Units in Electricity

Electrical units include: current, voltage, power and energy.

Current

Electric current, I, is the rate at which charge flows past a particular point in a circuit. If ∆Q represents the amount of charge that passes through this point in a time

The unit of charge is a Coulomb (6.24151x10^18 protons). Current is measured in amperes (A), where 1 A = 1 C/s. In other words, 1 A is equivalent to 1 Coulomb charge passing through a point in 1 s.

Example: If the amount of charge that passes through the filament of a light bulb in 3 seconds is 1.8 C. What is the current in the light bulb?

Voltage

Electrical potential difference or voltage, V, is a measure of energy per unit charge. In other words, electrical potential is the work done to move a charge from one point to the next, divided by the magnitude of the charge. The unit of electric potential is joules per coulomb.

Resistance:

Resistance, R, is defined as the ratio of the voltage across the conductor to the current it carries:

The unit of resistance is measured in Ohms (Ω).

Power

Power, P, is defined as the rate at which electric energy is transferred by an electric circuit. One unit of power is equal to 1J/s and is known as a watt, W. Given different combinations of information about current, voltage, and resistance, the following equations can be used to calculate power.

or

To better understand power, consider the units of the different components in the following equation. Here we have

 

.

 

 

Energy

Often times, we confuse power with energy consumption. Power is actually a measure of how fast energy is consumed. Consider the following example about power companies.

Example: Power Companies

Power companies charge consumers for electric energy consumption, not power. A kilowatt-hour (kWh) is the unit used to measure energy consumption.To make things easier for consumers, the amount of power used by different appliances is reported on the appliance. The consumer decides how much energy they consume based on how they use their equipment.

For example, assume your laptop uses 50 Watts per hour. This means if you use you laptop for 1 hour, then you consume 50 Watts of energy. If you use your laptop for 8 hours a day, in each month you will use 12 kilowatt-hours of energy. *Note that 1 kilowatt = 1000 Watts, so we will divide by 1000 to convert watts to kilowatts.

(50 W * 8 Hrs/Day*30 Days)/ (1000W/1kW) = 12 kWh

If you use this method to find the amount of energy consumption for each appliance in your house (i.e. TVs, lamps, microwave, laptops, etc..), then add them all together, you will find the total amount of energy consumption for your house in a given month.

The following table shows the kWh usage history of a typical single family house "usage" table on customer statements.

For Example:

Meter number
NC2432

Readings:

October 17
September 17

 

39449
- 38599

kWh usage
850
Days in period: 31
Average use per day: 27.4kWh

The meter counts the number of kilowatts used. Data from the meter is cumulative, meaning it is continuously rising. For more information on data types visit "Working With Data Types".The values given for readings on October 17 and September 17 give the start and end values, respectively. The difference between these two values gives the total amount of energy consumption for a period (in this case a 31 day period).

If each kilowatt-hour is about $0.11, then the charges related to electric services for this time period is:

850 kWh * $0.11 = $93.50

The British thermal Unit (BTU):

The British Thermal Unit (BTU) is a unit for energy. By definition, it is the amount of energy needed to increase the temperature of one pound of water by one degree Fahrenheit. This unit is commonly used heating, cooling, and steam generation.

 

Example: How many joules are in 2 BTU?

2 BTU * 1054 = 2180 J

 

How many BTU are in 3000 J?

1 J ~ 1/1054 BTU

3000 J ~ 3000/1054 = 2.85 BTU

Apply it...

1. How many watts are in a Kilowatt?

1 kW = 1000 W

2. How many watts are in 2.3 Kilowatts?

1 kW = 1000 W

2.3 kW = 2.3 kW* 1000 = 2300 W

3. A 23 W light bulb is left on for 20 hours.

a.) How many Watt-hours of energy does this light bulb consume?

(23W) * (20 hr) = 460 Whr

b.) How many kilowatt-hours does this light bulb consume?

1 W = 0.001 kW; therefore 23W = 0.023 kW

(0.023 kW) * (20hr) = 0.46 kWh

4. A 60 Watt light bulb is left on for 10 hours.

a.) How many watt-hours of energy does the light bulb consume?

(60 W) * (10 hr) = 600 Wh

b.) How many kilowatt-hours energy does the light bulb consume?

1 W = .001 kW

therefore:

60 W = 60 x .001 = .06kW

.06kW x 10hr = .6kWh

5. How many joules are in 2 BTU?

                                                          1 BTU ~ 1054 J

                                                          2 BTU ~ 2 x 1054 J = 2108 J

6. How many BTU are in 3000 Joules?

                                                          1 BTU ~ 1054 J

                                                          1 J ~ 1/1054 BTU

                                                           3000 J ~ 3000/1054 = 2.85 BTU

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