Electric potential difference, or voltage, is about how much energy per unit charge is gained or lost as charges move between two points in a circuit. The unit is the volt, and one volt equals one joule per coulomb (1 V = 1 J/C). In simple terms, if a charge of q coulombs moves through a potential difference of ΔV, the energy change is ΔE = qΔV. So moving 1 coulomb through 1 volt gives 1 joule of energy. This is why a battery or power source with a higher potential difference can push charges to do more work as they flow through the circuit. The other ideas—power (rate of energy transfer, measured in watts), opposition to current (resistance in ohms), or energy stored per unit charge—don’t define what the potential difference itself measures.

A short circuit is a path of very low resistance that bypasses the intended load in a circuit. When such a low-resistance path forms, most of the current can flow through it instead of through the load. Because current is determined by I = V/R, a small resistance means a large current. This surge can overheat wires, blow fuses, or damage components, and the load may not receive the voltage or current it’s supposed to. In short, it’s a low-resistance shortcut that draws excessive current.

The amount of resistance in a conductor depends on how easily charges can flow through it. For a given material and a fixed temperature, resistance is proportional to the length and to the resistivity, and inversely proportional to the cross-sectional area: R = ρL/A. This means that increasing the cross-sectional area provides more pathways for current to flow, so the same amount of charge moves through a larger area with less opposition, lowering the resistance. Conversely, narrowing the area makes it harder for charges to pass, increasing resistance. The idea that resistance stays the same or increases with larger area isn’t consistent with this relationship, and color has no effect on resistance.

Measuring current in a circuit requires a device that becomes part of the path the current travels. An ammeter is designed for this purpose: it is connected in series with the circuit, so all the current flows through it. Because an ideal ammeter has near-zero resistance, it doesn't change the current much, and it displays the current in amperes. A voltmeter, on the other hand, is placed in parallel to measure voltage, not current. A galvanometer is a sensitive current detector and is the basic sensing element inside an ammeter, but by itself it isn’t typically used to measure the circuit’s current unless configured as part of a larger instrument. A multimeter can measure current, but it’s a versatile tool that can measure several quantities; when used to measure current, it serves that function through an ammeter-like path, not specifically a dedicated current meter. Therefore, the instrument best suited to measure current in a circuit is the ammeter.