Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: labview1958 on 17/02/2009 09:29:20

Title: How does temperature of a resistor vary with resistance?
Post by: labview1958 on 17/02/2009 09:29:20

If a 1 kΩ and 2 kΩ resistance i parallel with a 1.5V battery, which resistance have a higher temperature? Is it IxIxR ? Which is true, higher current, higher temperature or higher resistance, higher temperature?

Title: Re: How does temperature of a resistor vary with resistance?
Post by: justaskin on 17/02/2009 09:45:50

Higher current, higher temperature.
If not short circuits would not be a problem.
Have a look at the size of the cable going to the starter on your car as opposed to the rest of the wiring.

Cheers
justaskin

Title: Re: How does temperature of a resistor vary with resistance?
Post by: labview1958 on 17/02/2009 10:04:28

What about electric water heater. Which heats up the water faster, long or short heater filament? Longer filament, higher resistance.

Title: Re: How does temperature of a resistor vary with resistance?
Post by: justaskin on 17/02/2009 10:17:41

Which heats up the water faster, long or short heater filament?
The one that has the greater current flow.
More current more heat.
More resistance less current less heat.
Water analogy
Open tap(less resistance) more water(current).

Cheers
justaskin

Title: Re: How does temperature of a resistor vary with resistance?
Post by: Vern on 17/02/2009 11:13:03

In an electrical circuit power (heat) is equal to current times resistance. P = I * R. A fellow named Ohm figured that out awhile back so we call it Ohm's law.

Title: Re: How does temperature of a resistor vary with resistance?
Post by: justaskin on 17/02/2009 11:23:35

Quote from: Vern on 17/02/2009 11:13:03

In an electrical circuit power (heat) is equal to current times resistance. P = I * R. A fellow named Ohm figured that out awhile back so we call it Ohm's law.

Power(Watts)= E(voltage)X I(current).I think?.

Cheers
justaskin

Title: Re: How does temperature of a resistor vary with resistance?
Post by: LeeE on 17/02/2009 20:38:37

Yeah.  P = I * V, but you need to get I from V/R or V from I * R, depending on which ones you know, usually V & R.

Title: Re: How does temperature of a resistor vary with resistance?
Post by: lyner on 17/02/2009 21:08:27

It also depends upon the actual construction of the resistor. Take a 1k resistor, made out of wire. To get a 2k resistor, you can either make it twice as long or half the cross sectional area.
SO, the power dissipated by the 2k resistor would be half that dissipated by the  1k resistor (v squared / R) but the thinner resistor might be just as hot because its surface area would be less.

The example of water heaters is a matter of optimum design - available wire sizes, length of heater element that can be fitted nto the water tank, thickness of ceramic insulation desirable contact area with the water, desirable operating temperature of the wire when its inside the insulating ceramic etc. etc.

Temperature is difficult to predict - power is totally predictable.

Title: How does temperature of a resistor vary with resistance?
Post by: labview1958 on 04/03/2009 13:32:09

The filament of a bulb is extremely long. Which bulb will be brighter, a long or short filament. Remember resistance changes with temperature.

Title: How does temperature of a resistor vary with resistance?
Post by: lightarrow on 04/03/2009 15:14:58

Quote from: labview1958 on 17/02/2009 09:29:20

If a 1 kΩ and 2 kΩ resistance i parallel with a 1.5V battery, which resistance have a higher temperature? Is it IxIxR ? Which is true, higher current, higher temperature or higher resistance, higher temperature?

It's not possible to answer this question with these data only.

Title: How does temperature of a resistor vary with resistance?
Post by: lyner on 04/03/2009 19:46:00

Quote from: labview1958 on 04/03/2009 13:32:09

The filament of a bulb is extremely long. Which bulb will be brighter, a long or short filament. Remember resistance changes with temperature.

If the filaments are the same cross section of tungsten then the shorter filament will get hotter - brighter.

Title: How does temperature of a resistor vary with resistance?
Post by: labview1958 on 05/03/2009 08:09:44

Is it true that the filament of a 100Watt bulb is longer than a 60watt bulb? Is it also true that a 100Watt bulb is brighter than a 60watt bulb.

Title: How does temperature of a resistor vary with resistance?
Post by: lyner on 05/03/2009 09:57:02

Not if you take just any pair of bulbs.That's why I referred to identical filament thicknesses - you need a fair test, to come to any serious conclusion. A 2kW electric kettle element is not as 'bright' as a 60W light bulb.

Title: How does temperature of a resistor vary with resistance?
Post by: LeeE on 06/03/2009 00:14:18

A good point, nicely made Sophiecentaur.