Thermal and Electrical Conductivity

Thermal conductivity is the ability of a material to transfer heat, and electrical conductivity is the ability of a material to allow an electric current to flow. Materials that conduct well are called conductors, and those that conduct poorly are called insulators. In brief: metals such as copper and aluminum are good conductors of both heat and electricity, while wood, plastic and air are good insulators.

These two properties explain many daily choices: why pans have metal bases and plastic handles, why electric cables are copper wires wrapped in plastic, and why a winter jacket traps air. This article explains each type of conductivity, shows how to test them, compares common materials in tables and works through a design example.

What is thermal conductivity?

Heat flows from a hotter place to a colder one. Thermal conductivity describes how quickly a material allows this to happen. If you leave a metal spoon in hot soup, the handle soon becomes hot, but a wooden spoon stays comfortable to hold. The metal is a thermal conductor and the wood is a thermal insulator.

When you touch a metal surface and a wooden surface at the same room temperature, the metal feels colder. This is not because it is colder, but because it carries heat away from your hand faster.

What is electrical conductivity?

An electric current is a flow of charged particles. Electrical conductors let charge flow easily. Insulators do not. Copper wire is used inside cables to carry current, and the plastic coating prevents the current from reaching your hand.

Some substances are in between. Semiconductors such as silicon conduct only under certain conditions, which makes them valuable in electronics. Pure water is a poor conductor, but tap water and salt water conduct better because they contain dissolved ions.

Why do metals conduct so well?

Metals contain electrons that are free to move through the structure. These free electrons carry charge as an electric current and also carry energy as heat, which is why most good electrical conductors are also good thermal conductors. In insulators, the electrons are held tightly by their atoms and cannot travel through the material.

A notable exception is diamond, which is an electrical insulator but conducts heat extremely well. Air, on the other hand, is an excellent thermal insulator when it is trapped in small pockets, because gases transfer heat poorly.

How can you test conductivity?

Testing thermal conductivity

1. Place rods of equal size made of different materials in a container of hot water.
2. Attach a small piece of butter or wax near the end of each rod with a dab.
3. Note how quickly the wax melts on each rod. A faster melt means higher thermal conductivity.

Testing electrical conductivity

Build a simple circuit with a battery, a bulb and two gaps for test materials. Place a sample between the gaps. If the bulb lights, the material conducts. Use only low-voltage batteries and never test mains electricity.

Conductors and insulators compared

Approximate thermal conductivities at room temperature illustrate the large differences between materials.

The table shows that copper conducts heat about 15,000 times better than air. That ratio is why trapped air is so effective in insulation.

Worked example: choosing materials

A designer must choose materials for a saucepan, an extension cord and a winter glove.

1. Saucepan base: it must pass heat from the stove to the food, so aluminum or copper is chosen.
2. Saucepan handle: it must stay cool enough to hold, so heat-resistant plastic or wood is chosen.
3. Extension cord: the inner wire must carry current, so copper is used, and the outer coating must stop the current from escaping, so plastic or rubber is used.
4. Glove: it must keep heat in. A fleece or wool lining that traps air works better than a thin metal sheet.

In every case, the property of the material is matched to the job.

Where are conductors and insulators used?

• Cooking: metal pots heat food quickly, and oven mitts protect the cook.
• Buildings: foam, wool and double-glazed windows slow heat loss.
• Electronics: copper tracks carry signals and plastic boards separate them.
• Safety: rubber-soled shoes and insulated tools reduce the risk of electric shock.

How were these properties discovered?

In the early 1700s, the English experimenter Stephen Gray found that electricity could be carried along some materials but not others, which led to the distinction between conductors and insulators. Later, scientists measured heat flow quantitatively. In the early 1800s, Joseph Fourier developed a mathematical theory of heat conduction, and the unit of thermal conductivity is expressed in watts per meter per kelvin as a result of that work.

Which mistakes are common?

Mistake 1: Saying that metal is colder than wood

Both are at room temperature. Metal feels colder because it conducts heat away from your skin faster.

Mistake 2: Assuming that insulators block all heat or current

Insulators reduce flow but do not stop it entirely. A very high voltage can force current through an insulator, and heat still passes slowly through a wall.

Mistake 3: Believing that all conductors are metals

Graphite is a nonmetal that conducts electricity, and salt water conducts because of dissolved ions.

Mistake 4: Thinking that a good thermal conductor is always a good electrical conductor

Diamond is a clear counterexample: it conducts heat extremely well but is an electrical insulator.