Calculating Change in Thermal Energy: Formula & Examples

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  • 0:04 What Is Heat?
  • 0:30 Thermal Energy
  • 0:59 Kelvin Temperature Scale
  • 1:40 Specific Heat Capacity
  • 3:20 Sample Calculations
  • 4:39 Lesson Summary
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Lesson Transcript
Instructor: Stephanie Bryan

Stephanie has a master's degree in Physical Chemistry and teaches college level chemistry and physics.

In this video, we'll explore how kinetic energy gives rise to thermal energy and how the amount of thermal energy transferred to a substance can be calculated in situations where we have one phase changing temperature.

What Is Heat?

Does an object have to be hot to have heat? A physicist would say no. Ice cubes and even liquid nitrogen can contain heat. Heat is the measure of thermal energy being transferred from one object to another. It's designated with the variable, q. Because q is a measure of energy, we use energy units for this value. The most common energy units used are calories and Joules.

Thermal Energy

If heat is the transfer of thermal energy, what is thermal energy? Thermal energy is the kinetic energy of the atoms or molecules that make up a substance. These atoms and molecules move around by rotating, vibrating, and in the case of liquids and gases, even translating. The energy of motion, in general, is kinetic energy. When tiny particles, atoms, and molecules perform this kinetic energy, or motion, we call it thermal energy.

Kelvin Temperature Scale

The word 'thermal energy' probably sounds a lot like the word 'thermometer' to you. If so, good! The temperature of a substance, as measured by a thermometer, is actually the average thermal energy of the atoms or molecules in a substance. This is why the Kelvin temperature scale is so useful. At 0 Kelvin, there is no kinetic energy and no motion of the atoms in a substance. On the Kelvin temperature scale, as the temperature goes up from zero, the motion increases and thermal energy increases. Temperatures can be easily converted from Celsius to Kelvin using this relationship.

temperature conversion C to K

Specific Heat Capacity

As the temperature rises, the thermal energy of the substance increases. The opposite is also true. As the temperature decreases, the thermal energy of a substance decreases. The heat transferred, q, can be calculated as:

heat = m Cp deltaT

In this equation, Cp is the specific heat capacity of a substance, m is the mass, and ΔT is the change in temperature. Always calculate the change in temperature by subtracting the initial temperature from the final temperature, Tf - Ti . If the heat is transferred into the substance, the thermal energy of the substance will go up and so q will be positive. If heat is transferred out of the system, the thermal energy of the substance will go down and q will be negative.

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