← back

A thermal model for understanding electricity

Electricity is a subject that often isn't intuitive. The lack of understanding and intuition around electricity might come from the fact that it's intangible and needs specialized equipment to quantify. Furthermore, electricity is mostly presented quantitatively, not qualitatively. Contrary to many other physical phenomenon, there is no intuitive scale.

For example, if someone is studying hydraulics, a dam intuitively has a much larger volumetric flow than a faucet. You don't need to see a that $Q_{dam} = 5 700 m^3/s$ vs $Q_{faucet} = 0.0005 m^3/s$ to feel that the dam has a much larger flow of water than a faucet. The only thing that you can do with electricity is measure current with a multimeter and get a numerical value. People are first taught electronics through formulas and numbers, and I have felt that building an intuition for electricity wasn't a priority at all when teaching electronics.

This feels ridiculous, so I'll try to present a possible mental model for electronics. Keep in mind that plenty of these exist and are already presented online, but I recently thought of a model that I haven't seen anywhere else:

Modeling electricity through heat

People have an intuitive sense of heat because we interact and sense it daily. Here's how heat can be related to electricity:

Imagine an ice cube in the Sahara desert in the middle of the day. There is a very large difference in temperature, and the universe will try to reduce this temperature difference by transmitting some of the ice cube's cold to the sand and by transmitting the environment's heat to the ice cube. You can also imagine that the ice cube will melt much faster in the Sahara desert with a 40°C environment than in Ireland with a temperature difference that would be a lot less important.

Electrical potential is like temperature. An object has a certain temperature, but there's no energy that you can harness if there is no difference in energy. This is why we use voltage instead of potential in electronics. Everything around you has a temperature, and everything has an electrical charge. Most things around you have the same temperature, and in the same way, most things around you are charged in the same way.

The temperature difference is the equivalent of voltage. Voltage is defined as the "difference in potential". This simply means that there is voltage (a thing that can create current) when there's a difference in potential. When you dump 100Kg of 40°C sand into the desert, there is no difference in potential, so no energy is transmitted. In the same way, when you place an ice cube on the ground in Antarctica, it isn't going to do much because there isn't a big difference in temperature.

Electrical current corresponds to the amount of thermal energy transmitted between two objects every second. It's the rate of thermal energy transmitted. You can imagine current as being the amount of thermal energy being added to a brownie every minute as it's being cooked. After 2 minutes, the brownie might have heated up by two degrees. After two more minutes, maybe it will have increased temperature by a few more degrees. We could quantify this temperature transmission in degrees per minute $°C / min$. In the above example, $T_{transmission} = 1°C / min$.

Transient model

Here's when the model starts to be interesting: current isn't constant in the brownie screnario and neither would current be in the electrical equivalent. As the brownie heats up, the difference in temperature (∥ voltage) decreases. Heat transmission (∥ current) is proportional to the difference in temperature so as the brownie heats up, less energy gets transmitted per minute. Difference in temperature causes heat transmission. Voltage causes current. When there is no voltage, there is no current.

This is what goes on in a transient example. When there's a continuous difference in temperature (continuous voltage, like with a battery or generator), it corresponds to the continuous model:

Continuous model

This scenario simply corresponds to two isolated items, one of which has a magically constant temperature, and the other item's temperature increases. This corresponds to an open circuit in electronics. It's simply when something with electrical potential (let's say static electricity from rubbing a balloon against hair) touches an object that's less charged (like another person who gets slightly shocked). You can feel the current when touching another person who's a lot more charged (you can get charged by going down a slide for example) but the currently dissipates very quickly because the difference in potential comes down to 0.

This isn't only true for static electricity. If you stuck a finger into a socket, your body would get charged by being as charged as the main power supply (Ignoring the fact that it's AC for simplicity's sake). It only really becomes dangerous when you're touching ground, because there's a continuous difference in potential between the main power supply and ground. When you're touching the ground, you're closing the circuit and causing a big difference of electrical potential (high voltage) across your body. This creates current across your body and isn't great.

Resistance

Now one last thing that this model can help understand is resistance. People have an intuitive sense that touching hot metal will burn more than touching hot air. This is why exposing your hand to cold air is bad, but touching something more thermally conductive like snow is worse.

With the same difference in temperature, different materials will have different thermal flows. For example, the oven's door doesn't let (most) the heat through because it's resistive to the difference in temperature. To the contrary, a continuous piece of metal between inside and outside the oven will transmit a lot more of the energy. This is the same as a resistor in electricity. Some materials are far more conductive than others. It doesn't mean that some heat isn't transferred to the oven door, but far less of it is transmitted than to the metal. This means that for a given voltage, current (or temperature flow) is inversely proportional to the resistance of an object. This simply means that the higher the resistance, the less energy flows through in a given amount of time. In the same way, when the temperature difference is higher, there is more energy flow. No matter how good the oven door might be, if it were placed between a cold room and the sun, it would let thermal energy through. An oven mitten isn't going to do much between a the sun and a cold room, despite it's relatively high thermal resistance (at our scale).

We just recreated Ohm's law! It's the law that states that $V = I \cdot R$. Where $V$ is voltage, $R$ is resistance, and $I$ is current.

After-note

This model could probably be taken further, but I lack electrical knowledge to add to the model.