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I am just starting off with hobby electronics and can't get hold of the concept of Load. Do electrical appliances also "Draw" current. An air conditioner upon start dims the light. My point is that if the voltage difference is not enough to move whatever machinery is in there then the air conditioner should stay off or not work. How come it "draws" electricity upon start?

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Load is the generic term for something in the circuit that will draw power. The characteristics of a "Load" can vary widely. It is normally the largest power draw and most components in the circuit are there to support the Load. Anything that uses electricity to do work will draw current. The amount depends on how much resistance to current flow the device has and the amount of voltage applied to it, assuming the source has more power to give than the load will use.

The term is derived from a physical load, like carrying a bundle of wood. It requires work, when you are loaded down.

In your question about an air conditioner, you are running into non-simple loads. A large electric motor will behave differently at startup vs when it is running normally. An air conditioner has a fairly high load running the compressor. Imagine a car starting out from a stop, there is more energy needed to get it moving than keep it moving. This is the same with a motor and there is an inrush of current to get it moving. This is either due to starting coils, which are switched out when the motor is at speed, or changing electrical characteristics of the motor between stopped and running mode.

The reason your lights dim are because the extra current draw causes the house wiring to the AC unit to lose more voltage before it gets there. (Look up Ohm's Law and compare fixed resistance with varying current.) Once the initial inrush of current is over, the voltage drop subsides and the lights resume their normal power.

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Do electrical appliances draw current? Absolutely yes, they do. The amount of current drawn, multiplied by the voltage, is the amount of power used by the appliance.

If an air conditioner causes the lights to dim when it turns on, this is because the amount of current it is drawing on startup is causing the input voltage to sag slightly, meaning that other devices connected to the same circuit are also seeing the voltage sag. A light bulb (assuming incandescent) acts like a resistor - lower voltage means lower current, lower power and reduced light output (hence the dimming).

You've essentially answered your last question without realizing it. If the voltage applied to an air conditioner is 'too low', two things may happen:

1) The current drawn may be higher - this is because motors and compressors don't act like resistors - they're more like constant-power devices (lower voltage means higher current)

2) If the voltage is severely low, there may not be enough voltage to get the motors started, and the current draw will be low because the motors aren't working.

If there is sufficient voltage applied to an air conditioner, the motors and electronics will start operating, which results in current being drawn.

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Some good answers up here already but I thought I'd take a shot as well...

The term "load" is used in different contexts, and its meaning can change slightly depending on context.

In the most general sense, the load is the thing that is connected to the output of an electrical circuit. The load can basically be anything. For example, it can be a resistor, or a capacitor or an inductor, or a transistor, or a motor, or an air conditioner etc.

The term "load" can also mean "the amount of power" or "the amount of current" drawn by the thing that is connected to the output of the circuit. For example, think about a battery. If we were to say that there is a heavy load connected to the battery, or that the battery is supplying a heavy load, we would mean that the thing (load) connected across the battery's terminals was drawing a lot of current (or power). Conversely, if we said that it was a light load, we would mean that there was not a lot of current (or power) being drawn from the battery.

"Load" can also be used as a verb. We might say that the graphics card in a computer was "loading down" the power supply too much, and by that we would mean that the graphics card was drawing too much current (or power) from the power supply.

I guess it would be correct to say that all loads draw current (or power) from whatever circuit they are connected to. It would not be correct to say that they draw voltage or otherwise "take" voltage from their supplying circuit. Voltage (electric potential) is required to move charge (electrons) through the load or circuit, but voltage doesn't actually "go" anywhere.

Other people have already explained the light dimming thing. Here is another little blurb on it in this PDF...

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A device consumes power if the electrons that leave it are less energetic than those which enter it. For various historical reasons, current is generally described as flowing in the direction opposite the way electrons actually move, and so more energetic electrons are indicated by a voltage which is less positive or more negative.

One may view electrons as water, and energy level (voltage) as elevation. Given a chance, water will flow downhill. A load would represent a place where water is allowed to flow downhill; the amount of energy is proportional to the quantity of water that flows and the distance that it falls. A power supply would represent a place where water is (by introduction of outside energy) pumped uphill. The water leaving a power supply has a higher energy level (elevation) than the water enters. Note that the amount of water coming out of anyplace will be the same as the amount of water flowing in; the difference is in the energy level.

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  • \$\begingroup\$ "A device consumes power if the electrons that leave it are less energetic than those which enter it." - This statement confuses me. First of all, considering AC, AFAIK there is no net movement of electrons. So I wouldn't speak about electrons "entering a device". Secondly I am not sure if it is correct to say that electrons are more or less energetic. I understand your analogy to water and potential energy, but somehow the comparison seems flawed. \$\endgroup\$ – Rev1.0 Aug 28 '14 at 7:51
  • \$\begingroup\$ @Rev1.0: When a DC current is flowing through a device, electrons will enter via one wire and leaving via the other. There will not be any significant net immigration or emigration, but that doesn't mean that there aren't any electrons entering and leaving--merely that the number of electrons that for every electron that enters via one path, another must leave via some other path. As for the term "energetic", that's an attempt to use a non-technical English term to relate a concept; it's not quite accurate, but I don't know what adjective would be more suitable. Among hydraulic concepts,... \$\endgroup\$ – supercat Aug 28 '14 at 15:08
  • \$\begingroup\$ ...voltage most directly correlates with pressure, and current with volume flowing per unit time. \$\endgroup\$ – supercat Aug 28 '14 at 15:11
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Lots of fine answers, here, and nothing I disagree with. Would like to add two things...

1.) Load means power; but in the home consumer arena, where the 'V' of 'P=IV' is set, many have come to equate load with current; since, if 'V' remains relatively constant, then you can only vary power by varying 'I' (or the angle between I and V, but we won't get into that).

2.) When electric motors run, they develop a "back EMF" which acts to oppose the voltage applied to the motor windings. This in turn reduces current through the motor windings. When a motor first starts, this back EMF is not there (not built up yet) so there is an initial large inrush of current. You might notice that when your AC starts, the lights get very dim, but then get brighter (almost as bright as before motor start) very quickly, even though th AC is still running. This is the inrush event of the AC motor taking so much current that it temporarily forces 'IR' voltage drops in the home wiring, lowering effective 'V' to itself and the lights.

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The exact meaning of a load: A load is any device that lets-through the energy. It dissipates or delivers the energy to elsewhere. The elsewhere may be surrounding in the form of heat/light/flux or to other device as mechanical energy/shaft energy.

The other question: how current draw? Actually current is driven by the higher voltage overcoming through impedance. If by any means the impedance is reduced, the current will flow even at the decreased voltage. If we imagine with respect of load side, with the decrease of impedance the current will flow in, as if drawn by the load. The concept of current draw is actually the perception from load-side, not from source.

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protected by Olin Lathrop Nov 19 '15 at 11:52

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