## Resistance

Current is the flow of electrons traveling through some medium around a circuit. The magnitude of the current is determined by the voltage across the circuit. There is another parameter that determines the magnitude of the current across the circuit, known as resistance. The higher the resistance lesser the electron flow or current.

The resistance of a given medium depends primarily on two factors, such as the material it is made of and its shape. For a given material, the resistance is inversely proportional to the cross-sectional area. For example, a thick copper wire has lower resistance than other identical thin copper wires. The resistance of a given material is proportional to the length. I.e., a long copper wire has higher resistance than other short copper wires.

Electrical resistivity and its inverse, electrical conductivity, is a fundamental property of a material that quantifies how strongly it resists or conducts electric current. A low resistivity (therefore, high conductivity) indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho).

A simple analogy for an electric circuit and its attributes, current, voltage, and resistance, is water flowing in a closed circuit of pipes, driven by a mechanical pump. The rate of water flow is analogous to current. The pressure difference between the two points is similar to voltage. The higher the pressure difference between two points, the faster the water flow between those two points. Just as a higher voltage would result in a higher current. The diameter of the pipe is analogous to resistance. For the same pressure difference, if the pipe is narrowed offers greater resistance and reduces the speed of the water flow. Based on resistivity, materials can be classified into three main groups, Conductors, Insulators, and Semiconductors.

### Ohm's Law

Ohm's Law defines the relationship between voltage, current, and resistance in an electrical circuit. It states that the current through a conductor between two points is directly proportional to the voltage across the two points. The constant of proportionality is the resistance. This law can be expressed by a mathematical formula, such as: I = V / R Where I is the current through the conductor in amperes, V is the voltage measured across the conductor in volts, and R is the resistance of the conductor ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

It can be interpreted as one volt of pressure is required to push one amp of current through one ohm of resistance. Resistance cannot be measured in an operating circuit, so Ohm's Law is especially useful when it needs to be calculated rather than shutting off the circuit to measure resistance. A technician can determine R using the above variation of Ohm's Law.

Ohm's Law can be used to validate the static values of circuit components, current levels, voltage supplies, and voltage drops. If, for example, a test instrument detects a higher than normal current measurement, which means the resistance has decreased, or that voltage has increased, causing a high-voltage situation. It indicates a supply or circuit issue. In direct current (DC) circuits, a lower than normal current measurement, which means either the voltage is decreased, or circuit resistance has increased. Possible causes for increased resistance are poor or loose connections, corrosion, and damaged components.

### Kirchoff's Law

**Kirchoff's Current Law**

Kirchoff's circuit laws deal with the current and voltage in electrical circuits. This law states that, for any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node. A fundamental concept in physics says that the charge will always be conserved. In the context of circuits, it means that, since the current is the rate of flow of charge, the current flowing into a point must be the same as the current flowing out of that point.

**Kirchoff's Voltage Law**

This law states that the directed sum of the potential differences (voltages) around any closed loop is zero. As electrons flowing through a circuit pass through a component, they lose electrical energy. It is because work is done on them by the electric forces inside the circuit components. The negative work done by these electric forces on a unit of charge passes through a component is called the potential difference, or voltage across the component (also referred to as the voltage drop across the component). Since no energy can be lost, the work done by the electric forces around any closed loop in the circuit must be zero. It means that the sum of all potential differences across the component involved in the loop must be zero.