Final answer:
One battery terminal in cars is connected to the metal body to create a chassis ground, allowing the car's frame to serve as the return path for electric current. This eliminates the need for a second wire for each device, simplifying the electrical system and reducing weight and cost.
Step-by-step explanation:
In automobiles, the connection of one battery terminal to the metal body of the vehicle is part of the design of the vehicle's electrical system referred to as a chassis ground or grounding the vehicle. This system allows the metal frame of the vehicle to serve as a return path for electric current, which eliminates the need for a second wire running from the electrical devices back to the battery to complete the circuit. Every electrical device is connected with just one wire to the power source (the battery) and relies on the car's metallic structure to complete the return path to the battery's other terminal.
This is possible due to the principle that an electrical circuit must form a loop for the current to flow. By using the chassis as a conductor, it effectively becomes part of the circuit. This greatly simplifies wiring and reduces the amount of copper wire needed, which in turn reduces the weight and overall cost of the vehicle's electrical system. This design also simplifies repairs, because there is less wiring to deal with. However, for this system to work effectively, all parts of the vehicle intended to conduct electricity must have a good electrical connection to the body of the vehicle to prevent issues like poor conductivity or electrical resistance.