2. A tungsten filament in an incandescent bulb has the following properties when

Question

2. A tungsten filament in an incandescent bulb has the following properties when the bulb has been connected to an ideal 1.5 volt battery for a long time:

* temperature = 2800 K

* resistivity at this temperature = 8.47E-7 m

* length = 0.30 m

* diameter = 0.52 mm

You may assume that any other parts of the bulb and the wires have no resistance.

1. Determine the resistance of the tungsten filament.

2. Determine the current through the tungsten filament.

3. Consider the time when the circuit was first connected as t = 0. Describe what happens to the resistance and current as a function of time. Be specific and include calculations when possible.

Explanation / Answer

1)

resistance, R=rho*l/A

resistance, R=rho*l/(pi*r^2)

R=8.47*10^-7*0.3/(pi*(0.26*10^-3)^2)

R=1.19 ohms

2)

I=V/R

I=1.5/1.19

I=1.26 A

3)

Rt=Ro*(1+alpa*dT)

resistance icreases

and

current decreases

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