1.1.2 Energy Transfer & Work Done

• Energy can be transferred between energy stores in one of the following ways: Mechanical Work – when a force moves an object at a distance.

Heating – when energy is transferred to a colder object from a hotter object.

Electrical Work – when energy is transferred by a moving charge due to a potential difference.

Radiation – when energy is transferred as an electromagnetic wave e.g. gamma-rays or UV radiation

Examples:

Energy Transfers

Pendulum - The Law of Conservation of Energy

System Definition

• System: An object or a group of objects.

  • Example: Pendulum
  • Components:
  • Fixed point
  • String
  • Mass
  • Air particles

• Closed System: No energy can enter or leave.

Energy Transfer in a Pendulum

• Pendulum Motion:
  • At the highest points: Maximum Gravitational Potential Energy (GPE)
  • At the lowest point: Maximum Kinetic Energy (KE)

Friction in the Pendulum

• Sources of Friction:

• Friction in the fixed point

• Friction as the pendulum passes through air particles

  

• Effect of Friction:

  • Energy transferred to thermal energy
  • Fixed point and air around the pendulum become warmer
  • Thermal energy is less useful
  • Energy dissipation causes the pendulum to swing with less energy and eventually stop

Reducing Unwanted Energy Transfers

Bungee Jumper

Energy Transfers During a Bungee Jump

• At the start of the jump:
• All energy in the system is the store of gravitational potential energy (GPE).

• As the jumper falls:

• Energy is transferred from the GPE store to the kinetic energy (KE) store.

• When the bungee rope just starts to tighten:

• The kinetic energy store is now at its maximum.

Maximum Kinetic Energy

• When the rope is fully extended:
• The KE store is zero.
• The jumper is not moving.
• All energy has been transferred to the elastic potential energy (EPE) store.

Elastic Potential Energy

• As the rope recoils:
• Energy is transferred from the EPE store back to the KE store.
• During the ascent:
• Energy transfers from the KE store back to the GPE store.
• At the top of the ascent:
• All of the energy is now in the GPE store.

Bungee Jump

Energy Dissipation

• Observation: The jumper never returns back to its original position.

Reasons for Energy Dissipation

• Energy is dissipated as thermal energy due to:
  • Friction with air particles
  • Stretching effects in the bungee rope, which is not fully elastic

Work Done by a Force

Formula

$$\text{Work done (J)} = \text{Force (N)} \times \text{Distance (m)}$$

Example