Photoelectric cell and its uses
A photoelectric cell turns light into an electric current. Its saturation current grows with the intensity of light, so it can sense light, count objects and replay film sound.
Construction of a photoelectric tube
- 1An evacuated glass bulb holds two electrodes.
- 2The cathode C is a semi-cylindrical metal plate. It is coated with a metal of low work function, chosen so that the light to be sensed can free electrons.
- 3The anode is a straight wire of nickel or platinum, placed along the axis of the cathode.
- 4★A battery (with a variable voltage) and a microammeter are joined between anode and cathode, with the anode positive.
★ key step
How it works
When light above the threshold frequency falls on the cathode, it gives out photoelectrons. The positive anode collects them, so a current flows. A few electrons reach the anode even at zero voltage, because they leave with some kinetic energy. As the voltage rises, the current rises and then saturates. The saturation current is tiny, of the order of 10⁻⁹ A, and it is proportional to the intensity of light. So the current tells us how much light falls on the cell. Cut off the light and the current stops at once.
Uses of photoelectric cells
- Sound in cinema films: light passes through the sound track along the edge of the film and falls on a photocell. The varying current is amplified and fed to a loudspeaker.
- Sending pictures over long distances (photo-telegraphy) and scanning scenes in television cameras.
- Burglar alarms and fire alarms: an intruder or smoke cuts a light beam, the current stops and an alarm rings.
- Counting people, vehicles or articles that pass through a light beam, for example on a factory belt.
- Catching traffic offenders and switching street lights on at dusk.
- Finding tiny holes or flaws in metal sheets: light leaking through a hole shows up as a current.
Worked example: counting the electrons
A photocell gives a saturation current of 3.2 nA. (a) How many electrons reach the anode each second? (b) The small lamp lighting it is moved from 100 cm to 50 cm away. What happens to the current? (e = 1.6 × 10⁻¹⁹ C)
- (a) Current I = (electrons per second) × e, so electrons per second = I/e.
- I/e = 3.2 × 10⁻⁹ A ÷ 1.6 × 10⁻¹⁹ C = 2 × 10¹⁰ electrons per second.
- (b) For a small lamp, intensity ∝ 1/(distance)². Halving the distance makes the intensity 2² = 4 times larger.
- The saturation current is proportional to intensity, so it becomes 4 × 3.2 nA = 12.8 nA.
(a) 2 × 10¹⁰ electrons per second; (b) the current becomes four times as large, 12.8 nA
How to draw a photoelectric tube
- Draw an oval evacuated glass bulb.
- Inside, draw a curved, half-cylinder plate labelled cathode C (coated with a low work function metal).
- Draw a thin vertical line at its centre labelled anode (wire).
- Draw wavy arrows of incident light falling on the cathode.
- Outside, join the anode to the + of a battery B and the cathode to the −, with a microammeter (μA) in series. Label the two leads P₁ and P₂.
- Beside it, draw the graph of current against voltage, rising and then flattening, with a higher flat curve for greater intensity.
The automatic doors at a railway station or a mall work like a photocell. So does the sensor that turns on street lights at dusk: light controls a current.
2 marks: “State three uses of a photoelectric cell.” Reproducing sound from cinema film; burglar and fire alarms; counting people or articles; switching street lights; detecting flaws in metal sheets (any three).
2 marks: “Describe the construction of a photoelectric tube with a labelled diagram.” Evacuated glass bulb; semi-cylindrical cathode coated with a low work function metal; wire anode of nickel or platinum on the axis; battery with the anode positive and a microammeter in series. Diagram with light falling on the cathode.
1 marks: “How does the saturation current of a photocell change if the intensity of light is doubled?” It doubles, since it is proportional to intensity.
Light in, current out: coated cathode, wire anode, current ∝ intensity.