Electrical Principles Chapter 3 Resistance, Power, Coulombs, Ohm’s Law, Power Formula, Power – Current Relationships, Impedance, Multimeters, Calculating-Measuring Energy and Power

Resistance and Ohm’s

Resistance and Ohm’s Resistance is the opposition to current flow by the dissipation of heat. The Resistor is a device that is included within electrical and electronic circuits to oppose current flow by introducing a certain value of circuit Resistance. Resistance is measured in Ohms and is represented by the Geek letter Omega, the symbol for Omega is Ώ.

Resistance and Ohm’s The larger the resistance, the larger the value of Ohms and the more the resistor will oppose current flow. As the resistance in the circuit is increased the current will decrease and, conversely, as the resistance of the resistor is decreased the circuit current will increase.

Ohm’s Law In 1827 George Ohm proved there was a direct relationship between Voltage (E), Current (I), and Resistance (R) in an electrical circuit. This relationship is known as Ohm’s Law. Ohm’s Law states that current in a circuit is proportional to the voltage and inversely proportional to the resistance.

E I R Ohm’s Law E = Voltage - Volts I = Current - Amps R = Resistance or Reactance (Impedence) - Ohms Direction: Cover what you want and perform the mathematical process with what’s left over. Example: Cover E = I X R

Ohm’s Law Voltage/Current Relationship. Ohm’s Law states that if the resistance in a circuit remains constant, a change in current is directly proportional to a change in voltage. Example: A heat shrink sealing gun connected to a variable power supply.

Ohm’s Law Current/Resistance Relationship. Ohm’s Law states that if the voltage in a circuit remains constant, a change in resistance produces an inversely proportional change in current. The current in a circuit decreases with an increase in resistance, and the current in the circuit increases with a decrease in resistance. Example: The dimmer/brightness switch for lights inside a car.

Ohm’s Law In engineering applications, Ohm’s Law is used to solve for the proper values of voltage, current, or resistance during circuit design and to predict circuit characteristics before power is applied to a circuit when two of the three electrical values are know.

Ohm’s Law In troubleshooting applications, Ohm’s Law is used to determine how a circuit should operate and how it is operating under power. Resistance measurements can not be taken when a circuit is under load (power). Current and Voltage can be taken when a circuit is under load (power).

Power

Power Transforming energy from one form to another is called work. The greater the energy transformed, the more work that is done. There are six basic forms of energy and they are light, heat, magnetic, chemical, electrical, and mechanical energy. The unit for measuring work is called the Joule (J).

Power Power (P) is the rate at which work is performed and is measured by the unit called Watt (W). Watts = Joules per second. The output Power, or power ratings of electrical, electronic or mechanical devices can be expressed in Watts (W) and describes the number of Joules of energy converted every second.

Power The output of Power of rotating machines is given in the unit horsepower (hp). The output of Power of heaters is given in the unit British Thermal Units per hour (BTU/h) The output of Power of cooling units is given in the unit Ton of Refrigeration.

Power Despite the different names, all can be expressed in the unit of Watts (W) by using the following conversions: 1 hp = 746 W 1 BTU/h = 0.293 W 1 ton of refrigeration = 3520 W The amount of energy stored (W) is dependent on the coulombs of charge stored (Q) and the voltage (V) and can be expressed mathematically W = Q x V.

Power Power is the rate at which electric energy (W) is converted to some other form and can be expressed mathematically as P = I x V. This formula states that the amount of power delivered to a device is dependent on the electrical pressure (or voltage applied across the device) and the current flowing through the device.

Power Formula The Power Formula is the relationship between Power (P), Voltage (E), and Current (I). P E I P = Power -Watts E = Voltage - Volts I = Current - Amps Direction: Cover what you want and perform the mathematical process with what’s left over. Example: Cover E = I X R

Power Formula The Power Formula states that if the voltage in a circuit changes, the current in the circuit also changes. The power required from a circuit changes any time loads are added (power increases) or removed (power decreases). The Power Formula is used when troubleshooting and to predict circuit characteristics before power is applied.

Combining Ohm’s Law and Power Formula Ohm’s Law and the Power Formula may be combined mathematically and written as any combination of Voltage (E), Current (I), Resistance (R), or Power (P). Ohm’s Law and the Power Formula are limited to circuits in which electrical resistance is the only significant opposition to the flow of current. This limitation includes all DC circuits and AC circuits that do no contain a significant amount of inductance and/or capacitance – which we will learn about later.

Combining Ohm’s Law and Power Formula

Review Ohm’s Law states that current in a circuit is proportional to the voltage and inversely proportional to the resistance. The Power Formula states that if the voltage in a circuit changes, the current in the circuit also changes. The power required from a circuit changes any time loads are added (power increases) or removed (power decreases).

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