question about fixed resistors in a circuit?

Question

recently we've been doing physics coursework at school. we had to make sensors using fixed resistors as part of a potential divider circuit. since we had to choose a resistor of a specific value on our own, some kid didn't choose the right one and soon after he turned his power pack on at 6 Volts and connected it to his rotary potentiometer, smoke started coming out of the fixed resistor and it stank really badly.
my question is, what makes a fixed resistor burn like that? was it because he chose a very small resistance or a large resistance? was it because of excess electric charge in the circuit or excess electric current? what makes a component in an electric circuit burn like that? could i have some kind of scientific explanation pls? i need it for my coursework.

Answer 1

When a current flows through a resistor, power is dissipated as heat. In other words, the resistor warms up.

Apart from their resistance value, resistors are manufactured in different power ratings according to the amount of power that they need to dissipate safely for a particular job. So, for example, you can get 1/2 Watt resistors or 10 Watt resistors.

The amount of power dissipated = I^2R where I is the current and R is the resistance.

In this case the power dissipated was more than the resistor could safely handle, and it got too hot.

So we can say that I was too big for that particular resistor.

When resistors overheat they usually burn off the paint or protective coating on the surface. This usually causes quite a stink. If they are made of carbon, the carbon track can glow red hot and also start to burn.

Answer 2

A fixed resistor, which I'll just refer to as a resistor throughout, limits current across it in a circuit.

As such it is assumed that the current is within the limits of its power capabiity. 'Power' being the electrical power: voltage multiplied by current. This is measured in Watts. Most resistors are 1/4 Watt, but there are higher rated ones.

Apparently there was too much current passing thru that particular resistor, and it couldn't handle it. This was because the resistir was under-rated, or there was more then the appropriate current passing thru it, or maybe a little of both.

I'd say, check the curcuit and voltages and currents being used, then check the value of a resistor (make sure you're using the right one), and/or maybe use one with a higher power rating.

Try that and see if that helps.

Good luck, and remember...the magic smoke in any electrical device is what makes it work......once you release the magic smoke.....it won't work anymore!

ha ha

:)

Good luck with your project. Hopefully I won't see you in the medical section asking about how to take care of burns! yikes

Answer 3

Here's my take on it:

The current through a resistor is I = E/R So it would be big if either E is very big and/or R very small. Too much voltage would cause a large flow of current through the resistor, or a low resistor would too.

Now the heat or power generated is P = IE. So lots of E and/or I would cause lots of power. And the power loss through the resistor is I^2 * R. Since I is squared here, anything from the above that causes lots of I to flow would cause a big heat loss and overpower the resistor and smoke it.

Your resistor was too small for the voltage it saw, or putting in another way, your voltage was too high for that resistor. It generated more heat than it could dissipate. It's kind of like you overheating while jogging if you can't cool as quick or quicker than you generate heat - 'cept that you would pass out instead of smoke.

Answer 4

For a fixed potential difference, current is inversely proportional to resistance. It's a variant of Ohm's law, which says

V = IR

(Where V is potential difference, I is current and R is resistance).

That means, smaller the resistance, greater the current and vice-versa.

Heat generated is directly proportional to current in a circuit. So, for a larger current greater heat should be generated. That would explain the smoke coming out.

Physical explanation:
H = I^2 x R x T

Answer 5

The resistance chosen was too small thus allowing too much current to flow. V(volts)=I(current in amps)*R(resistance in ohms),

Power(in watts)=Volts*I(current in amps)

Once the current was too great, the power it needed to dissipate was greater than the rating of the resistor. It was probably only a 1/4 watt resistor, drew too much current and smoked it.

At 6Vdc and a 1/4W resistor ...you'd draw 41.67mA of current
So, 6V/41.67mA = 144 ohms (smallest resistor you can use without exceeding the 1/4 Watt rating at 6Vdc)

Answer 6

All fixed resistors have power ratings...indicating what level of currents they can safely handle.
You need to look for those with the highest power ratings available.