Physics Revision Notes - Thermal Energy Transfers PDF

Thermal energy: - Thermal energy is the average kinetic energy of particles. - The lowest achievable temperatures is Absolute 0 (-273^o^C). - At this temperature, particles have the minimum kinetic energy. Absolute zero temperature and the Kelvin scale - The Fizzics Organization Thermal e...

Thermal energy: - Thermal energy is the average kinetic energy of particles. - The lowest achievable temperatures is Absolute 0 (-273^o^C). - At this temperature, particles have the minimum kinetic energy. Absolute zero temperature and the Kelvin scale - The Fizzics Organization Thermal energy transfer can take place by 3 different ways: - Conduction - Convection - Radiation Conduction: Conduction is the transfer of thermal energy through a substance by the vibration of the atoms within the substance. This is the main method of thermal energy transfer in solids. It mainly happens in solids because the particles are very close together so lots of collision can happen. Metals are good conductors of thermal energy, where there is a positive metal, surrounded by negative delocalized, free electrons. The electrons are free to move around the they can quickly move through the metal passing on the vibrations. This creases the rate of heat transfer.  - As a part of a substance is heated, the particles in the space gain more kinetic energy. - This causes the particles to **vibrate more -- more atomic vibration, causing them to collide more with adjacent particles.** - **These collisions transfer kinetic energy to the neighbouring particles causing them to vibrate more and collide with their adjacent particles.** - **This process repeats and energy is transferred along the object from the hot parts to the cooler parts chain reaction of TET.** **Thermal conduction in liquids and gases:** Thermal conduction can happen in liquids and gases; however, the particles need to be close enough together so when they vibrate, the vibrations can be passed along the substance. This can be difficult to achieve in fluids as: - In liquids particles are close but slide past each other. - In gases particles are widely spread apart and will not nudge each other when they vibrate. - This is why both types of fluid: liquids and gases bad conductors of heat through conduction. - This means they have a low thermal conductivity. However, in **solids**, the particles are closer together, so can pass vibrations on quicker. **Examples: Heating a pan for cooking or heating skewers** **Worked example: Heating a pan for cooking salmon.** - As the pan is heated up, the particles inside the pan gain more kinetic energy and **vibrate more (more atomic vibration)** - As they collide with adjacent particles, they transfer some kinetic energy to those particles. - The energy transfer goes on throughout the pan. - This transfers thermal energy from the hot parts to the cooler parts of the pan heating the salmon piece up evenly. **Investigating conduction:** **Variables:** **Independent variable**: Type of metal **Dependent variable**: Time taken for the drawing pin to drop. **Control variables:** - Same diameter of rods. - Same amount of wax used. - Ball should be placed the same distance from the heat source. - Same starting temperature of the rods **Method:** 1. Take 4 metal rods each made of a different metal (iron, copper, brass, aluminium). 2. Using some Vaseline, attach a drawing pin to the end of each of the rods.  *Note: Try to make it the same amount of petroleum jelly per rod.* 3. Bring together the opposite ends of the rods (without pins), so that they can each be heated the same amount, at the same space. 4. Using a Bunsen burner, begin heating the ends of the rods without the pins and start the stopwatch. 5. Record the time taken for the pins to fall off the end of each rod and use this to determine the order of conductivity of the metals. *Note: The first pin to fall will be from the rod that is the best conductor.* **Results:** - Copper conducts heat the best because the ball attached to the copper rod dropped off first. - Iron conducts heat the words because the ball attached to the iron rod dropped off last. - This is because materials with **high** thermal conductivity **heat up faster** than materials with **low** thermal conductivity. **Tips:** - Try to avoid handling the rods and the jelly too much before heating. - Allow the rods to cool to room temperature before heating so that they all begin at the same temperature and the results are more accurate. **Convection:** Convection is the transfer of heat through fluids -- liquids and gases, by the upward movement of warmer and less dense region of fluids. Convection cannot happen in solids -- only in fluids (liquids or gases). **Process of convection:** - A part of a fluid is heated. - The particles in that part gain kinetic energy. - The fluid expands because the particles move faster and take up more volume *-- the particles remain the same size but become further apart.* - This makes the hot liquid or gas less dense, so it rises into colder areas. - The colder liquid or gas moves to take its place. - Eventually the hot fluid at the top cools increases in density and causing it to sink back down. - This motion is called a convection current. Preventing the circulation of fluid can prevent convection currents and unwanted energy transfer by convection. IMPORTANT: You must know that: - Hot air (heat) rises. - Cold air sinks.  For convection to work at its best: - The heat source must be closest to the floor. - Increase the surface are of the heat source.