Theme 2: Energy and the Environment Background Material

Ohm's Law

Resistors
In electronics, resistance is provided by a technological device called a "resistor."

Inside a fixed resistor Diagram from "Ultimate Visual Dictionary of Science," Stoddart 1998.

A resistor regulates the flow of electric current in a circuit. If a battery is attached to a closed circuit, a voltage differential is established. Resistors can control current from a battery by producing opposing voltages that increase with the current flow. When the power is switched on, current increases until the resistor voltages balance the battery voltage. This limits the current flow through the resistor. Resistance is measured in units called Ohms.

This effect may be used directly in electrical circuits. For example, a light-emitting-diode (LED) may require 3.6 volts to operate. If a 9-volt battery is used, the LED would quickly burn out. However, if the appropriate resistor is added to the circuit, the voltage over the LED will be lowered. Lowering the voltage will prevent the LED from being damaged or destroyed. The resistor lowers the voltage in the entire circuit.

Resistors get hot when they are working because they exert control by converting power to heat. They can only reduce the power of an electric signal, never increase it.

Ampere - unit of flow
The term ampere is the unit of electric current flow and is simply a measure of the rate at which electrons are flowing in a circuit. For example, a current flow of 2-Amps is flowing twice as fast as a flow of 1-Amp. A current flow of 4-Amps is flowing four times faster than a flow of 1-Amp.

Voltage - unit of pressure
The pressure behind the electrons affects their rate of flow or amperes. The pressure, called voltage, is the electric pressure applied to electrons to make them flow. The volt is the common name or unit used to measure the electric pressure.

A volt is a certain amount of electric pressure, or force, tending to cause any electrons present to flow in an electric current. For example, 4 volts produces four times the number of amperes that would result from 1 volt. 10 volts produces ten times the number of amperes compared to 1 volt.

Ohm's Law
In order to understand Ohm's Law, the following points should be reviewed:

• Current flow is measured in Amperes (A).
• Voltage is measured in volts (V).
• Resistance is measured in Ohms (O or Greek symbol "omega").
• Electric power is measured in Watts (W).

• Current flow is measured in Amperes (A) and is designated as I which stands for Intensity of flow.
• Voltage is measured in volts (V) and is designated with an E which represents the Electromotive Force of pressure of flow.
• Resistance is measured in Ohms (O or Greek symbol "omega") and is designated with the symbol R for resistance.
• Electric power is measured in Watts (W).

• Electric current flows from high electrical pressure to low electrical pressure.
• The greater the voltage, the greater the current flow. The more resistance, the less current flow through the circuit.

Ohm's Law is a simple equation that makes it easy to:

• calculate the number of Amperes in a circuit when the voltage and the resistance are known
• calculate how many volts pressure is needed to push a certain current through a known resistance
• calculate how many ohms of resistance will keep the current to a certain amperage in a circuit of known voltage

According to Ohm's Law, voltage and current are directly related - a change in voltage produces a change in amperage.

1. To calculate the resistance required, the following formula is used:

This formula can also be described as: Ohms = volts/Amps or with symbols: R = V/I

2. To calculate the number of volts required, the following formula is used:

This formula can also be described as: Volts = Amps X Ohms or with symbols: V = I X R

3. To calculate the amperage of current flow in a system, the following formula is used:

This formula can also be described as: Amps = volts/ohms or with symbols: I = V/R