Conservation of Energy
The Conservation of Energy is one of the most important laws in all of Science, not just Physics! It states that: __“Energy cannot be created or destroyed, only transferred from one store to another”. __Remember this!!
Using this law we can answer the following:
If 100J of chemical energy from petrol in car is transferred to 80J of kinetic energy, how much energy is wasted?
The answer is 100J - 80J = 20J
This must be the answer since no energy is destroyed. In this example, the 20J of energy is thermal energy which is released to the surroundings.
- Calculate the efficiency of a device that uses a total of 400J compared to the 800J it was supplied with
- What are the energy changes involved as a car accelerates forwards and what is the wasted energy?
- Your answer should include: chemical to kinetic / thermal
- What is the efficiency of a student who does there 30 min homework in 50 mins?
- Your answer should include: 67% / 66.7%
Since energy is always conserved it means that we can always show and/or predict the changes in the energy stores. Let’s look at some examples:
- An object projected upwards or up a slope: Kinetic to Gravitational
- A moving object hitting an obstacle: Kinetic to Elastic
- An object being accelerated by a constant force: Chemical __to __Kinetic
- A vehicle slowing down: Kinetic to Thermal
- Bringing water to a boil in an electric kettle: Chemical to Thermal
There is no system which is 100% efficient. Energy is always wasted somewhere along the line.
When this happens the energy dissipates to the surroundings, meaning that it spreads out from the object. The most common wasted energy is thermal energy. As such, we can say that a rise in temperature around the object is caused by thermal energy dissipating from the object.
Efficiency of a system can be improved by a variety of ways. The key part is trying to limit the amount of energy that is wasted.
- Lubricating two objects that are moving past each other (like a piston) to reduce friction.
- Insulating __a house to prevent __thermal energy from escaping to the surroundings by keeping it inside.
As described above, insulating a house by having thicker walls helps to improve a home’s efficiency rating. The thick walls help to reduce wasted energy.