Chp 9 Energy, Work Done, and Transfer of Energy

Energy

• Definition: Energy is defined as the capacity (or ability) to do work.

• SI Unit: Joule (J)

• Work in Science: Doing work involves the action of a force and movement of an object in the direction of the applied force.

What is Work Done?

• Definition: Work done (W) by a constant force on an object is the product of the force (F) applied and the distance (d) moved by the object in the direction of the force.

  • W=Fd

• SI Unit: The SI unit of work is the Joule (J).

  • 1 Joule is defined as the work done by a force of one Newton (N) which moves an object through a distance of one metre (m) in the direction of the force.

  • 1J=N divided m

Conditions for Work Done

• Work is done when:

  • The direction of the applied force and the direction of motion coincide.

• No Work Done when:

  1. The direction of the force is perpendicular to the direction of motion (e.g., carrying a load horizontally while walking).

  2. A force is applied but the object does not move (e.g., pushing against a solid object like a tree).

Negative Work

• Negative work occurs when a force acts against the motion of an object, hindering its displacement.

  • Example: A car skidding to a stop.

Types of Energy

• Kinetic Energy (KE): Energy due to motion.

  • Formula: KE=0.5mv square

  • mass (kg), v = speed (m/s).

• Gravitational Potential Energy (GPE): Energy due to position above the ground.

  • Formula: GPE = mgh where m = mass (kg), g = acceleration due to gravity (9.8 m/s²), h = height (m).

• Elastic Potential Energy: Energy stored as a result of deformation (stretch/compression) of an elastic object.

• Chemical Potential Energy: Energy stored in food and chemicals.

• Thermal Energy: Energy transferred due to temperature differences.

• Light Energy: Energy emitted as electromagnetic waves.

• Sound Energy: Energy associated with the vibration of matter.

• Electrical Energy: Energy produced by electric charges.

Law of Conservation of Energy

• Principle: In a closed system, energy cannot be created or destroyed, only converted from one form to another. Total energy remains constant.

• Examples of conversions:

  • Hydro-electric power: Gravitational potential energy (GPE) converts to kinetic energy (KE) of falling water, then to electrical energy.

  • A pendulum converts potential energy to kinetic energy and vice versa.

Applications of Energy

• Renewable Energy Sources (e.g., solar, wind)

  • Solar energy converts to electrical energy via photovoltaic cells.

  • Wind energy harnesses kinetic energy to generate electricity.

• Non-renewable Energy Sources (e.g., fossil fuels)

  • Chemical potential energy from fuels is converted to thermal energy to generate electricity.

  • Fossil fuels are finite and contribute to pollution.

Problem Solving with Energy

• Use the conservation of energy principle to solve problems involving the conversion and transfer of energy, especially in closed systems.

• Example Problem: A box is lifted, find the work done.

  • Calculate using GPE formula when lifting against gravity.