Physics P1

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Solar System Through History

Ancient Greek Astronomers - Sun, Moon, planets and stars orbited Earth in perfect circles - the Geocentric Model.

1600s - Heliocentric Model - Earth and planets all orbit the sun. Sun = at the centre of Universe. Introduced by Copernicus in 1543. It was not popular and was condemned by the Church. However, in 1610, Galileo observed Jupiter w/ telescope and saw three stars. Next evening, they moved in the wrong direction. Then, a fourth star appeared. They turned out to be moons orbiting Jupiter - officially disproving the Geocentric Model.

Today - planets orbit the sun, but they are elliptical orbits, not circular orbits. Advances in technology have led to the discovery of new planets, e.g. Uranus.

Most of what scientists know comes from detecting waves from objects in space. Stars, for example, are hot and very far away, but are visible to the naked eye - this is how early astronomers made their discoveries.

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Waves

  • All waves have wavelength, frequency, amplitude and speed.
  • Wavelength - distance from one peak to the next.
  • Frequence - how many complete waves there are per second. Measured in hertz (Hz).
  • Amplitude - height of the wave.
  • Speed - how fast it goes.

Formulas:

  • Speed (m/s) = Freq. (Hz) x W.length (m) OR v = f x lambda.
  • Wave Speed (m/s) = distance (m)/time (s)
  • Waves can be transverse or longitudinal.
  • Most waves are transverse. In these waves, the vibrations are at 90* to the waves' direction of travel. Examples are Light and other EM waves, Ripples on water, Waves on strings and springs and S-Waves.
  • In longitudinal waves, the vibrations are along the same direction as the wave is travelling. Examples are sound, ultra-sound, P-Waves and slinkies when you push and pull the end.
  • Oscilloscopes show everything as transverse waves.
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Reflection and Refraction

  • All waves can be reflected and refracted.
  • When a wave hits a boundary between one medium and another, some of its energy is reflected.
  • Angle of reflection (r) = Angle of incidence (i).
  • The light is reflected because of the change in density.
  • Waves travel at different speeds in substances with different densities. EM waves usually travel slower in denser media. Sound waves travel faster in denser substances.
  • When a wave crosses a boundary between two substances, it changes speed.
  • Light shines on a glass window pain = as it passes from the air into the glass, it slows down. When it passes into air on the other side, it speeds up and bends away from the normal. Hence, the light emerging will now travel in the same direction it was to begin with. It was refracted to the normal and back again by the same amount.
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Lenses

  • A converging lens is convex - it bulges outwards, causing parallel rays of light to converge to a focus.
  • The axis of a lens is a line passing through the middle of the lens.
  • The focal point is where the rays all meet.
  • Each lens has a focal point in front, and one behind the lens.

Ray Diagram:

  • Choose point on top of object. Draw ray going from object to lens parallel to axis of lens. Draw another ray from the top, going through lens' middle. Draw refracted ray passing through focal point. Ray passing throguh middle doesn't bend. Mark where rays meet at top of image. Repeat for point on bottom of object.

Focal Length: Clamp lens at one end of track, and clamp white card further down track. Set it up near window, directing lens at a distant object. Make sure you can see the object on the white card. Make sure all lights in the room are turned off. Move card along track until image is focused.  Measure distance between ctr. of lens and the card.

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Simple and Reflecting Telescopes

Refracting telescope uses two converging lenses - an objective lens and an eyepiece lens. Rays are coming from so far away, that when they reach the objective lens, they're parallel. Objective lens converges the rays to form a real image @ the focal point of the objective lens. The rays of light from the real image enter the eyepiece lens. The lens spreads them out so they leave at a wider angle than they entered it. This way, the light rays fill more of your retina, making the image look bigger.

Concave Mirrors and Reflecting Telescopes:

1). An incident ray parallel to the axis will pass through the focal point when it's reflected.

2). An incident ray passing through the focal point will be parallel to the axis when it's reflected.

Pick a point on top of object. Draw ray going from object to mirror, passing through focal point. Incident ray passing through focal point is reflected parallel to axis. Draw reflected ray passing parallel to the axis. Mark where the two reflected rays meet, at the top of the image. Repeat for a point at the bottom.

