
Waves that pass though a material are vibrations of that material eg. Sound waves, seismic waves, strings 2) Electromagnetic Waves Vibrating electrical or magnetic fields through space (no material needed) eg electromagnetic spectrum

TYPES OF WAVES 1. Longitudinal Waves E C
Direction of Travel E C 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 Depending on the motion of the medium compared to the movement of the wave, waves are classified as either transverse or longitudinal. Transverse waves have sideways vibration. Vibrations are at 90* to the direction of travel of the wave. Most waves are transverse. Longitudinal waves have vibrations along the same direction as the wave is travelling. 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……. Compression (C) Expansion (E) 7

TYPES OF WAVES 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 Depending on the motion of the medium compared to the movement of the wave, waves are classified as either transverse or longitudinal. Transverse waves have sideways vibration. Vibrations are at 90* to the direction of travel of the wave. Most waves are transverse. Longitudinal waves have vibrations along the same direction as the wave is travelling. 9

Parts of a Wave Wavelength Peak Amplitude A Trough x 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).

Amplitude and Frequency
Low amplitude, low frequency: Low amplitude, high frequency: High amplitude, low frequency: High amplitude, high frequency:

WAVE SPEED D T S We know the relationship between distance, speed and time. Speed = Distance / Time Therefore… Wave Speed = Wavelength / Period V = λ / T But we know that T = 1/f So we can substitute….

Wave speed (v) = frequency (f) x wavelength ()
The Wave Equation relates 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 = 2Hz Speed = 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 = 3Hz Speed = 4m x 3Hz = 12m/s

WAVE PROPERTIES 1. Reflection
Waves will bounce off a surface under certain conditions eg the surface must be shiny for electromagnetic waves Reflective surface All waves can be Reflected Incident Ray Reflected Ray 16

Angle of Incidence = Angle of Reflection
WAVE PROPERTIES Angle of Incidence = Angle of Reflection Angle of Incidence Normal When waves are reflected from a flat surface, the Angle of Incidence (i) = the Angle of Reflection (r) Angle of Reflection 17

WAVE PROPERTIES 2. Refraction Air Glass Block Normal Normal
Waves cross a boundary causing a change in speed and consequently wavelength Depends on the refractive index of different substances Normal All waves can be Refracted. Refraction is a change of direction due to a change of speed. The example I used involves light rays hitting a glass block. Glass is a medium which is more dense than air. so the light will travel more slowly within the glass block than it would do in air. As the light ray crosses the boundary from one medium to another, it bends. Air Glass Block Normal 18

WAVE PROPERTIES 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. Diffraction occurs when waves pass through a gap or around an object of roughly the same size or smaller than their wavelength. When parallel water waves move towards a large gap, the middle parts of the waves go straight through the gap, with a slight curving at the edges of the waves. If however, the gap is smaller than the wavelength of the waves, the waves fan out in circles from the gap. The gap seems to act as a vibrating source and waves spread out from the gap. Diffraction provides evidence for the wave nature of water waves, sound waves & light. A good example of Diffraction of sound waves occur when you can hear people talking, out of sight, on the other side of a doorway. When the sound waves reach the doorway, they spread out and become diffracted around the door way. 20

INTERFERENCE 1. Constructive Interference - when the crests (or troughs) of two waves coincide, they combine to create an amplified wave. All waves can be Reflected The two waves are in phase with each other – there is zero phase difference between them. 21

INTERFERENCE 2. Destructive Interference - where the crests of one wave are aligned with the troughs of another, they cancel each other out. All waves can be Reflected The waves are out of phase (or in antiphase) with each other – they are half a cycle different from each other. 22

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: Energy in the form of transverse magnetic
and electric waves. In a vacuum, these waves travel at the speed of light.  Electromagnetic radiation: Photon: a quantum of electromagnetic radiation, usually considered as an elementary particle that has zero mass and charge Wave-Particle duality: Electromagnetic radiation exhibit wave like and particle like properties

How are energies and frequency related?
Using the formula E=hf What is the relationship between energy and frequency? i.e. if the frequency goes up, what happens to the energy? (h always stays the same it is constant) Recall In excited atoms an electron from a lower energy level is given energy to move to an outer shell.

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

Energy level number Orbital letter 1 K 2 L 3 M 4 N 5 O 6 P Maximum
electrons 2 8 18 32 50 ….. P O N M L K