1. HW!
Energy Mind map…

2. Energy – the ability to do work
- measured in joules (J)

3. Difference Between Energy Transfer & Source
Energy transfer – energy changing from one form to another
Energy source – anything that can give useful energy

4. Sources of Energy Chemical energy Electrical energy Nuclear energy
Energy which is produced as a result of a chemical reaction.
Electrical energy
Energy due to electrical charges moving.
Nuclear energy
Energy due to changes in the nuclei of atoms.
Sound energy
Energy carried by sound waves.

5. Elastic potential (strain) energy
Thermal (heat) energy
Energy due to fast-moving particles in hot objects.
Elastic potential (strain) energy
Energy stored in materials that have been bent or stretched.
Gravitational potential energy
Energy stored in materials that have been moved upwards so that they can now fall down.
Kinetic energy
Energy in objects that are moving.
Radiated energy
Energy carried as electromagnetic waves.

6. Energy calculations Gravitational potential energy = weight x height
Ep = mg x h
e.g. An athlete of mass 50kg runs up a hill. The foot of the hill is 400m above sea-level. The summit is 1200m above sea-level.
By how much does the athlete’s Ep increase? (g = 10m/s2)
Assume Ep is zero at the foot of the hill.
Calculate the increase in height:
h = 1200m – 400m = 800m
= 50kg x 10 m/s2 x 800m
= J (=400kJ)
So the athlete's Ep increases by 400kJ

7. Kinetic energy = ½ x mass x speed2
Ek = ½ mv2
Example: A van of mass 2000kg is travelling at 10m/s. Calculate its kinetic energy. If its speed increases to 20m/s, by how much does its kinetic energy increase?
Step 1: Ek = ½ mv2
= ½ x 2000kg x (10m/s)2
= J (= 100kJ)
Step 2: The van’s Ek at 20 m/s
Ek = ½ mv2
= ½ x 200kg x (20m/s)2
= J (= 400kJ)
Step 3: Change in the van’s Ek = 400kJ – 100kJ
= 300kJ
So the van’s Ek increases by 300kJ when it speeds up from 10m/s to 20m/s

8. The Law of Energy Conservation
Energy must come from somewhere – it is never created out of nothing.
Energy may change form but the amount is constant
Energy can never be destroyed, only transformed.
For example, what is the chemical energy being changed into in this running tiger?
Kinetic, sound and heat energy

9. Energy changes
For the following examples, identify the energy input (energy source) and the energy outputs (types of energy produced):
For example, a match that has been lit:
Energy input Energy output
Chemical potential energy  heat energy + light energy
A light you have just turned on in your house.
A tree burning
A sky rocket (fireworks) being let off
A car traveling up a hill

10. What is the impact Velocity?
dropped from rest V?

11. Energy efficiency
The efficiency of an energy conversion is the fraction of the energy that ends up in the desired form.
Below is a Sankey diagram for a filament lamp

12. Calculating efficiency
Efficiency = useful energy output x 100 energy input e.g. A filament lamp is supplied with 100J of electrical energy and it produces 10J of useful light energy. Calculate its efficiency. Efficiency = 10/100 x 100 = 10%

13. Questions
Calculate the efficiency of an energy-efficient lamp that gives out 15J light energy when supplied with 25J of electrical energy.
A coal-fired power station produces 100MJ of electrical energy when it is supplied with 400MJ of energy from its fuel. Calculate its efficiency.
A lamp is 10% efficient. How much electrical energy must be supplied to the lamp each second if it produces 20J of light energy per second.

14. Answers Efficiency = 15/25 x 100 = 60% (2) Efficiency = 100/400 x 100
= 25%
(3) Electrical (input) energy
= useful energy/(efficiency/100)
= 200J