A student is building a simple indicator circuit for a model railway signal. The circuit uses a 9V battery, an LED and a switch. The student connects the LE...
Question 1 Report
A student is building a simple indicator circuit for a model railway signal. The circuit uses a 9V battery, an LED and a switch. The student connects the LED directly to the battery without any other components.
(a) State why the LED must be connected the correct way round in the circuit (with the correct polarity). [1]
(b) Name the component that must be connected in series with the LED to prevent damage. [1]
(c) State what would happen to the LED if it were connected directly to the 9V battery without this protective component. [1]
Answer Details
(a) The LED must be connected with the correct polarity because it is a semiconductor diode that only allows current to flow in one direction (from anode to cathode). [1]
If connected the wrong way round (reverse-biased), the LED blocks current flow and will not emit light. Unlike a filament bulb, which works regardless of polarity, an LED's internal P-N junction only conducts when the anode is connected to the positive terminal and the cathode to the negative terminal of the supply.
(b) A resistor (current-limiting resistor) must be connected in series with the LED. [1]
(c) Without the current-limiting resistor, too much current would flow through the LED, causing it to overheat and burn out almost immediately. [1]
A 9V battery can supply far more current than a standard LED can safely handle (typically 20 mA maximum). The LED's forward resistance is very low once it begins conducting, so without a resistor to restrict the current, the power dissipated in the LED's junction would exceed its rating, permanently destroying the component.
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