I've learned that a resistor converts some electrical energy into heat energy while the current flows through it and thus causes a power loss, but what if there's not any resistance in a circuit. Will current still flow?

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    \$\begingroup\$ If there is no resistance, it is called superconductor. Current will still flow but no heat is dissipated. \$\endgroup\$ – jippie Jul 3 '13 at 18:06
  • \$\begingroup\$ So then what is the use of a resistor in a circuit. I mean what does it do? \$\endgroup\$ – Syed Sahl Jul 3 '13 at 18:10
  • \$\begingroup\$ Resistors are used to control the flow of current. For example to prevent too much current burning-out a LED. There are more uses. \$\endgroup\$ – RedGrittyBrick Jul 3 '13 at 18:18
  • \$\begingroup\$ And here the voltage is constant. \$\endgroup\$ – Syed Sahl Jul 3 '13 at 18:22
  • \$\begingroup\$ mmm uses for resistors - just off the top of my head they used to set time delays (RC), obtain voltages (potential dividers), set op amp gain, eliminate reflections in transmission lines (matched termination), prevent oscillation by 'damping', match impedances between circuits and lets not forget incandescent lamps and cooking rings, electric 'fires' ... \$\endgroup\$ – JIm Dearden Jul 3 '13 at 18:31

A wire is just a very small resistor. The limiting case is a superconductor. So, ask yourself what happens in these circuits:


simulate this circuit – Schematic created using CircuitLab

You know that the current in a resistor is given by Ohm's law:

$$ I = \frac{E}{R} $$

So as \$R \to 0\$, then \$I \to \infty\$. When \$R=0\$, then you are dividing by zero and the universe explodes. Fortunately, all wires and things we use as voltage sources (batteries, lab power supplies, wall warts...) have some (very small) resistance, so this doesn't happen in practice.

To know what the use of resistance is, consider what would happen if we had none. Voltage is the result of current that wants to flow, but can't. If there were no resistance, then all the current in the universe could flow, and pretty soon, all the electrical energy in the universe would be depleted, leaving you with no voltage anywhere, and no way to do any electrical work.

As long as we are considering what happens as the resistance gets smaller, we might as well consider what happens as it gets bigger:


simulate this circuit

That is, as the resistance gets higher, less current flows. As \$R \to \infty\$, \$I \to 0\$. When \$R = \infty\$, you have an open circuit, and no current flows. This is just the case of a battery sitting on your desk. There's also no work being done in this case, because although the voltage is exerting a force on the charge in the circuit, it can't move it.


In short: yes, the current will flow and the heat will be dissipated.

The resistor, the capacitor and the inductor, as described in basic text on electrical engineering, are just an ideal models. In reality there is no such thing as perfect R, C or L - each component has all three quantities at some degree. Component's "name" (how we call it) depends on which one of these three quantities is dominant.

For example: C = 1pF, L = 1uH, R=10Ohm is "resistor". The capacitance and the inductance of this component may (usually) be ignored in the first order analysis.

However, R, L and C depend on various operation conditions. For example: the component which we call "inductor" may become "capacitor" at sufficiently high frequency of operation.

Now to your question: no matter which components you use, once you assemble an electrical circuit it will have some resistance and will dissipate heat while the current is flowing through it.

NOTE: There is very interesting exception to the above rule though - superconductors. You can find some information on superconductivity on internet, but we can exclude this special case from our discussion, because it requires very special conditions and/or material to observe this effect.


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