across the LED element remains constant and the rest is across the resistor. Visible light corresponds to LED element threshold voltages of 1.7 V to 3 V. To make our analysis simpler, let's use 2 V for our threshold voltage value. Most of the resistance in the LED bulb is there to limit the current and (as we will soon see) a reasonable value to choose for resistance would be 300 Ohms. An incandescent bulb is simply a long, thin filament that acts like a resistor that gets so hot that about 20% of the energy dissipated in it is dissipated as visible light rather than as heat or infrared light. Consider the LED bulb: How much power is converted to heat in the resistor of the bulb, how much to light in the LED element of the bulb? Why would we be better off producing the same light output with a lower voltage battery and smaller resistor? Now consider the incandescent bulb: Recall, only 20% of the electric power appears as visible light for an incandescent bulb! How much power do we need to provide in order to produce the same amount of visible light as produced by our LED bulb? What should the resistance of the bulb’s filament be, if, as stated, we use a 9 V battery?
across the LED element remains constant and the rest is across the resistor. Visible light corresponds to LED element threshold voltages of 1.7 V to 3 V. To make our analysis simpler, let's use 2 V for our threshold voltage value. Most of the resistance in the LED bulb is there to limit the current and (as we will soon see) a reasonable value to choose for resistance would be 300 Ohms. An incandescent bulb is simply a long, thin filament that acts like a resistor that gets so hot that about 20% of the energy dissipated in it is dissipated as visible light rather than as heat or infrared light. Consider the LED bulb: How much power is converted to heat in the resistor of the bulb, how much to light in the LED element of the bulb? Why would we be better off producing the same light output with a lower voltage battery and smaller resistor? Now consider the incandescent bulb: Recall, only 20% of the electric power appears as visible light for an incandescent bulb! How much power do we need to provide in order to produce the same amount of visible light as produced by our LED bulb? What should the resistance of the bulb’s filament be, if, as stated, we use a 9 V battery?
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Transcribed Image Text:You design two flashlights, both using a 9V battery for power, with one using an LED bulb for the
light and the other an incandescent bulb.
The LED bulb contains an LED element that converts electrical energy directly to light and a small
resistance in series. Current will flow through the bulb once the voltage across this combination is
equal to or greater than the threshold voltage corresponding to the energy of the emitted photons
per electron (1240 eV nm / wavelength in nm.) Once that threshold has been reached, the voltage
Transcribed Image Text:across the LED element remains constant and the rest is across the resistor. Visible light corresponds
to LED element threshold voltages of 1.7 V to 3 V. To make our analysis simpler, let's use 2 V for
our threshold voltage value. Most of the resistance in the LED bulb is there to limit the current
and (as we will soon see) a reasonable value to choose for resistance would be 300 Ohms.
An incandescent bulb is simply a long, thin filament that acts like a resistor that gets so hot that
about 20% of the energy dissipated in it is dissipated as visible light rather than as heat or infrared
light.
Consider the LED bulb: How much power is converted to heat in the resistor of the bulb, how much
to light in the LED element of the bulb? Why would we be better off producing the same light
output with a lower voltage battery and smaller resistor?
Now consider the incandescent bulb: Recall, only 20% of the electric power appears as visible light
for an incandescent bulb! How much power do we need to provide in order to produce the same
amount of visible light as produced by our LED bulb? What should the resistance of the bulb's
filament be, if, as stated, we use a 9 V battery?
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