Heat change

[rktpro]

I have two wires of same length. One coiled and one straight. Which would offer more resistance and thus, would produce more heat.

[lemur]

Is the magnetic field generated by the current relevant?

[J.C.MacSwell]

I have two wires of same length. One coiled and one straight. Which would offer more resistance and thus, would produce more heat.

 

Same total length of wire?

 

Are they in series, parallel, or what type of circuit are they in?

 

Higher resistance does not necessarily mean more heat. Usually it means less current flow...and often that means less heat.

[Ramesh ss]

I think both will have the same resistance because it depends on length , area of cross-section , temperature and substance of the wire.

So by coiling the wire, none of the above changes and so does the resistance. But if we talk of heat production , the coiled wire will produce more heat than the straight wire as it has a lesser surface area for heat dissipation the the straight wire.

[imatfaal]

If the circuit is such that resistance will cause temperature rise and if the coils are close enough to stop heat dissipation as Parth suggested above then as the material is less able to lose heat, the temperature increase will be greater, which for most metals at room temperature will raise resistance and generate more heat...

[swansont]

If it's AC, coiled wire will probably have a higher impedance from the inductance.

[rktpro]

Same total length of wire?

 

Are they in series, parallel, or what type of circuit are they in?

 

Higher resistance does not necessarily mean more heat. Usually it means less current flow...and often that means less heat.

 

They are connected separately with a cell.

Higher resistance surely means more heat. Q=IRT

 

I think both will have the same resistance because it depends on length , area of cross-section , temperature and substance of the wire.

So by coiling the wire, none of the above changes and so does the resistance. But if we talk of heat production , the coiled wire will produce more heat than the straight wire as it has a lesser surface area for heat dissipation the the straight wire.

 

It also depends upon average relaxation time, number of electrons per unit volume. Also, I don't want to know the further consequence. I wanted to know about the heat generated without considering heat dissipation. What I want to know is- Would resistance and thus heat change increase? If yes, Mathematically how?

[DrRocket]

They are connected separately with a cell.

Higher resistance surely means more heat. Q=IRT

 

No.

 

Given a voltage source (like a cell) with an internal resistance [math] R_0[/math] maximim power dissipation in a load resistor [math] R_L[/math] occurs when [math]R_L=R_0[/math] .

 

This results from the fact that the current is dependent on the resistance [math]E=I(R_0+R_L)[/math] and the power disipated in the load is [math]I^2R_L[/math]. A simple application of calculus shows that the maximum power dissipation occurs when [math]R_L=R_0[/math].

 

This is the resistive counterpart of "impedance matching" which maximizes the power to the speakers in your stereo.

Edited May 18, 2011 by DrRocket

[J.C.MacSwell]

They are connected separately with a cell.

Higher resistance surely means more heat. Q=IRT

 

 

 

It also depends upon average relaxation time, number of electrons per unit volume. Also, I don't want to know the further consequence. I wanted to know about the heat generated without considering heat dissipation. What I want to know is- Would resistance and thus heat change increase? If yes, Mathematically how?

 

As per Dr. Rocket:

 

As an example: A very high (or infinite) resistance would mean very little (or no) heat.

[rktpro]

If the circuit is such that resistance will cause temperature rise and if the coils are close enough to stop heat dissipation as Parth suggested above then as the material is less able to lose heat, the temperature increase will be greater, which for most metals at room temperature will raise resistance and generate more heat...

 

From wikipedia, the definition of dissipation in terms of physics - In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system.

 

This means that heat dissipation would increase the temperature of the wire? Then the straight wire which has more area for heat dissipation should get more heated.

If no, then how come coiled wire provide less surface area for heat dissipation?

[imatfaal]

Rktpro - if you had searched Wikipedia on heat dissipation which is what I typed then there would be no confusion.

[TonyMcC]

We seem to be making a meal of this one!

The two pieces of wire will have the same resistance (which is opposition to direct current flow).

The coiled wire will have greater impedance (which is opposition to alternating current flow). This is because it also has inductance .

If the question concerns resistance then it is fair to assume a d.c. circuit and so the temperature of both wires will be the same.

The amount of power taken and given off as heat can be calculated :- Power = (V^2)/R.

Thus higher resistance produces less heat. In this I am assuming the power source (eg battery) has negligible internal resistance, otherwise we shall have to consider maximum power transfer. This occurs when the battery's internal resistance equals the load resistance. I don't think the OP wants to go as far as this.