Circuit Analysis Part 3: Ohm’s Law, Power and Energy
By: Jason Hall
Tags: Electrical Engineering, Electricity, Circuit Analysis, Electronics, Physics

In part two of this series I provided an introduction to electrical resistance. In this article I will reiterate some principles of Ohm’s law that were featured in previous articles and introduce electrical power and energy. I will also briefly discuss SPICE programs and their use in circuit analysis.

Ohm’s Law

Georg Ohm determined that current (I) in a circuit is directly proportional to the applied voltage (E) and inversely proportional to the resistance (R). Ohm’s law states that I = E/R. The equation can be rearranged to solve for any of the three variables:

Normally the voltage source is shown on the left and the load (in this case, a resistor) is shown on the right, with an arrow indicating the direction of current (conventional direction):

Voltage Symbols

In circuit diagrams you will often see voltage represented as both E and V. The distinction here is that voltage sources are identified by E, while loads are identified by V. The voltage polarity is often identified as well:

E identifies voltage source, while V identifies the load

Power

Electric Power (P) is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt.

It is easy to calculate the power supplied in watts to a load using the equations for power. Let’s say that you have a circuit with a 120 V source and a load that has a resistance of 15 ohms. As you can see below, there is more than one way to calculate the power supplied:

Energy

In terms of electricity, energy is simply a measure of power supplied per unit of time. For example, if a 100 watt light bulb is lit for fifteen hours the energy used by the bulb is (100 W)(15 hours) = 1500 Wh (watthours).

In North America, energy is measured in kilowatthours, therefore the equivalent is 1500 Wh/1000 = 1.5 kWh.

Computer Aided Circuit Analysis

SPICE (Simulation Program with Integrated Circuit Emphasis) software is often used in industry to design circuits and simulate their behavior. There are many products available that make use of SPICE, and in future articles I will use one of these programs called Multisim to help illustrate the points being made. LTspice is an open source alternative to Multisim.

I have created the simple resistive circuit from above in Multisim and connected a wattmeter to measure the power supplied to the load. Note that the current is measured in series with the load (red wires) and the voltage is measured across the load (blue wires):

Summary

Current in a resistive circuit is proportional to the applied voltage, and inversely proportional to the resistance.

Ohm’s law can be used to solve for voltage, current, and resistance.

Voltage sources are often identified by E, while loads are identified by V.

Electrical power is the rate at which electrical energy is transferred in a circuit and is measured in watts.

Energy is a measure of electrical power per unit of time and is measured in watthours.

SPICE software is used in industry to aid in circuit design and analysis.

In the next series of articles I will introduce basic DC circuit analysis of series circuits, parallel circuits, and series-parallel circuits.