Physics P5 - P8 Flashcards
Wave types and their properties
Transverse waves have sideways vibrations - the vibrations are perpendicular (at 90*) to the direction the wave travels (S waves and ripples). All EM waves are transverse.
Longitudinal waves have parallel vibrations.
Longitudinal waves squash up and stretch out the arrangement of particles in the medium they pass through, making compressions
Wave speed (m/s) = frequency (Hz) x wavelength (m)
Wave Experiments
Use an oscilloscope to measure the speed of sound
To measure the frequency, you’ll need a cork and a stopwatch
To measure the wavelength, use a strobe light
Use a pencil and a stopwatch to measure the wave speed
Refraction
How to draw ray diagrams for refraction:
Draw a normal where any ray meets a boundary
If the light ray is travelling into a more dense material, it will slow down, making it bend towards the normal
If the light ray is travelling into a less dense material, it will speed up, making it bend away from the normal.
If a light ray is travelling through a rectangular block, the emerging ray and the incident ray will be parallel
Remember to use a ruler and add arrows to your rays to show directions
Red bends the least, violet bends the most
Sound Waves
Sound waves are caused by vibrating. These vibrations are passed through the surrounding medium as a series of compressions and rarefactions. They’re a type of longitudinal wave. Sound travels at different speeds in different media. The frequency of sound doesn’t change when it passes from one medium into another. But because v = f x wavelength, the wavelength doesn’t change - it gets longer when it speeds up and shorter when it slows down
Sonar and Ultrasound
When a wave reaches a boundary between two media it can be absorbed, transmitted (and possibly refracted) or reflected. If a wave is transmitted, some of the wave is usually reflected off by the boundary too. This is partial reflection. If you know the speed of the wave in the medium, you can use the time it takes for the reflections to reach a detector and the formula distance = speed x time to find how far away the boundary is.
Ultrasound waves can pass through the body, but are partially reflected at boundaries between different tissues (e.g. between the muscles in a pregnant woman’s stomach and the fluid in her womb, and between the fluid in the womb and the skin of the foetus)
If you know the speed of ultrasound in the different tissues, you can calculate toe distance to the different boundaries. The reflections are processed by a computer to produce an image.
Sonar is used by boats and submarines to find out the distance to the seabed or to locate objects in deep water.
Electromagnetic Waves
Electromagnetics (EM) waves are transverse waves. They travel at the same speed through a vacuum, but at different speeds in different materials.
It goes:
Radio waves - Longest wavelength, highest lowest frequency
Microwaves
Infrared
Visible light
UV
X-Rays
Gamma rays - shortest wave length, highest frequency
Gamma rays are also the most ionising, meaning that they can cause damage to human tissue
Waves in Medical Imaging
X-rays can be used for imaging bones
They are usually transmitted by soft tissue, but denser materials like bones and metal absorb. This helps locate issues with the bones. However, a little bit of absorption does happen and could potentially harm the patient
Gamma rays are used to see how things move through the body. Gamma rays are transmitted by skin, soft tissues and bones, so if gamma rays are produced inside a patient, they can be detected outside the body. Radiotracers are put into the body and emit gamma rays. The gamma rays are detected outside by a gamma camera so an image can be formed
Infra-red waves can tell you about injuries and infections (change in temperature)
Visible Light and Colour
Different objects absorb, transmit and reflect different wavelengths of light in different ways. The colour of an opaque object depends on which wavelengths of light are reflected. A rep apple appears to be red because the wavelengths corresponding to the red part of the visible spectrum are reflected.
Lenses and Images
Concave lens - A concave lens caves inwards. It causes light to diverge (spread out)
The principal focus (focal point) of a concave lens is the point where rays hitting the lens are parallel to the axis appear to come from - trace them back until they all appear to meet up at a point behind the lens
A convex lens bulges outwards. It causes rays of light to converge (move together)
The principal focus of a convex lens is where rays hitting the lens parallel to the axis all meet.
Lenses - Correcting vision
Short-sighted people can’t focus on distant object. This is corrected with concave lenses
Long sighted people can’t focus on near objects. This is correct with convex lenses
Isotopes and Radioactive Decay
Isotopes are atoms of the same element - they have the number of protons but a different number of neutrons. So isotopes have a different nuclear mass but the same nuclear charge. Most elements have different isotopes, but there are usually only one or two stable ones. The other unstable ones are radioactive and give out nuclear radiation and may decay into other elements.
When a nucleus decays, it will spit out on or more of four types of radiation.
Alpha particles - Two neutrons and two protons - similar to a helium nucleus. They have a relative mass of 4 and a charge of +2
Beta particles - An electron. No mass and a charge of -1. Beta particles move quite fast and are quite small. For every beta particle emitted, a neutron turns to a proton in the nucleus.
Sometimes gamma rays are emitted. They have no charge and mass. They’re just energy, so they don’t change the element of the nucleus that emits them.
If a nucleus contains a lot neutrons, it may just throw out a neutron. This creates isotopes of an element
Alpha particles are blocked by paper
Beta particles are blocked by thin aluminium
Gamma rays are blocked by thick lead
Electron Energy Levels
Electrons in an atom sit in different energy levels or shells. Each energy level is a different distance from the nucleus. An inner electron can move up one or more energy levels in one go if it absorbs EM radiation with the right amount of energy
An atom is ionised if it loses an electron
Alpha particles have the highest ionisation power
Half life
The Half-life of a source is the average time taken for its activity (or count rat`e) to half
Fission and Fusion
Nuclear Fission is a type of nuclear that is used to release energy from large and unstable nuclei by splitting them into smaller nuclei. This can happen:
Spontaneously - the fission is unforced and happens by itself
By absorbing a neutron - If a nucleus absorbs a neutron then it becomes unstable and splits. When a large nucleus splits it forms two new smaller nuclei (usually radioactive) and possible a few neutrons. A nucleus splitting gives out a lot of energy - some of this energy is transferred to the kinetic energy stores of the fission products. There is also a lot of extra energy which is carried away by gamma radiation.
