Volts Electronics Mathematics Formulas

Alternate titles

  • How to Figure Out Volts
  • How many volts in watts, ohms, and/or amps.
  • Volts - How to Calculate and Convert Power aka Watts / Amps / Ohms to Volts - A Beginners Guide

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This is one of four pages having to do with Ohm's Law and its derivatives.

  1. How to calculate Power aka Watts
  2. How to calculate Current aka Amps
  3. How to calculate Electromotive Force aka Volts (You Are Here)
  4. How to calculate Resistance aka Ohms

Volts Equals... The Quick, Easy Answer.

Electromotive force aka volts equals watts divided by amps. For examples and other formulas that will give you the answer, or more information in general; continue reading.

Converting Watts and Amps to Volts

  • E = P/I Ohm's Law derivative for volts.

Converting Amps and Ohms to Volts

  • E = IR Ohm's Law for volts.

Converting Watts and Ohms to Volts

  • E = √(PR) Ohm's Law derivative for volts.

How to convert amps / ohms / watts to volts.

Source

Volts Is the Unit of Measurement for Electromotive Force

The amount of voltage in a circuit, or being directed to a device, can be determined by knowing any two of either power, current, or resistance.

In other words, the number of volts can be determined by knowing any two of either watts, amps, or ohms.

Each set of two has its own formula. There are three formulas.

The algebraic term for "symbol" is "variable".

"E" and "V" are both used to designate electromotive-force by the unit of measurement, volts. The industry standard used to be "E", but now both "E" and "V" are being used interchangeably.

"I" is the industry standard to designate current by the unit of measurement, amps.

"R" is the industry standard to designate resistance by the unit of measurement, ohms.

"P" is the industry standard to designate power by the unit of measurement, watts. Sometimes "W" is used.

How to Convert Watts and Amps to Volts

#1 Calculating Volts Using Watts and Amps

All one does is divide watts by amps to get volts. The next four statements all say the same thing in different ways and have the same meaning. Whichever one you like is fine.

  • Electromotive Force equals Power divided by Current
  • Electromotive Force equals Wattage divided by Amperage
  • Electromotive Force equals Watts divided by Amps
  • Volts equals Watts divided by Amps

The next two formulas say the same thing and have the same meaning. It's just a matter of preference.

  • E = P/I
  • V = P/I

Some everyday examples:

A typical 100-watt, incandescent light bulb has .8333 amps running through it. Is that bulb being used in Europe or the US? Dividing the 100 watts by .8333 amps gives us 120 volts. The bulb is being used in the US (European household voltages run in the 210 to 240 volt range).

A 720-watt appliance consumes 3 amps. Is it feeding off a 120-volt or a 240-volt power source? Dividing the 720 watts by 3 amps gives us 240 volts.

Some circuit / formula examples (there are a 1000 milliamps in 1 amp, there are 1000 milliwatts in a watt, there are a 1000 millivolts in a volt):

A 300-milliwatt (.3 watts) circuit board uses 100 milliamps (.1 amps). Dividing 300 milliwatts by 100 milliamps indicates it is connected to a 3-volt input.

A 60-watt circuit uses .2 amps. Dividing 60 watts by .2 amps means a 300-volt input.

Some math / algebra examples:

200 volts = 300 watts / 1.5 amps

120 volts = 25 watts / .20833333 amps

120 volts = 100 watts / .833333 amps

Power substation voltages can range from 69 kV to 1500 kV

1 kV is equal to 1000 volts
1 kV is equal to 1000 volts | Source

How to Convert Amps and Resistance (Ohms) to Volts

Table of most-searched-for Questions and Answers

How many volts is in a D / C / aa / aaa battery?

  • All are 1.5 volts. However, the larger the battery the more current it can provide. Most devices use multiple batteries connected in series. To calculate the voltage of the device, multiply the number of batteries used times 1.5 volts. As an example, a 4 D-cell flashlight would be 4 x 1.5 = 6 volts; those spiffy, little, 3-battery, aaa LED flashlights are 4.5 volts.

How many volts is a car battery?

  • Historically, and still mostly so, the answer is 12 volts.

How many volts in an amp?

  • To find the answer, one would also have to know the number of watts used (see #1) or the resistance of the device or circuit (see #2).

How many volts in a watt?

