Energy Stores & Systems

• Energy is never used up
• Energy is transferred between different energy stores and different objects
• Energy stores include: thermal, kinetic, gravitational potential, elastic potential, chemical, magnetic, electrostatic, nulear
• Energy is transferred mechanically by a force doing work
• Energy is transferred electrically (work done by moving charges)
• Energy is transferred by heating or radiation e.g light or sound
• When a system changes, energy is transferred
• A system is a fancy word for a single object or a group of objects that you're interested in
• Energy can be transferred into or away from the system, between different objects in the system or between different types of energy stores
• Closed systems are systems where neither matter nor energy can enter or leave
• The net energy in the total energy of a closed system is always 0
• e.g boiling water in a kettle, 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 (increasing the temperature)
• e.g boiling water in a kettle, think of the water as the system - You could also think of the kettle's heating element and the water together as a 2-object system.
• e.g boiling water in a kettle, think of the water as the 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
• Work done is another way to say energy transferred
• Work can be done when current flows (work is done against resistance) 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 of the ball and arm
• A ball dropped from a height is accelerated by gravity. The gravitational force does work
• When a ball is dropped from a height 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 wheels does work as it slowes down. It causes an energy transfer from the wheel's 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
• In a collision between a car and a stationary object, the car's kinetic energy store is transferred to other energy stores such as elastic potential and thermal energy stores of an object and the car body
• In a collision between a car and a stationary object, some energy might also be transferred away by sound waves