VALVE GP-SELECTOR & PRESS CONT 1309434 - Caterpillar

Resistance
George Simon Ohm discovered that for a fixed voltage, the amount of current flowing through a material depends on the type of material and the physical dimensions of the material. In other words, all materials present some opposition to the flow of electrons. That opposition is termed resistance. If the opposition is small, the material is labeled a conductor. If the opposition is large, it is labeled an insulator.The Ohm is the unit of electrical resistance. The symbol to represent an Ohm is the Greek letter omega, Ohms. A material is said to have a resistance of one ohm, if a potential of one volt results in a current of one ampere.It is important to remember that electrical resistance is present in every electrical circuit, including components, interconnecting wires, and connections. Electrical circuits and the laws that relate to the electrical circuits will be discussed later in this unit.As resistance works to oppose current flow, it changes electrical energy into other forms of energy, such as, heat, light, or motion. The resistance of a conductor is determined by four factors:
Illustration 11 g01068936
Atomic structure is the amount of free electrons. The more free electrons a material has, the less resistance that is offered to current flow.
Length. The longer the conductor, the higher the resistance. If the length of the wire is doubled, as shown in Illustration 12(a), the greater the resistance between the two ends.
Width (cross sectional area). The larger the cross sectional area of a conductor, the lower the resistance (a bigger diameter pipe allows for more water to flow). If the cross section area is reduced by half, as shown in Illustration 12(b), the resistance for any given length is increased by a factor of 4.
Temperature. For most materials, the higher the temperature, the higher the resistance. Illustration 12(c) shows the resistance increasing as the temperature rises. Please note, there are a few materials whose resistance decreases as temperature increases.
Illustration 12 g01068937
Electrical Circuits and Laws
An electrical circuit is a path or a group of interconnecting paths, that are capable of carrying electrical currents. The electrical circuit is a closed path that contains a voltage source or sources. There are two basic types of electrical circuits: series and parallel. The basic series and parallel circuits may be combined to form more complex circuits, but these combinational circuits may be simplified and analyzed as the two basic types. It is important to understand the laws that are needed to analyze electrical circuits and to diagnose electrical circuits. They are Kirchoff's Laws and Ohm's Law.Gustav Kirchoff developed two laws for analyzing circuits. The two laws are stated below:
Kirchoff's Current Law (KCL) states that the algebraic sum of the currents at any junction in an electrical circuit is equal to zero. Simply stated, all the current that enters a junction is equal to all the current that leaves the junction.
Kirchoff's Voltage Law (KVL) states that the algebraic sum of the electromotive forces and voltage drops around any closed electrical loop is zero. Simply