What is the purpose of resistance in a circut?

  • Thread starter Aladin
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What is the main purpose of a resistance in a circut.
 
that depends on what the circuit is supposed to do. I old light switch dimmers the circuit was completed by varying the position in which the wire contacted the resistor. If the contact was placed near the start of the resistor the lights would be relatively bright, as the contact moves farther along the resistor, more of the electricity is dissipated into heat by the resistor to make the light switch dimmer. A toaster is another great example of a resistor. The metal coils inside have a high resistance, thus the electric energy is converted into heat energy via the resistance of the metal alloys. In these two example we make use of resistance in a positive way. A negative effect of resistance is static. Any cable line running into you house, or any speaker wire has internal resistance. Meaning the metal wires connecting devices have resistance (granted its usually quite small). If there is too much resistance in a wire your speakers or TV wont get a signal, thus you will hear hissing out of speakers or see white noise (not the movie) on your television screen.

In general resistors decrease the electric energy running through a circuit and convert it into thermal energy
 
26
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Consider a Rheostat. This is an instument that allows you to vary resistance by a mechanical means. They are used on new Fuel injected vehicles. The are built into the throttle position sensors. The farther the throttle plate is moved the more resistance is created in the circuit. A current is passed through the rheostat and a voltage drop is measured via the cars computer. The larger the resistance the larger the voltage drop. And thus this varying voltage can be used by a computer along with simple code to determine the cars throttle postion and how much fuel should be added at that time.

This is just one example. There are tons of uses. Resistance is just a term, lightbulbs would not shine if resistance didnt exist.
 

FredGarvin

Science Advisor
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Another practical use of resistors is a voltage divider.
 
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They are also used as "shunt resistors" in amp meters. I believe thats the name anyways. And they are used in volt meters as well.
 

Claude Bile

Science Advisor
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It depends on the circuit. In an anologue circuit, they are used to either create potential dividers or divert current (i.e. a shunt resistor). In a digital circuit they are used in conjunction with transistors to make various logic gates.

Claude.
 

russ_watters

Mentor
18,832
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Sometimes the resistor is the purpose of the circuit: ie, in a light.
 
They regulate voltages, divide currents, resistor can be in many machines the main purpose as russ mentioned..

No circuit can be functional without resistances...
 
1,675
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There is no single purpose. The fundamental action is to dissipate power.

Resistors are the only circuit elements which do anything in a DC circuit.

In an AC circuit rheostats can dim lights, but so can variacs which are not
resistors. Voltage dividers can also be made out of capacitors which would
dissipate no power and be more efficient.

Circuits can be functional without resistors. It's more accurate to say
that any practical circuit will have to take resistance into account
because (except for superconductors) every practical circuit element
exhibits some of it.
 
Last edited:
I didn't mean by functional that it wouldn't work without it, but i meant there would be no purpose for the circuit without a resistance, practically, most of the the machines, and electrical devices are regarded as resistances..
 
1,675
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Nomy-the wanderer said:
I didn't mean by functional that it wouldn't work without it, but i meant there would be no purpose for the circuit without a resistance, practically, most of the the machines, and electrical devices are regarded as resistances..
Only if you want heat (or to dump energy out of your circuit) do you
need a resistance. A motor for example is actually an inductance except
for the fact that real wires have unavoidable resistance. But if motor
makers could, they would in many cases prefer to make motors with
zero resistance in the wires. Same with solenoids. They are modeled
as resistances becuase they have unavoidable resistance but their
machine qualities would work with zero resistance.
 
Resistances in the wire, would produce a lot of un-needed heat, that would cause damage to the circuit, but inductance is a resistor current wise...
 
1,675
3
Nomy-the wanderer said:
Resistances in the wire, would produce a lot of un-needed heat, that would cause damage to the circuit, but inductance is a resistor current wise...
I know what you mean Nomey. What you are trying to say is that an
inductor can appear to act like a resistance to current. But this is
more language than math. An inductor opposes a change in current
while a resistor converts power into heat. The only way an inductor
can look even a little resistive is if some of its energy is pulled out to
do other work, other than heat.

For example a motor with a load will look like a resistor. But it is not
a resistor because the work is going out the shaft, and may or may
not turn into heat. It could turn another generator that charges
a capacitor.

The general way this is described is the concept of "impedance" which
can appear to be inductive, capacitive or resistive (or even negatively
resistive for sources of energy).

But electrical resistance is still not required for work to be performed.
(Ideal motor sawing a log for example.)
 
But it seems to me that the only function for a resistor to u is to convert power to heat, which is the most undesirable...
 

Q_Goest

Science Advisor
Homework Helper
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If you look at a typical piece of electronic hardware, you'll find dozens of resistors stuck all over the circuit board. I'd assume that none of these are used to create heat. So what are these resistors used for?
 

amt

52
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Bottom line is: It controls the flow of electrons.
For example if you are sending 5Amps of current, then a resistor can be used to make this smaller-2Amps, 1Amp or even 1 milli-Amp as needed by any circuit.
 
Q_Goest said:
If you look at a typical piece of electronic hardware, you'll find dozens of resistors stuck all over the circuit board. I'd assume that none of these are used to create heat. So what are these resistors used for?

That is exactly my question to antiphon..
 
1,675
3
Let me clarify.

What I mean is that the physical function of a resistor is to make heat.
In a particular circuit it could have many different engineering functions,
the most common perhaps (in a DC circuit) being voltage division as was
pointed out.

My point was only that resistance itself was not absolutely necessary
for a circuit to perform useful work. Of course any practical circuit
will likely have resitors in it.

I'll use the example I touched on earlier because it has all the elements
of the idea.

If you have an ideal (permanent magnet) DC motor running without a load,
the coils have zero resistance. Yet the motor will draw no current after it
reaches its operating speed. The back-emf of the coils is equal to the
voltage applied to the motor so no current flows. It's somewhat like
two batteries of equal voltage connected ++ and --.

If you put a load on the motor shaft, there will be a current flowing
into the motor and a voltage across the motor and will LOOK like a
resistor. This was what Nomy's point was. What I wanted to clarify
was that this arrangement was NOT a resistor, but it only looks like one
electrically at the motor terminals.

An actual resistor would have turned the electrical energy directly into heat while
the ideal motor converts the electrical energy into mechanical energy which
flows out the shaft of the motor. The work may then saw a log
(heat+work), or it may lift a weight (potential energy) or light a
lamp (heat).

Resistors are very practical circuit elements and are often NOT used to
produce heat. But that is what they fundamentally DO compared to other
types of basic circuit elements.
 
Antiphon said:
If you put a load on the motor shaft, there will be a current flowing
into the motor and a voltage across the motor and will LOOK like a
resistor. This was what Nomy's point was. What I wanted to clarify
was that this arrangement was NOT a resistor, but it only looks like one
electrically at the motor terminals.
and i agree on that point, i fully understand inductance has a quite different role, but so it it appears to be...

But in a practical not ideal motor, there'll be other circuits to regulate the voltage and current flaw, and hence there'll be resistances..But i udnerstand ur point.Thank for clarifying..
 
12
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One simple use of resistor to bring Voltage swing around the circuit for sharing the voltage across.
 

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