\\ *Note:* - Convection happens best in gases. - Convection happens less good in liquids. This is because: - Particles have more kinetic energy in gases. - So, convection currents are faster in gases. - When the container is filled with gas the mass is less than if the container was filled with water - This allows convection currents to happen faster in gases. **Examples: Radiators, water boilers, or hot air balloons** **Worked example -- Radiators:** - Radiators heat the air next to the radiator. - This causes it to gain thermal energy. - This means that the particles gain kinetic energy and move faster. - This causes the air to expand and decrease in density. - This causes the hot air to rise, creating a convection current. **Investigating convection:** **Independent variable**: Temperature of water **Dependent variable**: Rate of convection **Control variables:** - Same volume of water in beaker. - Same size/strength of flame (yellow or blue flame). - Mass of potassium permanganate crystal. **Method:** 1. Fill the beaker with water until it is ¾ full and place it on top of a tripod and heatproof mat. 2. Pick up the crystal using forceps and drop it into the centre of the beaker -- do this carefully to ensure the crystal does not dissolve prematurely. 3. Heat the beaker using the Bunsen burner and record observations. 4. Repeat experiment with hot water and record observations. **Results:**  - Energy is initially transferred from the Bunsen flame through the glass wall of the beaker by conduction. - The water molecules that are heated expand and the water becomes **less dense** and **rises.** - This causes the dissolved purple crystal to flow upwards with the water. - Meanwhile, when the water at the top of the beaker cools, there is less space between the water molecules and the water becomes **denser** again and **falls** **downwards**. - Leads to a convection current - The convection current is faster in hot water. - This is because the higher the **temperature**, the higher the** kinetic energy** of the water molecules. **Radiation:** Thermal radiation is the transfer of energy by infrared (IR) waves. These travel very quickly in straight lines. Infrared radiation is on the electromagnetic spectrum which mean that it does not require a medium (it can travel through a vacuum) to travel through. Essentially, radiation can travel through: - No medium (in a vacuum) - Or when changing medium (e.g., from solid to liquid) All objects give off thermal radiation. **Investigating Radiation:** **Variables:** **Independent variable** = Colour of surface **Dependant variable** = Temperature/intensity of infrared radiation **Control variables:** - Leslie cube should be same distance from the infrared detector. - Measurements should be taken at even intervals. **Method:** 1. Pour boiling water into the Leslie cube and replace the lid. 2. Leave for 1 minute to enable the surfaces to heat up to the temperature of the water. 3. Align the infrared detector with one side of the Leslie cube, 20cm away from the side, and take the initial temperature of the surface. 4. Measure and record the intensity of infrared radiation (or the temperature) emitted from each surface every 30s for 5 minutes. 5. Rotate the cube and repeat the experiment for a different surface. *Note: If plotting a graph Plot temperature (plot on y-axis, measured in °C) against time (plot on x-axis, measured in seconds) for each different surface.* **Results:** **Surface** **Absorber** **Emitter** -------------------- ---------------------- ---------------------- Shiny black Good Good Matt black Very good (the best) Very good (the best) Matt white Bad Bad Shiny white/silver Very bad (the worst) Very bad (the worst) - **Black** surfaces with a **matt** texture are the **best absorbers and emitters** of infrared radiation but are poor reflectors of it. - **White (light)** surfaces with a **shiny** texture **reflect the thermal radiation** so are the best reflectors of infrared radiation. This also means that they are the **worst absorbers and emitters.** - This means that placed next to a heat source, a dark object would heat up faster than a light one. - This also means that a hot object with a light shiny surface will emit less IR than a dark matt object at the same temperature. - **Shiny** surfaces can be used to **reduce unwanted energy transfer** such as on the **surface of a vacuum flask.** - **Matt** surfaces can be used to **maximise energy transfer such** as on the **surface of a cooking pan.** - The **higher the temperature** and the **greater the surface area** of a body the **more infrared radiation** emitted. *Notes:* - *Take repeated readings for each coloured flask.* - *Safety goggles should be worn when using a Bunsen burner.* Examples -- Infrared radiation from the sun, travelling through the vacuum of space. Worked example -- What happens to the temperatures of the ice cream and the surroundings? Why? The thermal energy store of the ice cream increases as it absorbs thermal energy from the surroundings as radiation. This causes the ice cream to melt. The thermal energy store of the surroundings decreases as it absorbs the thermal energy from the ice cream meaning it gets colder. *Note: When a question on heat transfer mentions the surface properties of an object, such as describing it as shiny, black or white, then you are being clued-in to write about **thermal radiation.