A reflecting telescope uses concave mirrors and a converging lens.  A large concave mirror collects the parallel rays of light from an object in space. The larger mirrror reflects this light onto a smaller second mirror placed in front of the large mirror's focal point. The smaller mirror reflects the light through a hole in the centre of the large collecting mirror. A real image is formed behind the mirror. A converging eyepiece lens is used to magnify this image.

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Electromagnetic Waves

EM Spectrum split into 7 groups:

Radio, micro, infra-red, visible light, UV, X-Rays, gamma. (In order of increasing freq. and decreasing WL).

They are all transverse and they all travel at the same speed.

Up until infra-red was discovered, scientists could only be sure about visible light. In 1800, Herschel found infra-red when experimenting with sunlight and a prism. White light + prism ----> spectrum of colours. He measured the colours' temperatures, and found that the heat just past the red light was the highest of all. It was infra-red.

A year later, in 1801, Ritter decided to look beyond violet. Ritter knew that when silver chloride was exposed to light, it turned black. He decided to test it with differened colours of light. He found that the strips changed colour fastest when exposed to the violet, decided to look beyond it, and had discovered ultra-violet.

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Electromagnetic Dangers

Properties of EM waves depend on their frequencies. The higher the frequency, the more energy the radiation has, the more harmful the radiation.

Microwaves heat human body cells. Are used in mobile phones, links with brain tumours, but no proof.

Infra-red waves also have a heating effect. If the body is exposed to too much, it can cause skin burns.

UV waves can cause sunburn, cell mutation and destruction, skin cancer and eye damage.

Gamma/X-Rays have high frequencies, and so are harmful. They can cause cell m+d, leading to skin cancer.

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Radio Waves + Microwaves

Radiowaves - used mainly for communications. Broadcast TV and radio signalsa, transmitting satellite signals. Long-wave radio (1-10 km) can be transmitted from London and received halfway around the world. RWs used for TV and FM radio transmissions have v. short WLs (10cm-10m). Get reception = be in direct sight of transmitter - signal doesn't bend round hills or travel far thru buildings. Short-wave radio signals (10m-100m) can be received at long distances from the transmitter. . They are reflected from the ionosphere.

Microwaves - communication to and from satellites. Satellite TV, signal transmitted into space, picked up by dish and transmitted back to Earth, although obviously as slight time delay from UK to Australia. Mobile phone signals also travel as microwaves. In communications, microwaves need to pass through Earth's watery atmosphere. In microwave ovens, the MWs need to be absorbed by water molecules in food to heat it up, so a different wavelength is used. The MWs penetrate up to a few cms into the food before being absorbed. The energy causes to the food the heat it - it is then conducted or convected to other parts of the food.

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Infra-Red

Infra-red radiation can be used to monitor temperature; it's also known as heat radiation. The hotter the object, the more IR it gives out. Hence, infra-red can be usedto meaure temperature. Infra-red is also detected by night-vision equipment. The hotter an object is, the brighter it appears, so the police and military use this.

Optical fibres carry data over long distances as pulses if IR radiatiom - e.g. in telephones. They bounce waves off the sides of a thin inner core of glass or plastic. The wave enters one end and is reflected repeatedly until it emerges at the other end.

Other uses include in cooking - grills and toasters use it, in remote controls, to transmit info between phones or PCs, and in security systems, as they can detect heat on an intruders body.

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Visible Light, UV and X-Rays

Visible Light - needed to see. Light enters eye, gets refracted, focused onto retina, sent to brain, interpreted into what you can see. Photography works in a similar way to the eye. The lens's aperture controls how much light enters the camera. The shutter speed determines how long the film or sensor is exposed to the light.

UV - Banks have special markings on their notes, that can be seen under UV light, to detect forgeries. Here, it's the amount of radiation emmitted that you are measuring. Fluorescent lamps also use UV radiation to emit visible light. The light in security pens are also only visible in UV light. UV radiation can also disinfect water, as it kills off any bacteria and/or viruses in the water, making it safe to use.

