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Energy Forms and Conservation

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Energy Forms and Conservation

Overview

Energy is one of the most important unifying ideas in physics. It allows many motion and system problems to be solved without tracking every force at every instant.

This page develops:

  • common forms of energy relevant to H2 Physics
  • transfer of energy between stores/forms
  • conservation of energy
  • mechanical energy conservation
  • losses and dissipation
  • choosing system boundaries

This page supports the main hub: Work, Energy and Power

Why It Matters

Energy conservation gives a system-wide method for solving problems where force details may be complicated or unnecessary.

Definition

Energy is a scalar quantity that measures the capacity of a system to produce change through work, heating, radiation, or other transfer processes. The SI unit is the joule.

Common forms in JC Physics include kinetic energy, gravitational potential energy, elastic potential energy, electric potential energy, internal energy, chemical energy, and thermal energy.

Key Representations

Scalar and Vector Distinction

Be precise.

Vector Quantities

  • Force:
  • Displacement:
  • Velocity:

Scalar Quantities

  • Energy:
  • Work:
  • Power:
  • Mass
  • Speed
  • Height

Energy is always treated as a scalar quantity in this chapter.

What Is Energy?

Energy is the capacity of a system to do work or to cause change.

Energy may be:

  • stored
  • transferred
  • transformed from one form to another

Energy is not “used up”; it is transferred or converted.

Forms of Energy (H2 Relevant)

Mechanical Forms

Kinetic Energy

Energy due to motion:

Gravitational Potential Energy

Near Earth’s surface:

Elastic Potential Energy

For Hooke’s law spring:

Non-Mechanical Forms

Thermal / Internal Energy

Microscopic kinetic and potential energy of particles.

Electrical Energy

Transferred by moving charge in circuits.

See Current Electricity Fundamentals

Chemical Energy

Stored in fuels, batteries and food.

Light / Sound

Energy carried by waves.

Energy Transfer vs Energy Store

This distinction helps conceptual clarity.

Energy Stores

Where energy is held:

  • kinetic store
  • gravitational store
  • elastic store
  • thermal store
  • chemical store

Energy Transfer Pathways

How energy moves:

  • mechanical work
  • heating
  • electrical transfer
  • radiation

Principle of Conservation of Energy

Energy cannot be created or destroyed.

It can only be:

  • transferred
  • transformed
  • redistributed

For an isolated system:

Mechanical Energy Conservation

Mechanical energy is:

If only conservative forces act:

Equivalent form:

Useful when friction and drag are negligible.

See Potential Energy and Conservative Forces

When Mechanical Energy Is Not Conserved

If non-conservative forces act, such as:

  • friction
  • drag
  • braking forces

then:

is not constant for the moving object alone.

Some energy transfers to:

  • thermal energy
  • sound
  • deformation
  • surroundings

But total energy of the wider system is still conserved.

Choosing System Boundaries

Very important in exam questions.

Narrow System

Consider only moving block:

Mechanical energy decreases if friction acts.

Wider System

Consider block + surface + surroundings:

Total energy remains constant.

Always decide:

What system am I analysing?

Typical Energy Conversion Chains

Falling Object

Gravitational potential → kinetic

Pendulum (ideal)

Gravitational potential ←> kinetic

Spring Launcher

Elastic potential → kinetic

Car Engine

Chemical → kinetic + thermal + sound

Light Bulb

Electrical → light + thermal

Worked Examples

Example 1: Falling Ball

A ball falls , neglect air resistance.

Loss in GPE:

Gain in KE:

Example 2: Rough Surface

A block with kinetic energy slides to rest.

Friction converts energy into thermal energy.

is transferred to internal energy of block + surface.

Example 3: Spring Compression

A moving trolley compresses a spring and stops.

Kinetic energy converts into elastic potential energy:

If losses negligible:

Example 4: Mixed System with Losses

A cyclist brakes while moving downhill.

Possible transfers:

  • loss in GPE
  • some KE change
  • thermal energy in brakes
  • sound

Need full energy accounting.

Useful Problem-Solving Method

Step 1: Identify System

Object only? Object + Earth? Whole machine?

Step 2: List Initial Energy Stores

Examples:

  • kinetic
  • gravitational
  • elastic

Step 3: List Final Stores

What changed?

Step 4: Account for Transfers/Losses

Friction? Heating? Sound?

Step 5: Write Equation

Example:

Relationship with Work

Work is a transfer of energy by force through displacement.

Positive work adds energy to system.

Negative work removes energy from system.

See Work, Energy and Power

Common Exam Pitfalls

1. Saying Energy Is Lost

Better phrasing:

Energy is transferred or dissipated.

2. Assuming Mechanical Energy Always Conserved

Only when non-conservative effects are negligible.

3. Ignoring System Boundary

Different systems give different bookkeeping.

4. Missing Thermal Energy

Friction usually means internal energy rise.

5. Using Wrong Height

Use vertical height difference for GPE change.

Summary

  • Energy is a scalar quantity.
  • Energy changes form but total energy is conserved.
  • Mechanical energy:

is conserved only when non-conservative effects are negligible.

  • Friction and drag transfer energy to thermal/internal stores.
  • Correct system boundary is essential.

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