basic question about resistance and Ohm's law

snipped-for-privacy@gmail.com schrieb:

Hello,

a resistor turns the electrical power into heat, that is true. But that does not mean more resistance will consume more power and produce more heat. It depends on the circuit and the source used. If you connect a resistor to a constant voltage source, more resistance will consume less power. More resistance means less current, the voltage stays the same, the product of current and voltage is power and gets smaller. But if you connect a resistor to a constant current source, more resistance will consume more power. The current stays the same, the voltage over the resistor rises and the power rises too.

Bye

Probably because common sense isn't all that common.

Perhaps a mechanical analogy might help. An electric circuit is much like a plumbing circuit. Voltage is the water pressure. Current is the water flow rate. Resistance is constriction to the flow. For this exercise, the light bulb can be any type of rotary "paddle" type of pump. I can't make water produce light, but I can extract energy my making the water drive a pump, which does some useful work such as lifting a weight.

When I connect the pump to a garden hose, and pump spins at some rate. This is your starting circuit. Like this:

Adding resistance is a bit tricky. What a resistor does is convert electron energy into thermal energy. In a hydraulic example, that would simulated by diverting some of the water (current) with a "T" pipe section. When I add a symmetrical "T" diverter pipe to the high pressure side of the pump, 1/2 of the water now flows into the pump and 1/2 flows out the diverter, and onto the ground. With half the flow rate, the pump now spins at half speed. The pressure (voltage) remains the same because nothing has changed at the water source.

More of the same on Ohm's Law analogies:

Ok, let me put my two cents in...

There are two concepts, Ohm's law about the relationship between voltage, current and resistance, and power formulas, that describe how much heat or whatever are being produced by a circuit. They are related, but not the same.

When you have a simple circuit like you described with a battery, light and resistor, you have to look at two things. First, Ohm's law will tell you what voltage and current are across each part. Then, you can take that voltage and current, and determine the power across each part.

Now, when you increase resistance, you reduce the current. Reducing the current causes the power to go lower. Remember, the formula for power is V^2/R, so increasing R lowers overall power. If you look at a heater, realize that the overall resistance of that element if often small to use a lot of power...

Charlie

It isn't removed from common sense:

Push something harder, it'll move faster.

It's easier to swim in water than in molasses.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

I think I understand now. Thank you. Too bad there is not an electronic equ ivalent to a water pipe faucet. It just reduces the flow without wasting en ergy (water) while a resistor does waste power instead of just making it ha rder for current to flow through, preserving the rest of the power in the b attery, rather than wasting it by turning it into heat. Thanks again.

A partly open faucet does warm the water that passes through it. Power is lost in the faucet.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

There *is* though. You switch hard on and off quickly with a variable mark space ratio to avoid resistive losses in the load controller.

It is the basis of all modern switched mode power supplies, lamp dimmers and cheap high power class-D amplifier designs.

The days of the power rheostat are long gone!

--
Regards, 
Martin Brown

I think I understand now. Thank you. Too bad there is not an electronic equivalent to a water pipe faucet. It just reduces the flow without wasting energy (water) while a resistor does waste power instead of just making it harder for current to flow through, preserving the rest of the power in the battery, rather than wasting it by turning it into heat.

** The OP reminds me of a guy I met in a components store once.

On being informed that rechargeable NiCd cells were only 1.2 volts each, not

1.5V, he exclaimed.

" That's ridiculous - how stupid are they not to make them the same. "

.... Phil

Hey, I broke the legs off my first transistor, then tried to replace it by wiring together two diodes :)

--

John Devereux

Everybody has done that. The younger you were, the more credit you get.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Speak for yourself. I started repairing tube based equipment, and knew enough about transistors by the time I built my first solid state amp to know it wouldn't work. I even made the chassis and case in metal shop in Jr high school.

The only transistors I remember breaking all the leads were in a HP

5245L Frequency counter that had been used in a salt air location. Touch 'ANY' of the TO-5 transistors, and they fell off their corroded leads.

Hmm... when I posted I had it in my head that it was early teens.

But thinking about it a bit more, must have been earlier, like 9 or

1. Precocious little brat.

It was one of those "101 circuit projects" kits with a solderless breadboard and a grand total of two 2N-somethings.

--

John Devereux

snipped-for-privacy@gmail.com schrieb:

Hello,

you may connect two 6 V bulbs in series to a 12 V source. Each bulb consumes 6 W, both together 12 W. 1 A flowing thru a 12 Ohm resistor on a 12 V source gives 12 W. You get twice the light as from one bulb only. But you can also use a 12 V 6 W bulb, 0.5 A is flowing and the power is

6 W, the resistance is 24 Ohms. You (hopefully) get the same light from a 6 V 6 W bulb and from a 12 V 6 W bulb.

Bye

Two bulbs of the same type. That means the same operating current and warm-up curve.

Me too, 9 or 10. I think I started actually understanding transistors when I was about 12, testosterone kicking in maybe. Up to then, I read PopTronics and fiddled with things. All good preparation for EE courses in college.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

There's one like that in every Radio Shack. They are usually behind the counter.

electronic

wasting

making it

in the

each, not

same. "

Not everyone; i never did. But before age 10 i knew better.

?-)

Nice explanation sir!