X-Rays - used in hospitals to take pictures of broken bones. They pass through flesh, but not through denser materials such as bones or metal, so it's the amount of radiation that's absorbed, or not, that gives you the X-Ray image. However, they can cause cancer, so radiographers try to limit their exposure to them. Airport security also use them on luggage and on passengers.

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Gamma Rays & Ionising Radiation

Radiotherapy - gamma rays can kill, so they can treat cancers. You have to have just the right amount; enough to kill all the cancer cells without killing too many normal cells. However, some normal cells do get damaged, making the patient very ill. Gamma rays can also be used to diagnose cancer, when a radioactive isotope is injected into the patient. A gamma camera is then used to dectect where the isotope is heading.

Sterilisation of food and surgical equipment also uses gamma rays. They will kill all germs, making them fresher and safer, and the food does not get damaged in any way, and stays perfectly safe to eat.

Ionising Radiation:

There are three types - gamma, alpha and beta. It is emmitted all the time by radioactive sources when their nuclei decay. . The emission is completely random. All three types transfer energy, and they are completely energetic. They all have their own uses, but can all be very dangerous, in some cases leading to cancer.

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The Solar System

Sun's radius - 696,000 km. Sun's mass - 1.99 x 10^30 kg.. 23,000 bigger than Earth's radius.

Earth's radius - 6378 km. Earth's mass - 5.97 x 10^24 kg.

Moon's radius - 1738 km. Moon' mass - 7.35 x 10^22 kg.

Radius of Mercury is 285 times smaller than Sun's radius. Jupiter's radius is 10 times smaller.

Neptune is about 30 times further away from the Sun than Earth is.

Manned spacecraft, reach the Moon in 3 days, Mars, 9 months, Neptune, 12 years.

Nearest star to Earth, after the Sun is 4 x 10^13 km away. 1 light year is 9.5 x 10^12.

Search for Extra-Terrestrial Intelligence, or SETI beams radio, TV and radar into space for an 'alien' to detect. Transmitters have been used.

Spacecrafts have carried probes, to investigate planets and moons, to record data. Robots can also take photos and send data back to Earth.

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Looking into Space

Various objects in space emit or reflect different frequencies of EM radiation.

Telescopes - if trying to detect light, the atmosphere gets in the way, absorbing lots of light from space. You need to go above the atmosphere. Light pollution makes it hard to pick out dim objects, and air pollution can reflect and absorb light coming from space. Ideally, a telescope should be used on top of a mountain away from any cities. Even better, put your telescope in space.

From 1940s onwards, telescopes developed for all parts of the EM spectrum, allowing us to learn more about the Universe. Cygnus A is a nearby galaxy. Looked at through an optical telescope, the galaxy is a small blob, surrounded by stars. WIth a radio telescope, you see two 'radio jets' - blobs of radiation. X-Ray telescopes are useful to see violent, high-temperature events in space, such as exploding stars. Radio telescopes discovered the Cosmic Microwave Background Radiation.

Bigger telescopes give better resolution, and now they are often linked to computers.

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Space and Spectrometry

Most large optical telescopes have spectrometers. A spectrometer is a tool used to analyse the light given out by stars and galaxies. It works by the telescope directing a beam of light into the spectrometer and through a slit, diffracting the light, and splitting it up into a spectrum - similar to a prism. The light spectra from stars and galaxies contain dark lines, which are caused by the light at those wavelengths being absorbed. Each element has it;s own absorption spectrum. Some spectra have bright lines - emission spectra. The spectra for galaxies further away appear more red than they should.

You can make a spectrum using a CD or DVD, a cardboard box and scissors:

Make a slit about 1mm wide on one end of the box, before making a slit for the CD at a 45* angle on the side of the box. Make a hole to look through - cut a slot by about 2cm by 6cm. Put the CD in the box so that the 'rainbow' side is facing your eye. Hold up the box in a dark room, to see the visible spectrum. Experiment with different sources of light.

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Stars - Life Cycle

1). Stars form clouds of dust and gas - nebulas.

