# Ohms Law and heat generation extended?

#### Back to school

Joined May 22, 2019
106
I hope no one minds if this is a stupid question but does the physical size of a resistor effect the measurable temperature of heat dissipated? I ask because I'm thinking of engines and a big block compared to small. One of each engines creating the same power theoretically creates the same amount of heat yet the small block will run hotter--all things being equal. Each engine has inherent pros and cons but because engines have a "Goldilocks" operating temperature the big block is generally considered more efficient. The total dissipated heat may be equal but the operating temperatures are not the same. Does the same hold true for a resistor? Can the physical size of a resistor effect its working life? Controlling engine compartment temperature to designed parameters is a fairly new design and engineering consideration for engine efficiency while at the same time electronic devices like a stereo or TV and get really hot. Is component selection and the design of an electronic device similarly a consideration when designing circuits?

After I retired I did some math and science teaching and still do occasionally for the local school district. While teaching I stressed regardless of the math or science subject how common solving the same problems becomes. This helped with the "why do I need this" kids. Does the resistor give me another example of commonality?

#### BR-549

Joined Sep 22, 2013
4,938
Yes....because of surface area.

#### nsaspook

Joined Aug 27, 2009
8,894
Size matters.

Remember:

#### crutschow

Joined Mar 14, 2008
28,153
The resistor gets rid of the heat generated by its power dissipation through conduction through the wires, conduction to the air, and radiation.
A larger resistor has more surface area, thus it can radiate away, and conduct more heat to the air for a given resistor temperature.
but because engines have a "Goldilocks" operating temperature the big block is generally considered more efficient.
I don't understand that statement, unless by "efficient" you mean something other than fuel efficiency.
For the same given load within the motors capability, a smaller engine is usually more fuel efficient than a larger engine (assuming the engine designs are otherwise essentially identical).

#### Back to school

Joined May 22, 2019
106
Interesting. How much then is surface area a design consideration? I'm just guessing but there seems to be a choice to be made between contained heat vs. localized environmental temperature. In automotive engineering circuit and module heat is a real issue for electronics life cycle.

I don't understand that statement, unless by "efficient" you mean something other than fuel efficiency.
For the same given load within the motors capability, a smaller engine is usually more fuel efficient than a larger engine (assuming the engine designs are otherwise essentially identical).
I was referring to heat efficiency. I concede I didn't make that clear.

Put a group of engine design engineers in a room and ask them to define fuel efficiency and you will get as many answers as there are engineers asked. How do you define fuel efficiency? Take two cars exactly the same and one is used daily to drive someone back and forth to work and gets 22 MPG. The other car is used for the same purpose but 4 grown men car pool in the car and it gets 18\19 MPG. Which one is more fuel efficient?

The engine temperature sensor is often referred to as the master sensor in conjunction with the O2 sensors because together they determine AFR (Air to Fuel Ratio) from a map in the ECU. Temperature is important because it determines a number of important parameters about how an engine is running and intake, cylinder and exhaust pressure depends on temperature. I used the term "Goldilocks" because too cold and ECU strategy will have the engine running rich while too hot effects how well fuel mixes with the air. Too hot and fuel condensing on cylinder walls isn't uncommon and leads to an uneven combustion in the cylinder. This just touches on how temperature effects an engine but generally overall engine efficiency is often expressed through VE (Volumetric Efficiency). VE is used because it can be scaled against what an engine is being asked to do. It can take in to account engine loading. For example, two identical pick ups but one is towing a trailer. What the engine is being asked to do in comparison the effects that it's having on the engine. Unfortunately VE is often expressed as a static value based on engine specs but accurate or real VE is either found on a dyno or in real time with a data logger.

Big block vs. small block; Although big block engines are usually larger displacement engines, engine size really has nothing to do with block size. For example, the chevy 350 and 400 are small blocks but the 400 is also available as a big block. The "big" refers to physical size and mass. The larger block size offers more options to coolant flow and water jacket size and location. The same with oiling. This just scratches the surface of how the block size can make a difference with all things being equal in terms of displacement. Of course a larger block size also offers bore and stroke options a small block may not. A "square" engine is where the bore and stroke are the same or really close, an over square engine is where the bore is larger than the stroke and the opposite is true for an under square engine. The are pros and cons to each design. F1 engines that exceeded 20,000 RPM's where super over square engines. This gave the engines a really short stroke with a large bore which resulted in lots of power but little torque. F1 engines are almost always revving high so speed is desired over torque. As you said though these were small block engines but I'm not sure I'd refer to them as fuel efficient. They run very rich. Also F1 racing isn't a racing style wanting to carry around the mass of a big block. NASCAR has no problem with a big block but NASCAR's expansion into road courses is fairly new and weight may become more of an issue for them. F1 engines because they're small blocks are continually being rebuilt between races and the engines aren't expected to last more than a season. Much of this short life expectations is because of heat. In top fuel drag racing engines are often referred to with terms like "it's a 15 minute engine" and they don't run with any coolant. Of course their races only last 3 to 4 seconds and most engine run time is given to staging. Ultimately what I'm getting at is engine efficiency is about context and intended use. Fuel efficiency without context or comparison can mean whatever you want it to.

#### WBahn

Joined Mar 31, 2012
26,398
Interesting. How much then is surface area a design consideration? I'm just guessing but there seems to be a choice to be made between contained heat vs. localized environmental temperature. In automotive engineering circuit and module heat is a real issue for electronics life cycle.
Like most things, electrical components are designed to be operated withing certain temperature limits (as well as other limits). If you operate them outside of those limits, all guarantees are off -- they may still work or they may fail catastrophically. Even if you operate them within limits, you can have a dramatic impact on their life expectancy by operating them close to the limits. A commonly used rule of thumb is to not operate most components at more than half their rated power. Good design practice will virtually never operate them without at least a 10% to 15% margin (unless some other factor is driving the decisions).