Principle of Conservation of Energy (DP IB Physics): Revision Note

Principle of Conservation of Energy

• The Principle of conservation of energy states that:

  Energy cannot be created or destroyed, it can only be transferred from one form to another

• This means the total amount of energy in a closed system remains constant, although how much of each form there is may change

• In physics, a system is defined as:

  An object or group of objects

• Defining the system in physics is a way of narrowing the parameters to focus only on what is relevant to the situation being observed

• When a system is in equilibrium, nothing changes and so nothing happens

• When there is a change in a system, things happen, and when things happen, energy is transferred

Types of Energy

• Kinetic energy, gravitational potential energy, and elastic potential energy are collectively known as mechanical energy types

Energy Dissipation

• No energy transfer is 100% efficient

• When energy is transformed from one form to another, some of the energy is dissipated to the surroundings

• Dissipated energy usually ends up as thermal energy transferred to the surroundings where it cannot be easily used for another purpose 

• Therefore, dissipated energy is usually regarded as wasted energy

• A kettle transforms electrical energy into thermal energy

• The thermal energy in the heating element is transferred to thermal energy in the water

  • Some thermal energy is also transferred to the plastic casing

  • Some thermal energy is also dissipated to the surrounding air

• The energy transfers that are useful for heating the water are considered useful energy transfers

• The energy transfers that are not useful for heating the water are considered wasted energy transfers

Applications of Energy Conservation

• In mechanical systems, the energy transferred is equivalent to the work done

  • A falling object (in a vacuum, where no energy is not dissipated into the surroundings) transfers its gravitational potential energy into kinetic energy 

  • Horizontal mass on a spring transfers its elastic potential energy into kinetic energy

  • A battery or cell transfers its chemical energy into electrical energy

  • A car transfers chemical energy from the fuel into kinetic energy of the car

  • A person bouncing on a trampoline is transferring energy from elastic potential to kinetic to gravitational potential 
    

Energy transfers whilst jumping on a trampoline

• There may also be work done against resistive forces such as friction

• For example, if an object travels up a rough inclined surface, then

  Loss in kinetic energy = Gain in gravitational potential energy + Work done against friction

Spring Energy Conservation

• When a vertical spring oscillates, its energy is converted into other forms

• Although the total energy of the spring will remain constant, it will have changing amounts of:

  • Elastic potential energy (EPE)

  • Kinetic energy (KE)

  • Gravitational potential energy (GPE)

• At position A:

  • The spring has some EPE because it is slightly compressed

  • Its KE is zero because it is stationary

  • Its GPE is at a maximum because the mass is at its highest point

• At position B:

  • The spring has some EPE because it is slightly stretched

  • Its KE is at a maximum as it passes through the equilibrium position at its maximum speed

  • It has some GPE because the mass is still raised

• At position C:

  • The spring has its maximum EPE because the spring is at its maximum extension

  • Its KE is zero because it is stationary

  • Its GPE is at a minimum because the mass is at its lowest point 

• For a horizontal mass on a spring system, you do not need to consider the gravitational potential energy because this does not change