2). Gravity makes them spiral in together. The gravitational energy is converted into heat energy, so the temperature rises.

3). If the temperature is high enough, the hydrogen nuclei undergo thermonuclear fusion to form helium nuclei, giving out massive amounts of energy. The star will enter it's stable period.

4). Eventually the hydrogen in the core will begin to run out, causing the star to swell into a red giant.

A smaller star will then become unstable, ejecting it's outer layer of dust and gas and a planetary nebula, leaving behind a white dwarf.

A larger star will glow brightly again, as it will undergo more fusion and expand and contract a few more times, forming heavier elements in nuclear reactions, before exploding in a supernova. Again, the outer layer of dust and gas will be ejected into space, leaving the core, known as a neutron star. If big enough, it will become a black hole.

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The Big Bang etc.

Light from other galaxies is red-shifted. Looking at light from distant galaxies, you find that the frequencies are lower than they should be, because they are shifted towards the red end of the spectrum. This is the red-shift; the same effect as when a car sounds lower-pitched when it's moving away from you. All the galaxies are moving away from us. Red-shift provides evidence for the whole Universe expanding.

Scientists have also detected Uniform Microwave Radiation coming from all directions in the Universe. CMB radiation is strong evidence for an initial big bang.

The Big Bang:

All the matter and energy in the Universe was compressed into a very small space - then it exploded, and started rapidly expanding. It is still expanding, even though the Big bang happened roughly 14 billion years ago. Without gravity, the Universe would expand at the same rate forever.

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Ultrasound and Infrasound

Ultrasound is sound with frequencies higher than 20,000 Hz. When a wave passes from one medium into another, some of the wave is reflected off the boundary between the two, and some is transmitted and refracted - this is partial reflection. This means that you can point a pulse of ultrasound at an object, and as long as there are boundaries, some of the ultrasound will get reflected back. The time taken for the reflections to reach a detector can be used to measure how far away the boundary is.

Ultrasound uses:

Ultrasound waves can pass through the body, but upon reaching a boundary, e.g. the fluid in the womb and the skin of the foetus, some of the wave is reflected back and detected, producing a video image of the foetus. Boats and submarines also use this in the form of sonar, to detect objects in the water around them. Many animals, such as bats, dolphins and frogs use it as a form of communication or as a sense of direction.

Infrasound is sound with a frequency lower than 20 Hz. It is used by elephants and tigers, and can be produced by volcanic eruptions.

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Ultrasound Formula and The Earth's Structure

Speed = Distance/Time - speed of the wave, and how long it takes to get there and back.

The Earth has a crust, mantle, outer and inner core. The crust is only about 20km thick.

The mantle has most properties of a solid, but can flow very slow. Radioactive decay takes place in the mantle. This produces a lot of heat, and it flows in convection currents. At the centre is the core, probably made of iron and nickel. The inner core is solid, but the outer core is liquid.

It's difficult to predict earthquakes and tsunamis. One way to predict them is to use probabilities based on previous events, say from the last century.

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Seismic Waves

Earthquakes and explosions cause seismic waves. They travel through the Earth. Seismologists work out the time it takes for the shock waves to reach each seisometer. Some parts of the Earth don't receive the shockwaves, and these are also noted down.

The two different types are P-Waves and S-Waves. P-Waves travel through solids and liquids. They are faster than S-Waves. S-Waves are transverse, not longitudinal - they only travel through solids. They're slower than P-Waves.

When seismic waves reach a boundary with Earth, some waves will be reflected. The waves also change speed,  causing them to also change direction, resulting in refraction. The wave speed usually changes gradually, but sometimes changes abruptly, resulting in a path with a kink. Scientists have been able to work out where the properties of the Earth changes.

Seismometer readings (seismograms) can be used to work out the distance to an earthquake's epicentre. PWs and SWs travel at different speeds - so you'll see two different tremors on the reading. Using the time difference between the two, you can calculate how far away the earthquake or explosion was. Then you draw a circle on a map centred on the location of your seismometer, with the above distance as it's radius - called a distance arc. The arcs from three or more seismometers will cross at one place - epicentre, and triangulation.

