Volt, Ampere and Ohm in real life

Question

Long story short, i am a what some would call a visual thinker; i think in photos and videos. If i can't visualize it, i'm having a very hard time understanding it. For that reason, i've never understood electricity, but i really want to, so i thought i'd give it a shot here.

I recently wanted to 'understand electricity' as in the terms and what they represent; Volt is potential difference, Ampere is current (as in the number of electrons passing a crosssection of a wire in a second) and Ohm is resistance. But today i realized that i don't know anything about the consequences of changing any of the values.

What happens if i increase or decrease the voltage or the current provided to a.. DC motor? LED? laptop? toaster? TV?

What happens if i try to charge a laptop through a thunderbolt port, instead of the designated charger port? What happens if i try to charge it with more/less Watts than what it should get?

I can of course calculate the numbers using ohm's law, but there's not much 'real life' about that. So, what would happen to the devices? would they heat up? explode? stop working? would the toaster toast.... less? Could a laptop battery take damage from more/less watts?

Sorry if these questions are stupid, but it would really help me understand what to think of when working, playing or experimenting with electricity.

Answer 1

In advanced electronics such for smart phones you often have internal switches that shut off the power when too low voltages are supplied.

So the scenarios are easier to grasp by considering simpler electric systems. Such as a toaster, as you mention.

A toaster in essense contains a long resisting conductor called a heat element made of a durable material that can withstand high temperatures. As you allow current to flow through this heat element, it heats up due to its internal resistance - this can be calculated in its simplest form via the power formula:

$$P=RI^2.$$

Now, think of voltage as an "electronic" pressure on the charges. As was it water pressure in a hose. Your electric wall plug (or battery or similar source) supplies this "pressure" which causes charges to flow (current). If the voltage suddenly reduces by, say, half, then the "pressure" is halved and the current will reduce. It might follow Ohm's law fairly well, where the heat element resistance $R$ is constant:

$$V=RI.$$

This now smaller current still runs through the heat element of the toaster. But with less current the power is greatly reduced (note above that the power is proportional to the square of the current) and might not be high enough for the necessary temperature increase of the material. Thus the toaster might work electrically but not practically, since it might not be able to toast a piece of bread now.

On the other hand, increasing the voltage may be disastrous. That would correspond to a higher current. With no relay switch to block to higher current, the power produced in the heat element will increase as well (again notice its square relationship with current). This might heat up the material to above what it can sustain and it will melt or simply start a fire.

Answer 2

Don't read too much into Ohm's Law. It's a common material property, but not a general law of physics. Consider a typical USB power supply/charger. The one I have here says 5.0V 2A. Those are its output limits. As long as the load is drawing less than 2A, the charger's output is 5V. If the load's ability to accept current at 5V exceeds 2A, the charger's output is 2A (and the voltage drops). This is not at all what Ohm's Law predicts.

The input side is even more interesting. The charger says it accepts 100-240V in. Imagine it's delivering 5V at 1A, 5W. If the input voltage is 100V, only 0.05A is needed for 5W, so that's what it draws from the mains (actually a little more: it isn't perfectly efficient). But if the input voltage is 200V, only 0.025A is needed, so that's what it draws. Ohm's Law actually works here, but with a negative resistance.

The best way to learn the practical realities behind the abstractions is to make things. There are lots of educational electronics kits around. Try things. Some will work, some won't, some will make smoke. Stick with low power until you learn to avoid smoke (but nobody is perfect here).