A tungsten filament used in a flashlight bulb operates at 0.20 A and 3.0 V. If its resistance at 20°C is 1.5Ω, what is the temperature of the filament when the flashlight is on?

The temperature of the filament when the flashlight is on is 2020 °C.

Explanation:

The resistivity varies linearly with temperature:

  (1)

Where:

T: is the temperature of the filament when the flashlight is on=?

T₀: is the initial temperature = 20 °C

α: is the temperature coefficient of resistance = 0.0045 °C⁻¹ 

R₀: is the resistance at T₀ = 1.5 Ω    

When V = 3.0 V, R is:

By solving equation (1) for T we have:

Therefore, the temperature of the filament when the flashlight is on is 2020 °C.

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