Understanding heat and temperature
- The temperature of an object tells us how hot it is and is measured in degrees but does not tell us how much heat it contains. This is different and is measured in Joules.
- If an object is warmer than its surrondings, then its heat flows to its surroundings and vice versa.
- A thermogram can be used to show this, it has different colours for different temperatures.
- Temperature- A measure of the average kinectiic energy of the particles in a substance
- Heat- A measure of energy on an absolute scale. It is not related to reference points.
Specific heat capacity
- When an object gets more energy, it gets hotter. When it cools down its losing energy. Different materials will hold different amonts of energy. This means that it takes more energy to raise the temperature of 1kg of water by one degree than 1kg of aluminum. Water has a higher Specific Heat capacity.
- The amount of heat transferred- energy (J) = Mass (kg) x SHC (J/kg'C) x Temperature Change ('C)
- The specific heat capacity of a material is the amount of joules it takes to raise 1kg of it by one degree. Water's SHC is 4200.
- Water is used in central heating systems because of it's high SHC, it needs a lot of energy to heat it up and therefore radiators can transfer a lot of heat energy to the room.
Specific latent heat
- When something melts it needs energy to change from a solid to a liquid. While it melts the temperture stays constant, even though its still gaining energy.
- When you heat a solid, the temperature rises at a steady rate until it reaches the melting point, its still using energy but there is no temperature change. Once it has melted the temperature will rise again until it boils and the same thing will again happen.
- The amount of energy (J) need to change the state of 1kg of a material is called its specific latent heat capacity.
- Energy (J) = SLH (J/kg) x Mass (kg)
- The specific latent heat of fusion (334 000) is NOT the same as the specific latent heat of vaporisation (2 260 000).
- The latent heat of fusion is the amount of energy need to break the intermolecular bonds between the particles in the solid so they can move around.
Conduction and Convection
- Energy is transferred through solid materials by conduction. The particles in a solid are always vibrating, but when they get heated they vibrate more and this causes the particles next door to start vibrating and this goes all the way through the solid.
- Some materials are good thermal conductors. When the particles are close together it's best because the vibrations are passed along quicker, this is why metals are good conductors.
- Some materials don't do this very easily and therefore are called thermal insulators. Wood and plastics are good for this.
- Fluids are poor conductors but they transfer heat by convection because the particles are free to move. For example, hot air rises and is replaced by cold air.
- When particles of air gain more energy, they move faster and expand and the density decreses, meaning it floats. When they cool the particles contract and become more dense so they sink.
Radiation and surfaces
- When you put your hands on a cup that is hot, energy is passed to them by conduction. If you take your hands off but still hold them close to it you can feel heat, this is transferred by radiation- known as infrared. When something hot glows it's transmitting radiation.
- The hotter something is, the more energy it will radiate in a certain amount of time.
- Objects can absorb radiation from the sun. For example, if you put an object in the sun it will absorb infrared and it's temperature will increase.
- The surface of the object affects this too. If there is a white or silver shiny surface, it won't absorb that much heat because the radiation reflects off the surface. Wheras if it has a black surface, it will absorb the waves and heat up.
- Infrared radiation is an electromagnetic wave, like light. it can travel through a vacuum and does not need a medium to travel through. This is why, although a cavity wall can prevent heat loss by convection and conduction, it can't by radiation.
Insulating your home
- Houses transfer energy to their surroundings. Energy flows from the hotter regions to the colder ones. This means the house cools down and the surroundings heat up. This isn't good because it costs money to heat.
- You have cavity walls (walls with air gaps (foam)) because air is a poor insulator and heat cannot be transferred by conduction or convection.
- You can cut back your energy bill by reducing the amount of energy that you use. To decide whether they are worth doing, you have to look at their cost and the payback time. The method with the shortest payback time is the most cost-effective.
- Some things in the home are designed around c+c+r. A boiler is designed to have the heating element at the bottom (convection). Other things are designed to minimise c+c+r; double glazing and cavity walls.
- Cost = Payback time x savings per year.
Useful energyy and efficiency
- In our homes we are always using appliances that transform energy from one form to another. for example, a boiler transfers chemical energy into heat, and the televison transforms electrical energy into light.
- A lightbulb produces heat as well as light. Light = useful but heat = not useful. You can show efficiency in a Sankey diagram.
