Circuit Concepts
Electric Current, voltage, and resistance are three of the fundamental electrical properties. Stated simply,
• current: is the directed flow of charge through a conductor.
• Voltage: is the force that generates the current.
• Resistance: is an opposition to current that is provided by the material, component, or circuit.
Electric Current, Voltage, and resistance are the three primary properties of an electrical circuit. The relationships among them are defined by the fundamental law of circuit operation, called Ohm’s law.
Electric Current
As you know, an outside force can break an electron free from its parent atom. In copper (and other metals), very little external force is required to generate free electrons. In fact the thermal energy (heat) present at room temperature (220C) can generate free electrons. The number of electrons generated varies directly with temperature. In other words, higher temperatures generate more free electrons.
The motion of the free electrons in copper is random when no directing force is applied. That is, the free electrons in copper are random when no directing force is applied. That is, the free electrons move in every direction, as shown in Figure 1. Since, the free electrons are moving in every direction, this net flow of electrons in any direction is zero.
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When an external force causes all of the electrons to move in the same direction. In this case, a negative potential is applied to one end of the copper and a positive potential is applied to the other. As a result, the free electrons all move from negative to positive, and we can say that we have a directed flow of charge (electrons). This directed flow of electrons is called electric current.
1.3 OHMS LAW
Ohm’s law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperature remain constant.
1.4 KIRCHOFF'S LAW
Kirchhoff’s Current Law - The total current entering a circuits junction is exactly equal to the total current leaving the same junction.
Σ IIN = Σ IOUT.
Kirchhoff’s Voltage Law - The algebraic sum of all the voltages around any closed loop in a circuit is equal to zero.
ΣV = 0.
1.5 VOLTAGE DIVIDER
Voltage Divider circuits are used to produce different voltage levels from a common voltage source but the current is the same for all components in a series cicruit
The simplest, easiest to understand, and most basic form of a passive voltage divider network is that of two resistors connected together in series. This basic combination allows us to use the Voltage Divider Rule to calculate the voltage drops across each series resistor.
1.6 CURRENT DIVIDER
Current Divider circuits have two or more parallel branches for currents to flow through but the voltage is the same for all components in the parallel cicruit
Current Divider Circuits are parallel circuits in which the source or supply current divides into a number of parallel paths. In a parallel connected circuit, all the components have their terminals connected together sharing the same two end nodes. This results in different paths and branches for the current to flow or pass along. However, the currents can have different values through each component.
1.7 Analogy between Electrical and Other Non-electric Physical Systems
The analogy, of course, is a mathematical one: that is, two systems are analogous to each other if they are described by similar equations. The analogous electric quantities for a mechanical system
1.8 Effect of temperature on resistance
the resistance of a conductor changes with the size of the conductor (e.g. thicker wires have less resistance to current flow than thinner wires), the resistance of a conductor also changes with changing temperature. This may be expected to happen because, as temperature changes, the dimensions of the conductor will change as it expands or contracts.
However, materials that are classed as CONDUCTORS tend to INCREASE their resistance with an increase in temperature. INSULATORS however are liable to DECREASE their resistance with an increase in temperature. Materials used for practical insulators (glass, plastic etc.) only exhibit a marked drop in their resistance at very high temperatures. They remain good insulators over all temperatures they are likely to encounter in use.
These changes in resistance cannot therefore be explained by a change in dimensions due to thermal expansion or contraction. In fact for a given size of conductor the change in resistance is due mainly to a change in the resistivity of the material, and is caused by the changing activity of the atoms that make up the material.