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Seismic Waves

Earthquakes and explosions cause seismic waves. They travel through the Earth. Seismologists work out the time it takes for the shock waves to reach each seisometer. Some parts of the Earth don't receive the shockwaves, and these are also noted down.

The two different types are P-Waves and S-Waves. P-Waves travel through solids and liquids. They are faster than S-Waves. S-Waves are transverse, not longitudinal - they only travel through solids. They're slower than P-Waves.

When seismic waves reach a boundary with Earth, some waves will be reflected. The waves also change speed,  causing them to also change direction, resulting in refraction. The wave speed usually changes gradually, but sometimes changes abruptly, resulting in a path with a kink. Scientists have been able to work out where the properties of the Earth changes.

Seismometer readings (seismograms) can be used to work out the distance to an earthquake's epicentre. PWs and SWs travel at different speeds - so you'll see two different tremors on the reading. Using the time difference between the two, you can calculate how far away the earthquake or explosion was. Then you draw a circle on a map centred on the location of your seismometer, with the above distance as it's radius - called a distance arc. The arcs from three or more seismometers will cross at one place - epicentre, and triangulation.

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Electric Current and Power

Electric Current is a Flow of Charge Round a Circuit - Current is the rate of flow of charge around a circuit. Electrons, being negatively charged, usually carry the charge. Voltage/Potential Difference is an electrical pressure giving a measure of the energy transferred. Current in a circuit flows from negative to positive, opposite to the flow of conventional current.

The mains electricity supply is a.c. - it keeps reversing its direction. A CRO can show current as a trace on a graph - an a.c. trace is awave.

d.c. always flows in the same direction - it has a constant value.

Electrical Power = Energy Transferred per Second

Measuring the Current and Voltage can tell you the power - make a circuit with an ammeter, switch, battery and test component in a series - i.e. all in one loop. Connect a voltmeter in parallel, across the component. Complete the circuit by closing the switch, and record the readings on the ammeter and the voltmeter. Calculate the power of the component using this formula: Power = Current x Voltage.

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Generating Electricity

Move a magnet in a coil of wire - induces a voltage. This is electro-magnetic induction. You can create this by rotating a magnet in or near a coil of wire, or by rotating a coil of wire in a magnetic field.

4 Factors affect the size of the induced voltage - they are the strength of the magnet, the area of the coil, the no. of turns on the coil and the speed of movement. To reduce the voltage, reduce one of these factors.

Generators work by alternating a.c. by electro-magnetic induction, either by rotating a magnet or by rotating a coil of wire, keeping the magnet fixed. All generators need something to do the turning. A dynamo is a type of generator that is often used on bikes to power the lights - here, the magnet is rotated instead of the coil.

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Non-Renewable Energy & Power Stations

12 different types of energy resource - fit into RENEWABLE and NON-RENEWABLE.

NR energy will run out one day. It includes: coal, oil, natural gas and nuclear fuels - uranium and plutonium.

NR energy - release CO2, causing global warming, can capture some CO2, and bury it underground, but very expensive process. Burning coal and oil releases sulfur dioxide, causing acid rain. Coal mining, particularly open-cast, makes a mess of the landscape. Oil spillages cause serious eco problems. Nuclear waste is dangerous. Overall cost of nuclear power is expensive, and carries risk of a Chernobyl-style disaster.

Most power stations use steam to drive a turbine. Fuel ----> Chemical Energy - Heat Energy - Kinetic Energy - Electrical Energy.

Nuclear Reactors are similar to boilers.

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Using Renewable Energy Resource

Renewable energy sources - never run out, damage the environment, but not as much as non-renewables. Don't all provide much energy.

Hydroelectricity - involves flooding a valley by building a big dam. Rainwater caught and allowed out through turbines. . Big impact on environment, loss of habitat? Immediate reponse to increased electricity demand. Initial costs oftem high, but minimal running costs and reliable.

Wave Power - Provide up and down motion, used to power a generator. Fairly unreliable - waves tend to die out when the wind drops. Useful on small islands etc. but not on a large scale.

