Temperature & Heat PP notes

Temperature and Kinetic Energy

Temperature is the average kinetic energy of moving atoms. The minimum temperature occurs when kinetic energy goes to zero. This is the basis of the Kelvin scale, where $0 K = -273.15°C$. The size of one Kelvin is the same as one degree Celsius, just offset by 273.

Heat and Energy Transfer

When objects of different temperatures come into contact, collisions between atoms cause an averaging out of kinetic energy. Energy flows from hot to cold. Heat is the flow of energy, not something objects contain. Objects contain internal energy, not heat itself.

Heat and Temperature Change

Heat causes a change in temperature, described by the equation:

$Q = cm\Delta T$

Where:

• $Q$ is heat.

• $c$ is specific heat capacity.

• $m$ is mass.

• $\Delta T$ is the change in temperature.

A calorie is the heat required to increase the temperature of one gram of water by one degree Celsius. The specific heat capacity of water is:

$c<em>{H</em>2O} = 1 \frac{cal}{g°C}$

Also,

$1 cal = 4.184 J$

A food Calorie (with a capital C) would raise a kilogram of water by one degree Celsius.

$1 Calorie = 1000 cal$

Thermal Inertia

Thermal inertia is the resistance to change in temperature. Water has a high heat capacity.

Islands have stable temperatures day and night because of the surrounding water. Deserts, with very little water, experience high temperatures during the day and very cold temperatures at night because the same energy from the Sun causes a greater temperature change.

Thermal Expansion

Most things become larger when heated due to thermal expansion. Bridges use expansion joints to allow for size changes as they warm during the day and cool at night.

A bimetallic strip consists of two different metals sandwiched together; because they expand at different rates, the strip bends when heated.

Water's Density and Freezing

Near the freezing point, some water molecules start to crystallize. Water is most dense at $4°C$. It expands when cooled from $4°C$ to $0°C$. As $10°C$ water cools, it becomes more dense and sinks to the bottom, circulating until all the water is $4°C$. Afterward, cooler water is less dense and stays at the top, where it can start to freeze.

Types of Heat Transfer

• Conduction: Heat transfer through contact between objects.

• Convection: Heat transfer through the motion of a fluid, usually due to differences in density.

• Radiation: Heat transfer via light.

Conduction Details

Conduction involves energy flow through a material. Conductors have free electrons that can transfer energy quickly, while insulators have electrons held in place, so heat flows slowly.

Standing on tile versus wood: Tile feels colder because it transfers heat away from your foot more quickly.

Air is a good insulator because its far-apart atoms can’t transfer energy through the air quickly.

Convection Details

Convection involves moving fluid.

An oven's heating element heats air near the element; heated air is less dense and rises. Convection ovens use a fan to mix air for more consistent heating.

Hot air rises.

Insufficient insulation can cause melting snow, leading to icicles.

Temperature Inversion

Normally, hot air near the ground rises because it is less dense. Lower pressure causes the air to expand as it rises, and this expansion causes a decrease in temperature.

A temperature inversion occurs when warm air above the ground prevents convection.

Radiation Details

Radiation is light emitted due to temperature. At low temperatures, the emitted light isn't visible (infrared). As temperature increases, an object will first glow with a red color.

When producing red, green, and blue light, the object will glow white.

The average frequency of light is proportional to temperature.

Applications of Radiation

A car parked at a mall on a summer day: Visible light enters the car, but infrared light can’t get back out, trapping heat inside.

A greenhouse uses this effect to keep plants warm.

The greenhouse effect occurs when chemicals in the atmosphere trap energy within the atmosphere.

Newton's Law of Cooling

Newton’s law of cooling states that the rate of cooling is proportional to the difference in temperature.

$Rate of cooling \propto \Delta T$

This applies when one thing is heating as another is cooling.

Igloos and Thermoses

An igloo is a domed structure made out of snow. Snow has many air pockets, making it a good insulator and reflector, which doesn’t absorb much radiation. Snow can fill in gaps, making it easy to have a sealed structure and reducing convection.

A thermos is a double-walled glass container with a silver lining.

• The vacuum prevents conduction and convection.

• The silver lining minimizes radiation.

• A double-walled plastic or steel container works well for keeping drinks cold.

Radiative Cooling and Dew

The radiative cooling of grass can cause the grass temperature to get lower than the surrounding air. The dew point is the temperature at which water in the air will start to condense.

Frost occurs when moisture in the air deposits as solid ice instead of liquid water.