# The relative resistance of a filament vs. a conductor

• erandall

#### erandall

1. I'm a middle school science teacher with no science background, and I'm trying to avoid teaching any misconceptions on this topic! However, I don't know that I grasp what's going on here myself. I'm trying to explain rationale for why, on a circuit diagram, the symbol for a resistor may sometimes also indicate a load.

My question is: would a light bulb filament have a lower resistance than any conductors in a circuit? Or would it be greater?

Here's what I think I know so far:

-Materials with lower resistance give off more light because electrons move through these materials more freely and the kinetic energy of their motion is transformed into thermal energy.

-Conductors have low resistance than other materials because they allow electrons to move through them freely.

3. At first I assumed that tungsten would have a higher resistance--this seemed like the only reason why it would be classified as a "resistor" on a diagram. But if this was true, wouldn't the copper wire or other conductors in the circuit be glowing even more brightly than the tungsten? Logic would seem to suggest, then, that tungsten would have a much lower resistance than the conductors. But then why is it classified as a resistor?

If I've gotten anything wrong, please disabuse me. Any clarity would be much appreciated.

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1. I'm a middle school science teacher with no science background, and I'm trying to avoid teaching any misconceptions on this topic! However, I don't know that I grasp what's going on here myself. I'm trying to explain rationale for why, on a circuit diagram, the symbol for a resistor may sometimes also indicate a load.

My question is: would a light bulb filament have a lower resistance than any conductors in a circuit? Or would it be greater?

Here's what I think I know so far:

-Materials with lower resistance give off more light because electrons move through these materials more freely and the kinetic energy of their motion is transformed into thermal energy.

-Conductors have low resistance than other materials because they allow electrons to move through them freely.

3. At first I assumed that tungsten would have a higher resistance--this seemed like the only reason why it would be classified as a "resistor" on a diagram. But if this was true, wouldn't the copper wire or other conductors in the circuit be glowing even more brightly than the tungsten? Logic would seem to suggest, then, that tungsten would have a much lower resistance than the conductors. But then why is it classified as a resistor?

If I've gotten anything wrong, please disabuse me. Any clarity would be much appreciated.

You can do some interesting experiments with a flashlight bulb, some batteries and a multimeter.

(1) Measure the resistance of the flashlight bulb by itself with the multimeter.

(2) Make a circuit with a fresh AA battery and the light bulb.
(a) measure the current flowing through the circuit -- using the ammeter function of the multimeter.
(b) calculate the resistance of the light bulb (V = i * R) Use the voltage of the battery.

(3) Make a new circuit with two AA batteries, etc...

You'll see that as the filament in the light bulb glows brighter, the resistance goes up. This is normal behavior. Electrical resistance goes up with increasing Temperature.

The power lost in a resistive element is P = i^2 * R

Filaments in light bulbs are very thin. Thin wires have higher resistance than thicker wires. The resistances of any conductors in a typical circuit are going to be very, very small. Heating elements, light bulbs, etc. will have higher resistances.

1. I'm a middle school science teacher with no science background, and I'm trying to avoid teaching any misconceptions on this topic! However, I don't know that I grasp what's going on here myself. I'm trying to explain rationale for why, on a circuit diagram, the symbol for a resistor may sometimes also indicate a load.

My question is: would a light bulb filament have a lower resistance than any conductors in a circuit? Or would it be greater?

Here's what I think I know so far:

-Materials with lower resistance give off more light because electrons move through these materials more freely and the kinetic energy of their motion is transformed into thermal energy.

-Conductors have low resistance than other materials because they allow electrons to move through them freely.

3. At first I assumed that tungsten would have a higher resistance--this seemed like the only reason why it would be classified as a "resistor" on a diagram. But if this was true, wouldn't the copper wire or other conductors in the circuit be glowing even more brightly than the tungsten? Logic would seem to suggest, then, that tungsten would have a much lower resistance than the conductors. But then why is it classified as a resistor?

If I've gotten anything wrong, please disabuse me. Any clarity would be much appreciated.

Welcome to the PF.

The filament in an electric light bulb is just a conductor with some special properties that let it carry enough current to glow brightly and not burn up quickly. See the "Electric Light" section of the wikipedia article on Edison, for example: http://en.wikipedia.org/wiki/Thomas_Edison

You can make a light bulb with copper or other conductors as the filaments, but they just burn out more quickly. :-)

1.

My question is: would a light bulb filament have a lower resistance than any conductors in a circuit? Or would it be greater?
No.
Here's what I think I know so far:

-Materials with lower resistance give off more light because electrons move through these materials more freely and the kinetic energy of their motion is transformed into thermal energy.

-Conductors have low resistance than other materials because they allow electrons to move through them freely.

3. At first I assumed that tungsten would have a higher resistance--this seemed like the only reason why it would be classified as a "resistor" on a diagram. But if this was true, wouldn't the copper wire or other conductors in the circuit be glowing even more brightly than the tungsten? Logic would seem to suggest, then, that tungsten would have a much lower resistance than the conductors. But then why is it classified as a resistor?

If I've gotten anything wrong, please disabuse me. Any clarity would be much appreciated.
"Glow" is basically proportional to the temperature of the filament. This in turn is mainly a function of the power dissipated in the filament. The power dissipated in a filament is V2/R where R is the resistance of the filament and V the applied voltage.
If copper, which has a greater conductivity than tungsten, were used as a filament in a light bulb, the filament would glow very brigthtly indeed - for a fraction of a second - then open up so no further current is conducted. The tungsten filament has a higher resistance such that, when excited by a 115V source such as obtains in a U.S. or Canadian home, will conduct enough current to glow, but not enough to self-destruct.

A resistor resists current in the sense that current I = applied voltage V divided by resistance R. So for a given applied voltage, the greater the resistance, the smaller the current, the smaller the power dissipated, and the less glow obtains.

If copper, which has a greater conductivity than tungsten, were used as a filament in a light bulb, the filament would glow very brigthtly indeed - for a fraction of a second - then open up so no further current is conducted. The tungsten filament has a higher resistance such that, when excited by a 115V source such as obtains in a U.S. or Canadian home, will conduct enough current to glow, but not enough to self-destruct..

Why doesn't copper self destruct, then, when used in other parts of the circuit?

Why doesn't copper self destruct, then, when used in other parts of the circuit?

If you put enough current through it, you can make copper self destruct in the same way. It is scary.

Why doesn't copper self destruct, then, when used in other parts of the circuit?