Lecture_2 - ECE103 Spring 2008 Lecture 2 Basic Laws- Ohms...

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ECE103 Spring 2008 Lecture 2 Basic Laws- Ohm’s Law Materials react resisting the passage of current through it. This is due to the microscopic interaction of the moving charges and the atomic structure of the material. This ability to resist the current of flow of charges is called resistivity. Resistivity is a characteristic of any material. Materials with very low resistivity are called conductors, materials with very small resistivity are called isolators and those in between are classified as semiconductors. When a piece of any material is “connected” to a voltage source (for example a battery), charges will flow through it. The amount of charge flowing through the material will depend on the geometry, and also on the intrinsic characteristics of the material (this is the resistivity). Intuitively, we can understand the importance of the geometry if we make an analogy with a water pipe: the flux (current) through a pipe (resistor) at a certain water pressure (voltage) will be proportional to the pipe section (cross section of the resistor). In a mathematical form the resistance R is related to the characteristics of the material and to the geometry through R A ρ = A where R is the resistance , ρ (rho) is the resistivity , A is the length of the resistor and A is the cross section. Resistance is measured in ohms (the symbol is ). So resistivity is expressed in .m (ohm per meter). In different materials this quantity can vary several orders of magnitude (~10 -8 m for conductors to more than 10 12 m for insulators). The relationship between the current and the resistance in an electric circuit is the Ohm’s law and states: Mathematically, this is expressed as vi R = I +- battery A I l l Ohm’s Law: The voltage across a resistor is directly proportional to the current i flowing through it, and the constant of proportionality is the resistance
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So the current through a resistor will increase increasing the voltage across it and will decrease increasing the resistance. Using our pipe and water analog we can say that the water flux (current) will increase increasing the pressure (voltage) and will decrease decreasing the cross section (increasing resistance). From Ohm’s law we can define 11 V A Ω= There are two extreme values: if an element has a resistance so small that we can consider R 0, we call a short circuit . On the other extreme, if an element has a resistance so big that we can consider R , we call this an open circuit . Resistors that are connected to real circuits and they come in different sizes and shapes. The shape and size of the resistors will depend on different characteristics as the power it can handle, the material, etc. . The photograph depicts several resistors with fixed value. In many cases a given circuit needs to have resistors with variable value (continuous variable value) that are known as potentiometers or most commonly named as pots . Next figure indicates the usual symbol and a photograph of some pots Resistors with a fixed value are called “linear”. These elements follow the Ohm’s law
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Lecture_2 - ECE103 Spring 2008 Lecture 2 Basic Laws- Ohms...

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