Edison Problem Solving Guide

Calculation

In Calculation problems, you must use the given parameters to calculate the indicated unknown value.

Most of the problems can be solved using only a few basic laws.

* Connecting resistors in series. When resistors are connected in series, the total resistance is increased. The total resistance of any number of resistors in series equals the sum of their individual resistances. For example: R=R1+R2=20+80=100 ohm.

* Connecting resistors in parallel. When resistors are connected in parallel, the total resistance is lowered. For any number of resistors in parallel, the reciprocal of the total resistance equals the sum of the reciprocals of their individual resistances. For example: 1/R = 1/R1 + 1/R2.

* Ohm’s law. Ohm's law is named after the German physicist Georg Simon Ohm. This law is used to calculate voltage, current or resistance in electrical circuits. If you double or triple the voltage across the terminals of a resistor, the current through the resistor will be two or three times greater. This phenomenon is expressed by Ohm's law, V/I = constant = R

* Electric power. Power is an indication of how much work (the conversion of energy from one form to another) can be accomplished in a specified amount of time; that is, power is the rate of doing work. The power delivered to or absorbed by an electrical device can be found in terms of the device's terminal current and voltage: P = W/t = QV/t = VQ/t. But since I = Q/t, P = VI (watts). By direct substitution of Ohm’s law, the equation for power can be obtained in two other forms: P = V*V/R, or P = I*I*R.

* Kirchhoff’s current law. Kirchhoff’s current law states that the sum of the currents entering an area, system, or junction must equal the sum of the currents leaving the area, system, or junction. For example, if we identify that at a junction where two paths with currents of 1 A and 2 A flow, and there is one and only one additonal path connecting to this junction, then the current through this third path will be 3 A.

* Kirchhoff’s voltage law. Kirchhoff’s voltage law states that the algebraic sum of the potential (voltage) rises and drops around a closed loop is zero. It means that the applied voltage of a series circuit equals the sum of the voltage drops across the series elements. For example if 1 V drops on one resistor, and 2 V drops on another one, and they are connected in series, then 3 V drops on the two resistors together.