Energy Through Our Lives

Units:

Section B. Energy Flow in Ecosystems

4. Watts, Volts, and Amps, Oh My!

Power and time of use are the factors that determine how much energy is used by an electrical appliance or piece of equipment. Power is the rate at which energy is used, or work is done, per unit of time. Electrical power is usually measured in watts; hence, electrical power is often referred to as wattage. The higher the wattage, the greater the amount of electrical energy that an electrical appliance or piece of equipment uses over a period of time. For example, a 1,200-watt microwave oven uses twice as much electrical energy and produces twice as much heat in one minute as a 600-watt microwave oven. However, an appliance with a higher wattage will not use much energy if it is used for only a few seconds, whereas an appliance with a lower wattage may use a lot of energy if it is used for a number of hours. For instance, a 1,200-watt microwave used for only 30 seconds uses less energy than a 600-watt microwave does in one-half hour.

The relationship between the wattage, time of use, and the energy used by an appliance or piece of equipment can be expressed by this formula:

Wattage (Power) x Time = Energy Use

By using this formula, we can compare the energy used by electrical appliances and equipment to see which ones use the most electricity.

Wattage and other electrical information is often listed directly on the appliance or equipment. For example, a label on a microwave oven may look like this:

ACME, Microwave Oven
Model No. X-15Z
120 Volts AC 5 A
600 Watts 60 HZ
Made in USA

The information on the label tells us that the microwave oven needs 120 volts of electricity in the form of alternating current (AC) to operate, and draws 5 amps (amperes) of current during its use. The 60 HZ number means that the current alternates at a rate of 60 times per second. The wattage of the microwave is 600 watts.

If the voltage and current are listed on an appliance but the wattage is not, the wattage can be calculated by multiplying the voltage by the current. Using the information on the microwave label, the wattage is equal to Voltage x Current = Wattage.

120 volts x 5 amps - 600 watts

If the microwave oven is used an average of a half hour each day, the average amount of energy is uses per day is
Wattage x Time = Energy Use

600 watts x 0.5 hours per day = 300 watt-hours per day

Because watt-hours are small units, electrical energy is more often measured in kilowatt-hours, where one kilowatt equals 1,000 watts. The energy used by the microwave oven each day in kilowatt-hours is Watt-hours per Day x (1 Kilowatt/1000 Watts) = Kilowatt-hours per Day

300 watt-hours per day x (1 kilowatt/1000 watts) =
0.3 kilowatt-hours per day

Volts, Amps, and Watts: What are they?

Voltage
All sources of electricity, such as batteries or generators, have the potential to do work (e.g. illuminate light bulbs, run electrical appliances). Voltage describes this potential. The greater the voltage, the more potential the electricity source has to do work.

The potential to do work should not be confused with actually doing work. For instance, a battery that is sitting on a table but not connected to anything has a voltage, or the potential to do work such as lighting a light bulb. However, the battery will not light the bulb unless it is connected to the bulb in an electric circuit. Only then will the battery actually do work.

The unit of voltage is the volt. One volt is defined as doing one joule (0.74 foot-pounds) of work to move one coulomb (6.25 x 10¹⁸) electrons.

Current

Electric current is simply the flow of electrons (or, in some cases, positive charges). In a circuit, current delivers energy from a source of electricity to an electrical device (e.g. a light bulb) or appliance.

The unit of current is the ampere, or amp. An ampere is defined as having one coulomb (6.25 x 10¹⁸) electrons flow past a point in an electric circuit every second.

The Relationship between Voltage and Current

The relationship between voltage and electric current is similar to the relationship between the height of a waterfall and the water that flows down it. A height is needed for the water to flow down the waterfall. The greater the height of the waterfall, the more energy the water has when it reaches the bottom. If no height exists, the water will not flow and it will not have any energy due to motion.

A voltage (similar to height) is needed to cause an electric current to flow (think of cascading water) so that it can deliver energy to an electrical device or appliance. It is helpful to remember that a current is a flow of electrons and electrons have mass (therefore current is a mass of flowing electrons!). The higher the voltage, the more work an electric current can do. If no voltage exists, a current will not flow and work cannot be done.

DC and AC Current

The current produced by sources of electricity comes in two main forms: direct current (DC) and alternating current (AC). Direct current is current that flows in one direction through a circuit. It is produced by sources of electricity whose positive (+) terminal always stays positive and negative (-) terminal always stays negative. For example, a battery produces direct current because the battery's terminals always remain the same; the negative terminal does not change to a positive terminal, and vice versa. Hence, the current will always flow from the negative terminal of the battery toward the positive terminal.

Alternating current is current whose flow in a circuit periodically reverses direction. It is produced by a source of electricity whose positive and negative terminals switch or alternate back and forth. In other words, one terminal will switch from positive to negative and back to positive, while the other terminal will switch from negative to positive to negative. Alternating the terminals from positive to negative cause the current to flow in one direction, then in the reverse direction, and back to its original direction, and so on. Electrical generators in power plants throughout the United States produce alternating current that reverses direction 60 times per second. The unit used to describe the rate at which current alternates is the cycle per second, or hertz (HZ).

Electric Power

In general, power is defined as the rate at which work is done, or energy is used, per unit of time. Electric power specifically refers to the rate in which a source of electricity produces energy, or refers to the rate in which an electrical device, appliance, or piece of equipment converts electrical energy into other forms of energy. The faster a source of electricity (such as a generator) produces electrical energy, the greater its power output. The faster an electrical device (such as a light bulb) converts electrical energy in light and heat energy, the greater its power consumption. Electric power is related to voltage and current by the following formula: Power = Voltage x Current

The unit of electrical power is the watt. One watt is defined as one joule (0.74 foot-pounds) per second, or one volt multiplied by one amp. Because the watt unit is used so frequently, electrical power is often referred to as wattage.

Did you know...

...many of our small and medium appliances never stop using electricity! Experts call this usage "standby consumption" or "leaking electricity" because people are often not aware that the appliance is using electricity.

Next reading: Leaking Electricity

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