energy transfer. what’s the difference between temperature & heat?? temperature is the measure...

Energy Transfer

What’s the difference between temperature & heat??

Temperature is the measure of the average kinetic energy of the particles in a substance.

Temperature unit of measurement is the Kelvin

(but you will see Celsius used)

a. K = C + 273 (10C = 283K)

b. C = K – 273 (10K = -263C)Thermal Energy – the total of all the kinetic and potential energy of all the particles in a substance.

Heat

• It is measured in joules

• Common usage is a heat unit called the calorie (the energy needed to raise the temperature of 1 gram of water by 1o C)

• Chemists use the kilocalorie (1000 calories=4200 Joules)

• Nutritionists call it a food Calorie

Thermal energy relationships

As temperature increases, so does thermal energy (because the kinetic energy of the particles increased).

Even if the temperature doesn’t change, the thermal energy in a more massive substance is higher (because it is a total measure of energy).

Heat

The flow of thermal energy from one object to another.

Heat always flows from warmer to cooler objects. Ice gets

warmer while hand gets

cooler

Cup gets cooler while hand gets

warmer

Heat Transfer• Heat flows from hot to cold.

– If you hold something cold, heat flows from hand to object.

– If you hold something hot, heat flows from object to hand

• Conduction- transfer of thermal energy through matter by the direct contact of particles – Occurs because particles are in constant motion

Conduction• Heating of metal pan-

– Particles in handle of pan move slowly– Fast moving particles from the bottom bump into

slower particles and speed them up– Occurs until all particles move the same speed

• Conduction works best in solids- especially metals- because particles are close together

Conduction and Convection• Metals- good conductors-because electrons move

easily • Fluid- any materials that flows• Convection- transfer of energy in a fluid by the

movement of heated particles • Convection currents transfer heat from warmer to

cooler parts of a fluid.• Convection vs. Conduction-

– Conduction involves collisions and transfers of energy. – Convection involves movement of the energetic particles

from one location to another

Convection• Convection- results in changes in density

– As particles move faster, they get farther apart– Fluid expands as temperature increases– Larger volume = smaller density– Decreasing density results in the rise of the warmer

fluid

• Lava Lamp- – Cool oil = dense = sits on the bottom– Warmer oil = less dense than alcohol & rises– As it rises, it loses energy through conduction

• Causes decrease in density = sinking

• lava lamps!

• When oil is cool

Oil is Oil is warm, so warm, so it risesit rises

Oil starts to lose heat by

conduction and falls

Convection Currents• Currents in which warm portions of the fluid

move through the substance- convection

• The warm portions transfer energy to the cool section through conduction

Heat Transfer on Earth• At equator- earth experiences the most heat from the

sun. – Result: evaporation of water and large accumulations of

clouds.

– As the water vapor rises, it cools and condenses, forming rain

• After the rain = dry air– Dry air causes moisture to evaporate, drying out the ground

– causes desert

• Convection currents create deserts and rain forests over different regions of Earth

Radiation• Transfer of heat to the earth – occurs through

radiation

• Radiation- the transfer of energy by electromagnetic waves. The waves travel through space even without matter

Specific Heat

Some things heat up or cool down faster than others.

Land heats up and cools down faster than water

Specific heat is the amount of thermal energy required to raise the temperature of 1 kg of a substance one degree (C or K).

This is why land heats up quickly during the day and

cools quickly at night and why water takes longer.

Specific Heat

• The higher the specific heat, the more energy is required to cause a change in temperature.

• Water is slower to heat but is also slower to lose heat

Why does water have such a high specific heat?

Water molecules form strong bonds with each other; therefore it takes more heat energy to break them. Metals have weak bonds

and do not need as much energy to break them.

water metal

Expansion of Water

• Remarkably interesting case

Expansion of Water

• This is why lakes and ponds and rivers freeze with the ice on top

• If they didn’t, no aquatic life would be possible