Energy: Electronic Products Word of the Week

We read a lot about energy: conservation of energy, alternative energy, EnergyStar, saving energy. Energy affects everything in our lives.

According to the Electronic Products article “Tamper detection in processor-based energy meters,” energy, as it refers to electronics, is the instantaneous product of ac voltage and ac current averaged over time.

Similarly, Wikipedia observes that “because energy is defined as the ability to do work on objects, there is no absolute measure of energy.” When discussing energy, the words “potential” and “kinetic” are very important. Potential energy is energy that is stored. Kinetic energy is energy in motion.

Work and heat are two categories of processes or mechanisms that can transfer a given amount of energy. The joule (J), the unit of energy in the Systeme Iinternationale, is based on the amount transferred to an object by the mechanical work of moving it 1 meter against a force of 1 newton. Work, a form of energy, is force times distance.

W = f s  (the “s” here means distance, whereas in other contexts it means seconds

We who sit at desks sometimes joke that we do no work if we are not somehow in motion.

Energy differs from power. Electrical power is measured in units for which we know we are billed: the watt, a unit named after James Watt, the 18th century developer of the steam engine. A watt is a joule per second. The joule is named for the 19th century English physicist James Prescott Joule. A watt is equal to volts times amperes:

W = VA

or 1 watt is the rate at which work is done when 1 ampere of current flows through an electrical potential difference of 1 volt. (These units are named after 18th century physicists Alessandro Volta and André-Marie Ampère.)
This can be confusing because W has multiple meanings in physics, but you have to remember the context in which the abbreviation is used.
Another relationship is known as Ohm’s Law, named for 19th century physicist Georg Simon Ohm:

V = IR

which signifies voltage equals current times resistance.
Voltage is electric potential energy per unit charge, so this equation is sometimes written

E = IR

Here’s an example from an Electronic Products article of how these relationships are expressed:

“The pulse energy for the pre-charge resistor is typically about 5 J for capacitance of 10 µF and dc voltage of 690 V.”

And in another article:
“The avalanche rectifier features 2.9-µA typ reverse current at 125°C, 5-mJ pulse-energy in avalanche mode, 1.0-A forward current, and 1.39-V forward voltage at 1 A and 125°C.”

To summarize, energy is expressed in joules and also, sometimes, in volts. In fact, an electron volt is a unit of energy equal to the energy acquired by an electron falling through a potential difference of 1 volt, approximately 1.602 × 10 -19 joules.