Lecture 3 Ohms Law 1 Ohms Law In
- Slides: 38
Lecture 3 Ohm’s Law 1
Ohm’s Law: In closed electrical circuit, the current that passes through a conductor between two points is directly proportional to the voltage across it. I 2
Volta Ampere Ohm Coulomb The common units: Volts, Amperes, Ohms, Coulombs are all named after people
Resistance is an electrical quantity that measures how the device or material reduces the electric current flow through it. The opposition of a material to current flow due to the collisions between free electrons and atoms. 4
Factors Affecting Resistivity 1. The length L of the material. Where longer materials have greater resistance. L 2 L 1 2 If the wire doubles in length, it doubles in resistance 5
2. The cross-section area of the material. Notice that the electrons seem to be moving at the same speed in each one but there are many more electrons in the larger wire. • This results in a larger current which leads us to say that the resistance is less in a wire with a larger cross sectional area. • It can be shown that R 1/A. 6
3. The higher temperatures result in higher resistances. For metals and a lot of insulators, when temperature raises, the lattice ions vibrate more vigorously, increasing the frequency of collision between electrons and the lattice. The resistance therefore increases. T ↑ ⇒ R↑ 7
4. The kind of material. Iron has more electrical resistance than a geometrically similar copper conductor. 8
Resistivity of a material 9
TYPES OF RESISTORS (Fixed and variable) 1. Fixed Resistors Fixed-composition resistors: (a) construction; (b) appearance. 10
Calculating Resistance Color No. Black Brown Red Orange Yellow Green Blue Violet Gray White 0 1 2 3 4 5 6 7 8 9 11
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Example: Calculating Resistance The first two bands correspond to Value. The third band tells you the number of zeros following. The fourth band tells you the percentage tolerance.
l l Since a device loses heat through its surface, the larger the surface area, the more heat a device dissipates. Heat dissipation is related to device surface area. Larger devices have a larger surface area. A physically larger device is able to dissipate more heat and handle more power. 14
Is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider ? Variable Resistors Potentiometers 15
2. Rheostats A variable resistance used to control current usually a two-terminal variable resistor potentiometer can be wired as a rheostat 16
Construction of a Potentiometer 17
Setting the Value of Resistance 18
Effect of Temperature on Resistivity (Figure a) Demonstrates positive temperature coefficient (Figure b) Demonstrates negative temperature coefficient 19
60 Resistivity (Ω. m) 50 ρ 40 Δ�� 30 �� 20 Δ�� 10 0 0 20 40 60 80 100 120 Temperature(℃) (Degree Centigrade) ����������� 20
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Temperature Coefficient For most materials, the resistance R changes in proportion to the initial resistance Ro and to the change in temperature DT. Change in resistance: 22
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Resistivity of various materials Material Class (. m ) Copper Good conductor 1. 72 10 -8 Aluminum Good conductor 2. 65 10 -8 Germanium Semiconductor 0. 6 Silicon Semiconductor 2300 Quartz insulator 5 1016 24
Ohm’s Law _ Every conversion of energy from one form to another can be related to this equation. In electric circuits the effect we are trying to establish is the flow of charge, or current. _ The potential difference, or voltage between two points is the cause (“pressure”), _ The resistance is the opposition encountered. _
Ohm’s Law Developed in 1827 by George Simon Ohm _ For a fixed resistance, the greater the voltage (or pressure) across a resistor, the more the current. _ The more the resistance for fixed voltage, the less the current. _ Current is proportional to the applied voltage and inversely proportional to the resistance.
Ohm’s Law (Golden Triangle) 27
Plotting Ohm’s Law
Plotting Ohm’s Law Insert Fig 4. 8
Example : When a 3 -V battery is connected to a light, a current of 6 m. A is observed. What is the resistance of the light filament? + I R = 500 R - 6 m. A V=3 V Source of EMF 30
In this first example, we will calculate the amount of current (I) in a circuit, given values of voltage (E) and resistance (R): What is the amount of current (I) in this circuit? 31
In the last example, we will calculate the amount of voltage supplied by a battery, given values of current (I) and resistance (R): What is the amount of voltage provided by the battery? 32
Power in Electrical Systems 33
Homework 34
WATT’S LAW ( Magic Circle ) 35
Efficiency
Efficiency _ The basic components of a generating (voltage) system are shown below, each component has an associated efficiency, resulting in a loss of power through each stage. Insert Fig 4. 19
Efficiency 38
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