basics and insulation Flashcards
Conservation of energy
Energy is always conserved: it can be transferred, stored or dissipated but never created or destroyed.When energy is transferred some energy is always dissipated. Dissipated energy is wasted energy. Because the energy isn’t being stored usefully. e.g. phones have energy wasted as thermal energy There are also closed systems e.g. a cold spoon is dropped in a a flask of warm soup which is then sealed, energy is transferred to make the spoon warmer and the soup slightly colder but no energy left the closed system so the net change is 0.
What is kinetic energy?
Anything that is moving, it is transferred to an object when the object speeds up and away from it when it lows down. How much energy is in a store depends on the mass and the speed, the more mass and the faster it is means the more energy there is.
When is energy transferred?
When a system changes energy is transferred. It can be transferred away from or into the system, between different objects in the system or between different types of energy stores.
Describe the energy transfers that occur when the wind causes a windmill to spin?
the chemical energy of the wind is transferred to kinetic energy of the windmill spinning, which is transferred to electrical energy which an be transferred to anything e.g. light, sound and thermal.
How is energy transferred?
Energy is transferred mechanically by a force doing work. Electrically, is the work done by moving charges. By heating or by radiation.
A motor transfers 4.8kj of energy on 2 mins what is the power output ?
2.4 w
What is Elastic potential energy?
Stretching or squashing an object transfers energy to an elastic potential store.
Lubrication
Whenever something moves there is usually at least one frictional force working against it. this causes some energy in the system to be dissipated. For objects that are being rubbed together lubricants can be used to reduce the friction between objects’ surfaces when they move. They are usually liquids like oil so they can flow easily between objects and coat them.
What are the energy stores?
1.Thermal (internal)energy stores2.Kinetic energy stores3.Gravitational potential energy stores 4.Elastic potential energy stores 5.Chemical energy stores6.Magnetic energy stores 7.Electrostatic energy stores 8.Nuclear energy stores
There is always waste energy…
No device is 100% efficient and the water energy is usually transferred to useless thermal energy stores. Electric heaters are the only exception to this. Overall all energy ends up as thermal.
Cavity walls
Cavity walls are made up of inner and outer walls with an air gap in the middle. The air gap reduces the amount of energy transferred by conduction through the walls. cavity wall installation is where the cavity wall air gap is filled with foam, this can also reduce energy transferred by convection.
Loft insulation
This can reduce convection current’s ( a cycle of particles constantly being heated rising cooling then sinking )being created in lofts.
Falling objects transfer energy…
Because the gravitational potential energy turns into kinetic energy. The energy lost in a GPE store is gained in the KE store. In real life energy would be lost because of air resistance like thermal.
Gravitational Potential
Raising an object creates gravitational potential energy, the higher, heavier or the strength of the gravitational field means the more energy it stores.
A 2.0kg object is dropped from a height of 10m. Calculate the speed of the object after it has fallen 5.0m, assuming there is no air resistance. Give your answer to two significant figures.
14 m/s
SHC practical
- Block of material with 2 holes for heater and thermometer(heater made by power supply and ammeter)2.measure block and wrap it in insulation. 3. Measure initial temperature turn on power supply at 10v4. Electric energy from power supply is transferred to heat. 5. Take readings of the current and the temperature every minute for 10 minutes 6. After 10 mins turn off power supply. using measurement of current and potential difference of power supply, calculate power supplied to the heater using p=v1. U can use this to find how much energy has been transferred to heater at time if each temperature reading using E= Pt. where t is time dice experiment began. 7. If all energy from heater went in to block then u can plot a graph of energy transferred to thermal store of block against temperature. 8. Find gradient of straight part of graph. This is /\Ø ➗ /\ E. So the SHC od the material is 1X (gradient ➗mass of block.
Conduction and convection can create heating.
When an object is heated energy is transferred to the kinetic stores of its particles. This causes them to vibrate more and collide, which transfers there kinetic energy this is called conduction. Materials that are most conductive have faster moving particles. If the particles are free to move like a gas or liquid the particles moving faster means that the space between individual individual particles increases. this causes the density of the region being heated to decrease. The warmer regions will rise above cooler and less dense regions. So energetic particles move from hotter to cooler regions. This is called convection.
Efficiency
Useful devices can transfer energy. Some is usually wasted. The less energy wasted the more efficient the device. U can improve efficiency by insulation, lubrication or making it more streamlined.
Find the Final temperature of 5kg of water, that an initial temperature of 5°C after 50 kJ of energy has been transferred to it. the specific keep capacity of the water is 4200kJ/Kg°c
.
A 2500 kg car is travelling 20 m/s how much energy does it have?
500000J
What is work done?
Energy transferred. It can be done when a current flows or by a force moving an object.
Specific heat capacities
More thermal energy is required to heat some objects then others. Materials that need more energy to heat up can hold a lot of energy. Specific heat capacity is the amount of energy needed to raise 1kg of a material by 1 °c.
Power is….
The rate of energy transfer, or the rate if doing work. Power is measured in watts. 1 watt = 1 joule transferred per second. A powerful machine can transfer lots of energy in a short time not just a strong force.
Energy can be transferred by heating by…
If the kettle and water are both systems then energy is transferred to the kettle via electrical energy then energy is given to the water by heat which is stored in the waters thermal energy stores.
What is the formula for efficiency?
Useful output energy transferEfficiency = —————————— Total input energy transfer Useful power output Efficiency =——————————— Total power input
Draught excluders
Put around doors and windows to reduce energy transfers by convection.
How to prevent houses from losing heat?
Thick walls made of low thermal conductivity material.
How does the initial force exerted by a person throwing a ball upwards do work?
It transfers from the chemical energy in the persons arm to the kinetic energy of the ball and arm then gravitational energy as it goes further up.
Double glazed windows
They work in the same way as cavity walls, they have an air gap between two sheets of glass to prevent energy transfer by conduction through the windows.
System
A single object or a group of objects
Closed systems
Closed systems are systems where neither mass nor energy can enter or leave. The net change in the total energy of a closed system is always zero