- If you know some things about a lightbulb, you can work ot how efficient it is:Efficiency = Useful energy output / Total energy input x 100%
Introduction to waves
- Waves are always around us, sea waves and sound waves. They are like ripples on a pond; contain a series of crests and troughs.
- Waves are made up of a series of occilations or vibrations travelling through space. In a Transverse wave, the occilations are always at right angles to the direction of wave motion.
- All waves have- Amplitude(m)- Maximum height of the wave measured from the middle, Wavelength(m)- Shortest distance from one point to the same point on the next wave, e.g. crest to crest, Frequency(Hz)- The number of waves passing a point per second.
- Wave speed(m/s) = Frequency(Hz) x Wavelength(m)
Electromagnetic waves and wave experiments
- Light, microwaves and x-rays are examples of electromagnetic waves. This means they don't need a medium to travel through, and they can travel through a vacuum, like space. The different waves form a family called the electromagnetic spectrum:
- Radiowaves - microwaves - infrared - visible light - ultraviolet - X-rays - gamma rays
- All electromagnetic waves travel in straight lines at the speed of light.
- Whenever we draw ray diagrams we must include a normal line, which is 90' from th surface. The law of reflection states that the angle of incidence is always equal to the angle of reflection.
- Angle of incidence- the angle between where the light hits the surface to 90'.
- Waves can also be refracted when they go from one medium to another. This leads to pencils looking bent, etc. As they enter another medium, their speed changes and they go in another direction. If they slow down then they bend towards the normal (90' from surface) and if they speed up they bend away.
- Whenever they move through a gap or an obstacle, they spread out, this is called diffraction. This is why we can hear around corners. This causes problems when using optical instruments like telescopes. when light enters it diffracts a little which can lead to a blurry image.
Total internal reflection and fibre optics
- When a light travels from one medium to another it refracts, however there is still a small amount of internal reflection. For example, if a light travels from a glass to air, the light will go out of the glass but a small amount will also reflect back into the glass. But if the light through the glass hits the inside of it in exactly the right place then it will all reflect. This is called Total internal reflection.
- This effect can be seen if the light is travel from a dense material to a less dense material.
- Total internal reflection can only happen if: The angle of the light is greater than the critical angle (45' to the edge), and if the light is travelling in the denser of the two materials.
- Optical fibres are very fine glass cables. They can be used to transmit lots of information or to help doctors look inside a patient.
- Waves travel along optical fibres using total internal reflection. They hit the cables and bounce off the other side at an angle greater than the critical angle.
- With fibre optic broadband, pulses of visible light or infrared are sent down optical fibres, which allows lots of data to be sent quickly.
- An endoscope is a medical instrument that can produce an image of the patients insides without cutting them open. Inside it there are several bundles of optical fibres. Visible light is sent down the cables and lights it up, then reflecting back up the cables, producing a picture.
Light has been used for centuries to send messages acros long distances. An example of this is Morse Code. It has flashes of light representing letters and words. It's thought of as an early type of digital signal because it contains on off pulses.
Lasers produce a narrow beam of light, which doesn't spread out much as it travels. The light from any particular laser is just one colour, usually red.
The waves on a laser light all have the same frequency and are in sync with each other (all the crests and troughs line up). We say that it is in phase. There are billons of pits on the average CD. This digital information is read by the laser. As the disc spins, a laser reflects from it's surface. when the light enters a pit there is no reflection, so the detector can read a series of light pulses.
Cooking with waves
- Infrared radiation is emmited by warm objects. If an object absorbs infrared it gets hotter. This makes it useful ffor cooking. if you think of a toaster getting hotter and hotter and emitting more and more infrared, it also emits red light. The infrard waves from the toaster are absorbed by the food, which heats up the surface. If the toaster has a shiny surface on the bottom it cooks quicker because the waves are reflected back up.
- A microwave uses microwaves, which have a longer wavelength and cook food in a different way. It produces a steady stream of microwaves. Instead of just heating the surface of the food, they travel about 1cm into it. They are absorbed by fat and water molecules, which heat up and the food is cooked by conduction. The fat and water gains kinetic energy as they absorb the waves.There are shiny surfaces in microwves for the same reason as in a toaster.
- Microwaves can cause burns to humans, because they are absorbed by body tissue same as they are absorbed by food. They can pass through glass an dplastics, so a wire mesh is placed inside the door.
- For both infrared and microwaves, the energy is associated with the frequency (how many waves there are each second) . Higher frequency waves have more energy.