Conservation of Energy
In a closed system, where no energy is added and none can escape, the total energy in that system must remain constant.
When energy is converted from one form to another, the total energy before the change has to equal the total of all energies after the change. It is not possible to create or destroy energy.
All cars are very similar in their energy transfer.
A fuel (petrol) is transferred into other energy types
The total energy used by a car on a journey from the petrol in its tank has to exactly equal the totals for the Kinetic, the light, the sound and the heat produced.
When a car brakes, the total kinetic energy it had is transferred into the heat energy from the friction of the brakes and the heat energy of the friction of the grip of the tyre on the road. The total Heat = the total Kinetic.
When you look closely at energy transfers and follow them to their conclusion, all energy changes eventual end with Heat. The movement of the car ends in heat when the car brakes and stops. The Light and sound will eventually be absorbed by atoms to make them move faster, thus increasing their thermal (heat) energy levels.
Examples of Energy Conservation
Example 1: A ball thrown upwards.
As the ball moves initially it has kinetic energy, as it climbs higher gravity is slowing it down so its kinetic energy is decreasing and being transferred mainly into Gravitational Potential Energy. If there were no air resistance then this transfer would be 100% efficient and all the kinetic energy would become GPE at the top of the climb.
In reality, some kinetic energy is lost to heat due to the friction between the ball and the air.
Note: In exam questions, the energy loss due to friction is ignored so the loss in kinetic energy is equal to the gain in GPE. The reverse is true for an object descending from a height.
Δ_KE = Δ_GPE
Example 2: A car crashing into a wall.
Initially, the car has kinetic energy, but that is lost in the collision. The collision transfers that energy in heat energy, which is then quickly dispersed into the air and surrounding materials.
Kinetic to Heat
Example 3: A jet plane on its take-off run along the runway.
When a constant unbalanced force is applied to an object it will accelerate. Initially, the energy is in the form of the fuel, as chemical energy, this is transferred to the kinetic energy of the air being pushed from the jet engine. Due to Newton’s Third Law of Motion (action and reactions), this produced a force on the plane which then moves. The plane is gaining Kinetic energy.
In common with all energy changes, there is a loss of energy as heat, but overall the energy stored in the fuel is converted to the kinetic energy of the plane.
Chemical to Kinetic
Example 4: A Vehicle braking.
When a vehicle brakes it is reducing its velocity, this reduced the vehicle’s kinetic energy. The kinetic energy is being transferred into heat energy by the brakes.
Kinetic to Heat
Example 5: Boil Water in a Kettle.
The initial energy source is the Electrical energy, this is transferred into heat energy by the element in the kettle. Some energy is lost to the heat the warms the kettle itself and in most modern kettles, some to light for the indicators etc that are fitted to the kettle.
Electrical to Heat
In almost all situations where there is an energy transfer, some energy will be lost as waste energy. This is energy that is converted into a type that is not useful for the purpose of the energy transfer.
A light bulb, for example, in an ideal world 100% of the electrical energy would be transferred into Light energy. In reality, some will be transferred into Heat too. The heat is the waste energy as it is not required for the bulb to perform its function.
When you are cycling you want as much of the kinetic energy from the movement of your legs to become the kinetic energy of the bike (and you). As you turn the pedals this turns the cogs, chain and gears that eventually turn the wheels and get you moving. At every point where there is contact between moving parts, there is friction. Friction always produced some heat as waste energy. (Try rubbing your hand on the desk, it will get warm.)
To reduce the heating effect of friction, oil is added to the moving parts as a lubricant. The same is true inside the engine of a vehicle. We add oil to lubricate the moving parts. Even humans have lubricants in their joints to reduce the friction, the loss of this causes pain and conditions like arthritis.
The dissipation of energy as heat to the environment is always going to occur. In some situation, this is intentional, when we pull the brakes on a bike for example. Here we want the kinetic energy to transfer to heat and dissipate.
When we are heating a house or pedaling hard on a bike, the dissipation of energy to heat into the air and other surrounding materials such as the road surface are a problem and we try to find ways to reduce this loss of energy.
Energy Conservation in Buildings
All new homes and any home up for sale has to declare how energy efficient it is.
One of the key factors in this will be the insulation of the walls, windows and the roof.
In this example, the house is rated as F, not very good. With a few changes, however, it could be a C - better but not perfect.
When a house is heated the energy heats up the air in the room and the materials of the house itself, the walls and windows. This allows heat to move from inside the house to the outside and dissipate into the air by thermal conduction.
The thermal property of the material affects how quickly this conduction will happen. Materials, such as brick and wood are relatively poor conductors of heat. Glass and metal are good conductors of heat.
A caravan, for example, is mainly constructed of metal walls and some glass for the windows. This makes them hard to keep warm as the heat can easily pass from the inside to the atmosphere.
A conventional house has two layers of brick, ideally filled with a foam to act as an insulator.
The gap (called a cavity wall) has trapped air in it because air is actually a poor conductor of heat, it slows down the escape of heat from the house to the outside. The foam traps more air and prevents it from moving inside the cavity, this improves the thermal insulating properties of the wall.
Double glazing for a window also traps a layer of air and this acts as an insulator, in some windows the air has been replaced with other gases that have even lower thermal conductivity.
The thickness of the bricks or other building materials will increase the time it takes for the heat to be conducted through the material.
Materials like breeze blocks are thick and have trapped in them. This makes them good insulators, and is one of the reasons they are commonly used to build houses.
The thicker a material is and the more air it traps the better it is as an insulator. That why you pick a thick padded jacket to wear on a cold day, its thick and the padding traps air to stop your body heat escaping into the atmosphere.
- State three ways a house can be constructed to improve its thermal insulating properties to reduce heat loss.
- Your answer should include: cavity walls / breeze blocks / double glazing / windows
Explanation: Cavity walls, foam added to the cavity walls, Use of thick materials that trap air such as breeze blocks, double glazing the windows.
- When a motor bike is travelling along the road, suggest two ways heat dissipation happens, one that is useful and one that is wasteful.
- Your answer should include: friction / breaks / moving / part / engine
Explanation: Friction on the brakes to slow down, friction with the moving part of the engine or the chain and gears (drive system).
- Describe the energy changes that occur as plane takes off and climbs away from the runway.
- gravitational potential energy
Explanation: Chemical energy to kinetic energy of the plane to gravitational potential energy of the plane as it climbs.