Topic 1- Energy

Annotations:

• Energy is never used up. Instead it's just transferred between different stores and objects...

1. 1. Energy is transferred between stores

   Annotations:

   • When energy is transferred to an object, it is stored in one of the object's energy stores- 1. THERMAL (INTERNAL ENERGY) 2. KINETIC 3. GRAVITATIONAL POTENTIAL 4. ELASTIC POTENTIAL 5. CHEMICAL 6. MAGNETIC 7. ELECTROSTATIC 8. NUCLEAR
   • Energy is transferred MECHANICALLY (through a force doing work), ELECTRICALLY (work done by moving charges), by HEATING or by RADIATION.
2. 2. When a system changes, energy is transferred

   Annotations:

   • 1) A system is just a fancy word for a single object (e.g. the air in a piston) or a group of objects (e.g. two colliding vehicles) that you're interested in. 2) When a system changes, energy is transferred. It can be transferred into or away from the system, between different objects in the system or between different types of energy stores. 3) Closed systems are systems where neither matter nor energy can enter or leave. The net change in the total energy of a closed system is always zero.
3. 3. Energy can be transferred by heating...

   Annotations:

   • 1) Take the example of boiling water in a kettle —you can think of the water as the system. Energy is transferred to the water (from the kettle's heating element) by heating, into the water's thermal energy store (causing the temperature of the water to rise). 2) You could also think of the kettle's heating element and the water together as a two-object system. Energy is transferred electrically to the thermal energy store of the kettle's heating element, which transfers energy by heating to the water's thermal energy store.
4. 4. ...or by doing work

   Annotations:

   • 1) Work done is just another way of saying energy transferred —they're the same thing. 2) Work can be done when current flows (work is done against resistance in a circuit, or by a force moving an object).
   • The initial force exerted by a person to throw a ball upwards does work. It causes an energy transfer from the chemical energy store of the person's arm to the kinetic energy store of the ball and arm.
   • A ball dropped from a height is accelerated by gravity. The gravitational force does work. It causes energy to be transferred from the ball's gravitational potential energy store to its kinetic energy store.
   • The friction between a car's brakes and its wheels does work as it slows down. It causes an energy transfer from the wheels’ kinetic energy stores to the thermal energy store of the surroundings.
   • In a collision between a car and a stationary object, the normal contact force between the car and the object does work. It causes energy to be transferred from the car's kinetic energy store to other energy stores, e.g. the elastic potential and thermal energy stores of the object and the car body. Some energy might also be transferred away by sound waves.