# Why can't passive components transform energy from one form to another [closed]

In parts of electrical components active components can control the flow of current in a circuit but passive components cannot. What is the factor that gives active components the ability to control the direction of flow of current in a circuit.

## closed as unclear what you're asking by tomnexus, helloworld922, Asmyldof, PeterJ, Andy akaSep 21 '15 at 7:33

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• Your title and your body are asking two different things. – Ignacio Vazquez-Abrams Sep 21 '15 at 4:24

"What is the factor that gives active components the ability to control the direction of flow of current in a circuit."

AFAIK what sets active components apart is that they can control the amount of current that flows.

The mechanisms that are used vary. Relays use a magnetic field to close a switch. An ancient precursor of the thyristor used inonized mercury. Vacuum tubes rely on the repulsion of free electrons by the electrons in the grid. (FETS use a similar mechanism.) Junction transistors use yet another mechanism, etc.

"Why can't passive components transform energy from one form to another"

If I understand that question correctly you wnat to know which common physical factor or mechanism is shared by the active components that differentiates them from the passive componnents? There is none. A components is active because it can use a small current to manipulate a larger current, that is the definition of active.

Passive components can transform energy from one form to another: inductors cant transform electrical energy to magnetic field energy and back, and resistors transform electrical energy to heat energy.

It's not clear what you mean by "control the flow of current", but passive components can effect the flow of current in a circuit. For example, Ohm's law dictates the current/voltage relationship of a resistor. Every passive component has a current/voltage relationship equation and can somehow effect the flow of current.

They can even increase voltage/current (though not at the same time), e.g. transformers.

One thing passive components cannot do is inject power into a circuit. Technically, active components can't create power from no-where and inject it into a circuit either, but the "convenient" lie often used is we ignore external power pins supplied active components. For example, we often approximate operational amplifiers with a voltage controlled voltage source, which produces power magically. However, every real op-amp has power supply pins, and any power outputted must come from these external power supplies.