- Heat Flow -


(The first law of thermodynamics)
When you put a hot object in contact with a cold one, heat will flow from the warmer to the cooler. As a result, the warmer one will usually cool down and the cooler one will usually warm up. Eventually, they will reach the same temperature and heat flow will stop.

Figure P3
Heat Flow

A Model of Heat Flow
When two objects are in contact, heat flows from the hotter object to the cooler. You will find it helpful to have a mental model of heat transfer when two objects are in contact. Think about the atoms in the hot and cold substances.

Figure P4
Cool and Warm

Imagine a bunch of hot atoms in violent motion next to a bunch of cool atoms that move less. Heat flows because hotter atoms, which have a greater random motion, hit cooler ones. When they collide, some of the energy of a hotter atom is transferred to a cooler one, speeding it up. This simplified model can help you remember the idea, but it is not entirely accurate. Actually, atoms do not have to collide to exchange thermal energy. Each atom has an electric and magnetic field around it and these fields can transfer energy. Metals conduct electricity because they have free electrons that bounce around like a gas; this electron gas interacts with atoms and can transfer energy, too.

Factors Influencing Heat Flow
We can develop an equation for the rate of heat transfer. Think about all the different kinds of factors that might influence the flow of heat. Imagine a material sandwiched between objects that are hot and cold. The amount of heat that will flow depends on the thickness, area, the temperature difference, and the nature of the material.
The effect of all these factors can be summarized in the Heat Flow Equation.

Feedback & Flow

Figure P5
Control Loop

Many things flow. Water flows in ditches and pipes, electric charge flows through wires, air flows over the wings of an airplane, and heat flows from hot to cold. The rate a which a flow can fill a container depends of the speed of the flow the effective cross section of the ditch or pipe or wire. Perhaps more interesting is the principle of negative feedback. In this control process, a changing condition of some system is regulated to keep it more or less constant. When the actuator increases the variable reaches some set maximum, a sensor sends a message to the actuator, and the cause of the change is shut off or reversed.

Figure P6
Feedback Loop

Controlling Heat Flow
In many situations it is important to control the transfer of heat. Some things get too hot unless heat can be removed. Examples include an elephant in the sun at noon, a car engine, a nuclear plant, and a microprocessor chip. Each of these need ways of removing heat quickly and easily. In other cases, you want to preserve heat by reducing heat loss as much as possible. Examples include a building in the winter, a jug of hot coffee, and a baby incubator. In these situations, the goal is to reduce heat transfer.

Incubators use general principles which appear in the physical world in many different forms. Flow and collection are controlled with negative feedback. In an incubator the flow of heat out (cooling) is compensated for by pouring new heat in. The inflow of new heat is controlled by sensor with a switch (thermostat) which turns the inflow off when the temperature is high enough and on again when the incubator is too cool.

Figure P7
Negative Feedback

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