  • To find the answer, one would also have to know the number of amps used (see #1) or the resistance of the device or circuit (see #3).



#2 Calculating Volts Using Amps and Resistance (Ohms)

Another easy one. To calculate volts, multiply current (amps) times resistance (ohms),i.e., amps times ohms. The next three statements all mean the same.

  • Electromotive-force equals Current times Resistance
  • Electromotive-force equals Amperage times Ohms
  • Volts equals Amps times Ohms

The four formulas say the same thing and have the same results. It is only a matter of preference.

  • E = I x R
  • V = I x R
  • E=IR
  • V=IR

Some examples:

Example #1:

You just bought this very odd, very old, very foreign electronics item at a garage sale. The spec plate is half worn off and isn't even in English. You are, however, able to tell that it says 1.6667 amps. So, you hook up the VOM (see note below) to the device to ascertain it's resistance measurement. The measurement comes back as 72 ohms.

The formula is E=IR. 1.6667 amps times 72 ohms equals 120 volts. You got lucky.

Example #2:

The thing uses a third of an amp and measures at 18 ohms. You multiply .3333 times 18 and know that it requires 6 volts worth of batteries.

Some circuit / formula examples (there are a 1000 milliamps in 1 amp, there are 1000 milliwatts in a watt, there are 1000 millivolts in a volt):

There are 1.5 amps running through a circuit with a measured resistance of 40 ohms. So that would be 1.5 x 40, which gives you 60 volts.

There are 600 milliamps running through a circuit with a measured resistance of 5 ohms. So that would be .6 amps times 5 ohms, giving you 3 volts.

Some math / algebra examples:

E = .096 x 500 = 48

V = .150 x 10 = 1.5

A general note about VOM's: A VOM is a multi-range, volt-ohm multi-tester meter. It is a test instrument to measure voltage, current, and resistance of an element, circuit, or even a device. Do not buy one until you really know what you are doing. If you just want a meter or meters for measuring volts and/or amps, you have more latitude. But when it comes to measuring resistance, cheaper meters are inaccurate within certain ranges. Really research the subject first.

How to Convert Power (watts) and Resistance (ohms) to Volts

Just a Tesla Coil demonstration at Fermilab.

Can click to enlarge.
Can click to enlarge. | Source

#3 Calculating Volts Using Power (watts) and Resistance (ohms)

This one is trickier. Break out the calculator or spreadsheet, square root is involved. The next four statements say the same thing and have the same meaning.

  • To calculate Voltage, multiply Power times Resistance, then derive the square root of the product
  • To calculate Voltage, take the square root of the answer to (Power times Resistance)
  • Volts equals multiplying Watts times Ohms, then deriving the square root of the product
  • Volts equals taking the square root of the answer to Watts times Ohms

The following formulas all say the same thing:

  • E=(P x R)
  • V=(P x R)
  • E=(PR)
  • V=(PR)

Some examples:

Example #1

How many volts are being imputed to a 2500 watt circuit that has a resistance of 25 ohms?

E=(PR)

The 2500 watts is multiplied by the 25 ohms giving an answer (aka product) of 62500. The square root of 62500 is 250. The final answer is 250 volts.

Example #2

How many volts is the 1500-watt space heater designed for that has a resistance of 9.6 ohms?

E=(PR)

The 1500 watts is multiplied by the 9.6 ohms giving an answer (aka product) of 14400. The square root of 14400 is 120. The final answer is 120 volts.

The Three Formulas for Volts

Summary

E=P/I (Volts equals watts divided by amps. Of the three formulas, this is the one that is used 90% of the time.)

E=IR (Volts equals amps times ohms.)

E=√(PR) (Volts equals the square root of (watts times ohms).)

If you have any suggestions on how this reference article can be improved, please let us know. There is a Comments Section further down the page.

The how to convert amps / ohms / watts to volts page.

E = P/I Ohm's Law derivative for volts.
E = P/I Ohm's Law derivative for volts. | Source

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Comments 2 comments

Kristen Howe profile image

Kristen Howe 19 months ago from Northeast Ohio

Great hub on this aspect of physics and how to calculate volts. Voted up for useful!


krbalram profile image

krbalram 12 months ago from Bangalore

enjoyed reading this hub on ohms and watts.It is all school stuff.Thanks for such wonderful information.

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