Final answer:
The voltage across a closed switch in a circuit can be determined using Ohm's law. In this case, a voltage reading of 1.2 volts across a closed switch in a 12-volt system with 21-ohms system resistance and a 3-ohms EOL indicates a bad switch.
Step-by-step explanation:
The voltage across a closed switch in a circuit can be determined using Ohm's law, which states that voltage is equal to the current multiplied by the resistance. In this case, the resistance is 21 ohms and the voltage is 1.2 volts. Therefore, the current flowing through the switch can be calculated as 1.2 volts divided by 21 ohms, which is approximately 0.057 amperes.
Since the voltage across the switch is significantly lower than the 12 volts provided by the system, it indicates a bad switch. A normal switch, when closed, would have the same voltage across it as the system voltage, which is 12 volts in this case. A weak battery would result in a lower system voltage, while a ground fault would typically result in no voltage across the switch.