Forces & Energy – Density, Heat, Energy Transfer, Evaporation

Vocabulary flashcards summarising key terms and definitions from the lecture notes on density, heat vs temperature, energy conservation, thermal energy transfer (conduction, convection, radiation), and cooling by evaporation.

Density

Mass per unit volume; calculated as mass ÷ volume (e.g., g/cm³ or kg/m³).

Volume (scientific)

The amount of space an object occupies, measured in units such as cm³ or m³.

Mass

The quantity of matter in an object, measured in grams (g) or kilograms (kg).

Regular Object

A solid whose faces meet at right angles, allowing volume to be found by multiplying its side lengths.

Irregular Object

A solid with unequal sides or angles; its volume is found by water-displacement in a measuring cylinder.

Hollow

Containing an internal space filled with air; the opposite of solid (full).

Solid (full)

Containing no internal air space; completely filled with matter.

Displacement Method

Technique for finding volume of an irregular object by measuring the rise in water level after immersion.

Formula Triangle (Density)

A visual aid showing density = mass ÷ volume, mass = density × volume, volume = mass ÷ density.

g/cm³

Common unit of density when mass is in grams and volume in cubic centimetres.

kg/m³

SI unit of density when mass is in kilograms and volume in cubic metres.

Average Density

Total mass of an object divided by its total volume, including air spaces (e.g., a ship’s overall density).

Float

To stay on the surface of water because the object’s density is less than 1.0 g/cm³.

Sink

To descend in water because the object’s density is greater than 1.0 g/cm³.

Heat (Thermal Energy)

Total energy of vibrating particles in an object, measured in joules (J).

Temperature

Indicates direction of thermal energy transfer and the average particle energy in a substance.

Absolute Zero

Theoretical lowest temperature (–273 °C) where particle motion stops; 0 K on Kelvin scale.

Kelvin (scientist)

Proposed the concept of absolute zero and the Kelvin temperature scale.

Conservation of Energy

Physical law stating energy cannot be created or destroyed, only changed or transferred.

Energy Input

Total energy supplied to a system (e.g., electrical energy entering a lamp).

Energy Output

Total energy leaving a system, including useful and wasted forms (e.g., light + thermal).

Conserved (energy)

When total energy remains the same before and after a process.

Created (energy)

To bring energy into existence—impossible by the conservation law.

Destroyed (energy)

To eliminate energy—also impossible; energy only changes form.

Dissipated Energy

Energy that spreads out into surroundings and becomes less useful, often as heat.

System (physics)

A chosen part of the universe for study of energy changes (e.g., an electric lamp).

Conduction

Transfer of thermal energy by particle vibration collisions, most effective in solids, especially metals.

Conductor (thermal)

Material that conducts heat well (e.g., metals).

Insulator (thermal)

Material that conducts heat poorly, slowing energy transfer (e.g., wool, plastic, foam).

Convection

Transfer of thermal energy by movement of warmer, less dense fluid rising and cooler fluid sinking.

Convection Current

Circular flow pattern in a fluid caused by heating, responsible for distributing heat evenly.

Radiation (thermal)

Transfer of thermal energy by electromagnetic waves; needs no medium and travels through vacuum.

Emit

To give out thermal radiation.

Absorb (radiation)

To take in thermal radiation from hotter objects.

Vigorously

With more energy, speed, and force; describes faster particle vibration when heated.

Expand

Increase in size as particles vibrate faster and take up more space when heated.

Vacuum

Region with no particles; conduction and convection cannot occur, but radiation can pass through.

Black, Dull Surface

Best emitter and absorber of thermal radiation due to color and texture.

Shiny, White / Silver Surface

Poor emitter and absorber; reflects thermal radiation effectively.

Evaporation

Change of state from liquid to gas at temperatures below boiling point when high-energy particles escape.

Boiling

Rapid change from liquid to gas at a fixed temperature where vapor forms throughout the liquid (100 °C for water).

Cooling by Evaporation

Process where escaping high-energy particles lower the average energy, reducing temperature of remaining liquid.

Random (motion)

Unpredictable particle movement directions and speeds in a liquid or gas.

Porous

Containing tiny holes that allow liquid to seep through (e.g., clay water cooler walls).

Perfume

Liquid with weak intermolecular forces that evaporates quickly, producing a cooling sensation.

Liquid Soap

Liquid with stronger particle forces; evaporates slowly, causing less cooling than water.

Conduction Insulation (Vacuum Flask)

Use of a vacuum layer to prevent particle vibration transfer and a silver surface to reflect radiation.

Double-Glazed Window

Two glass panes separated by argon gas layer to reduce conduction and convection heat loss.

Conduction Example – Saucepan

Metal pan conducts heat to food, while plastic or wooden handle insulates user’s hand.

Convection Example – Room Heater

Heater warms air; rising warm air and sinking cool air form convection currents heating the room.

Radiation Example – Sunlight

Thermal energy reaches Earth via radiation, travelling through space’s vacuum in ~9 minutes.

Humidity Effect

High water-vapour content in air slows sweat evaporation, reducing body’s cooling ability.

Helium Gas

Very low-density gas (0.00018 g/cm³) causing balloons to float in air.