***  Summary Example: Example: Boiling potatoes FLAME TO METAL PAN: Heat is transferred from the flame to the metal pan by radiation because of the change in medium from flame to solid metal. THROUGH METAL PAN: Heat is transferred through the metal pan by conduction because the thermal energy is going through a solid. The more kinetic energy the particles gain, the more they vibrate and pass that kinetic energy on to surrounding particles. FROM METAL PAN TO WATER: Heat is transferred from the metal pan to the water by radiation because of a change in medium. THROUGH THE WATER: Heat is transferred through the water by convection because the thermal energy is going through a liquid. The particles nearer the heat source gain more kinetic energy so they move more and become further apart and less dense, causing the particles to rise. Cold particles fill the space nearer the heat source. This is called a convection current and this process repeats. THROUGH THE POTATO: Heat is transferred from the outside of the potato to the inside by conduction because it is a solid (slowly as it is a non-metal). Thermal Equilibrium: - As an object absorbs thermal radiation it will become hotter. - As it gets hotter it will also emit more thermal radiation. - Eventually, an object will reach a point where it is absorbing radiation at the same rate as it is emitting radiation. - At this point, the object will be in thermal equilibrium. - At thermal equilibrium, an object has constant temperature. Example -- In the experiment above, both thermometers reached a steady temperature when next to the light bulb. Explain. The thermometer was releasing (emitting) heat at the same rate as it was absorbing heat radiation. This causes a thermal equilibrium, causing the steady rate. **Reducing unwanted energy transfer:** **Energy efficiency** -- Using as much energy as possible produced for a desired purpose. Vacuum Flask: A thermos flask reduces the rate of heat transfer by conduction, convection and radiation. **Cap:** **Reduces heat transfer by conduction and convection**: - The cap is made of an insulating material (like plastic or rubber) to minimize heat loss by conduction. - By tightly sealing the flask, it prevents air from escaping or entering, reducing heat loss by convection. **Vacuum:** **Reduces heat transfer by conduction and convection**: - The vacuum between the inner and outer walls is lack of air, eliminating conduction (since there are no particles to transfer heat) and convection (as convection requires a medium like air or liquid). **Silvered Glass Walls:** **Reduces heat transfer by radiation**: - The silver coating reflects infrared radiation, preventing heat loss (or gain) by radiation. - It keeps hot contents hot by reflecting heat back inside and cold contents cold by reflecting external heat. **Inner Wall:** **Reduces heat transfer by conduction and radiation**: - The inner wall is typically glass or stainless steel with a reflective coating. - Its smooth surface minimizes the transfer of heat by conduction, and the reflective surface reduces heat transfer by radiation. **Outer Wall:** **Reduces heat transfer by conduction**: - The outer wall, often made of plastic or stainless steel, is designed to remain cool to the touch. - It minimizes conduction from the inner layers to the external environment. **Insulated Support:** **Reduces heat transfer by conduction**: - The flask is supported by an insulated base or supports, which are poor conductors of heat. - This prevents heat from being transferred through the base to surfaces it is resting on. **Outer Case:** **Protects and reduces heat transfer by conduction**: - The outer case shields the vacuum flask from physical damage, protecting the bottle. - Its material is typically a poor conductor, further reducing heat transfer by conduction. **Ways to insulate homes:** Walls: The oldest houses have solid brick walls, so they lose a lot of heat by conduction through the walls. Newer houses have 'cavity walls', with an inner brick wall and an outer brick wall, and an air space between them. This reduces heat loss, because air is a very bad conductor of heat, so this reduces heat transfer by conduction. However, the air space still allows heat loss by convection; as convection currents can form. To prevent this, a solid foam ('cavity wall insulation') can be injected into the air space. This cavity foam is made of glass fibre insulating wool that reduces heat loss even further, because the gas bubbles in the foam are a very poor conductors of heat, and the bubbles are too small to allow convection currents.  To go 1 step further, you could replace the inner brick with special thermal bricks. None of these methods reduces radiation but a shiny and reflective material can be put behind radiators to reflect some thermal radiation back into the house. In addition, the cavity could be lined with a shiny and reflective material to reflect the thermal energy that has escaped. Windows: Heat is lost through windows in several ways: The frames lose heat, so steel or aluminum frames are no longer used because metals are very good conductors of heat. This is because for metals you just have to heat the delocalized electrons to transfer the thermal energy, while in non-metals you have to heat the whole atom. New windows have rubber seals around the edges to prevent heat loss from the frame.  A lot of heat may be lost through the glass by conduction. This can be reduced by using double glazing, but the air gap must be the right size. If it is too small, too much heat will be lost by conduction: if it is too big, too much heat will be lost by convection, as convection currents can form. None of these methods reduces radiation, but you can use special glass that is transparent to visible light but reflects some of the infrared radiation. This keeps houses warm in winter, but cold in summer. Roof: A lot of heat is lost through the roof (loft) by convection. This is because hot air rises, so the temperature will be hotter at the ceiling than at floor level. The heat loss through the ceiling can be reduced by fitting 'loft insulation'. The usual method is to put a 'blanket' of 'mineral wool' (fibre glass with lots of small air pockets in it) in the loft above the ceiling. This reduces heat loss because air is a very poor conductor, and the air pockets are too small to allow convection. If a house has a solid concrete floor, it will lose some heat by conduction. This is difficult to prevent after the house has been built, but new houses are built with a layer of insulation in the floor. Sometimes it is important to prevent heat losses inside the house. For example, the water in boiler will stay hot longer if the boiler is covered with insulation.