Tidal Barrages - Big dams built across river estuaries, with turbines in them. Tide comes in, fills estuary several metres. Water then allowed out through turbines at controlled speed. Reliable energy source.

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Using Renewable Energy Resources.2

WInd Power - each turbine has own generator, electricity is generated directly from wind turning the blades, which turn the generator. Unfortunately, they can spoil the view.

Solar cells generate electric currents directly from sunlight - they generate electricity on a relatively small scale. Used often in remote places, and sunny places.

Geothermal energy - only possible when hot rocks lie near to the surface. Water pumped down to hot rocks, returns and steam to drive a generator. No environmental disadvantages, but quite expensive, and not many appropriate areas.

Biomass - waste that is burnt in power stations to provide energy.

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Comparison of Energy Resources

Setting up a Power Station:

Set-Up Costs: Renewable resources need bigger power stations, so more expensive.

Set-Up Time: Affected by size and complexity.

Running/Fuel Costs: Renewables have the lowest running costs.

Reliability Issues: Depends on weather and finite resources etc.

Environmental Issues: Atmospheric, Visual, Noise Pollutions, using up resources, habitats and other problems such as nuclear, hydroelectric.

Location Issues:  Solar, gas, biomass etc. waves on coast, obviously. 

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Electricity and the National Grid

National Grid - network of pylons and cables, covers the whole of Britain. Enables power to be taken from anywhere on the grid, and then supplied anywhere on the grid. To transmit huge amounts of power, a high voltage or high current is needed. High current - loses heat. Cheaper to have high voltage instead.

To get voltage to 400,000 V, requires transformers, and big pylons with huge insulators. Transformers step alternating voltage up at one end for efficient transmission, bringing it down to safe, usable levels at the other end.

Transformers have two coils, primary and secondary, joined with an iron core. Voltage increased using step-up transformer. More secondary turns than primary. Reduced at the consumer end, using step-down transformer.

Calculate the output voltage: (Primary Voltage/Secondary Voltage) = (No. of Primary Turns/No. of Secondary Turns)

Problems with transmitting these amounts of electricity - high voltage is a risk to people, long-term health - links to leukaemia.

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Energy and Cost-Efficiency

Save Money - Insulate Your Home: payback time (years) = (initial cost/annual saving)

Loft Insulation - Initial Cost is £200, Annual Saving is £100 - payback time is 2 years.

Others include a hot water tank jacket, double glazing, draught-proofing, thick curtains, and cavity wall insulation.

Energy can be measured in Joules or Kilowatt-Hours.

Power = Joules/Time

Cost = Power (kW) x Time (hrs) x Cost of 1kWh

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Energy Transfer

Nine Types of Energy - electrical, light, sound, kinetic, nuclear, thermal, gravitational potential, elastic potential, and chemical.

Potential- and Chemical- are forms of stored energy.

Electrical devices convert electrical energy into sound, light heat etc.

Batteries convert chemical energy to electrical to run electric devices.

All Machines Waste Some Energy. For example, light bulb - energy supplied is 100J. Useful energy, 10J light. Wasted energy, 90J heat. Also, laptops when on for long periods of time - lots of wasted heat energy.

Efficiency = USEFUL Energy Output/TOTAL Energy Input x 100%

 

 

 

Principle of the Conservation of Energy - ENERGY CAN NEVER BE CREATED OR DESTROYED - ONLY TRANSFERRED.

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Heat Radiation

Thermal Radiation - Emission of Electro-Magnetic Waves.

Basically, all objects continually emit and absorb heat radiation. An object hotter than it's surroundings emits more radiation than it absorbs. If it's cooler, then it's the opposite.

Radiation depends on surface colour and texture. Dark matt surfaces absorb radiation much better than bright glossy surfaces. Silvered surfaces reflect nearly all of it. Solar hot water panels contain water pipes under a black surface. Survival blankets are coloured silver to stop their body heat radiating away - the difference between life and death.

Experiments:

Leslie's Cube: Matt black side emitts most heat, so that's the thermometer which gets hottest.

Melting Wax: Matt black surface absorbs most heat, so it's wax melts first and the ball bearing drops.

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