Watt per volt to EMU of current
W/V
EMU
Conversion History
| Conversion | Reuse | Delete |
|---|---|---|
1 W/V (Watt per volt) → 0.1 EMU (EMU of current) Just now |
Quick Reference Table (Watt per volt to EMU of current)
| Watt per volt (W/V) | EMU of current (EMU) |
|---|---|
| 0.1 | 0.01 |
| 1 | 0.1 |
| 5 | 0.5 |
| 10 | 1 |
| 20 | 2 |
| 100 | 10 |
About Watt per volt (W/V)
The watt per volt (W/V) equals one ampere, derived from the power relationship P = IV rearranged as I = P/V. A device consuming 60 W at 120 V draws 0.5 W/V = 0.5 A. The W/V form is most useful when calculating branch currents from known power ratings and supply voltages — for appliance load calculations, transformer secondary currents, or power budget analysis on a circuit board. Numerically identical to the ampere, it provides an alternative view emphasising the power-per-volt character of current and is common in power electronics and electrical installation design.
A 100 W light bulb on a 230 V supply draws approximately 0.43 W/V. A 60 W laptop adapter at 20 V delivers 3 W/V to the device.
About EMU of current (EMU)
The electromagnetic unit (EMU) of current equals exactly 10 amperes, numerically identical to the biot. It is the current unit native to the CGS electromagnetic (CGS-EMU) system, which dominated electrical physics from the mid-19th century until SI adoption in 1960. In CGS-EMU, the permeability of free space is defined as 1, giving the electromagnetic subsystem its characteristic form where magnetic force between parallel currents is expressed purely in dynes. The EMU of current appears in classical electrodynamics texts, historical measurement standards, and theoretical physics work using CGS-EMU conventions. All practical electrical measurement now uses SI amperes.
1 EMU of current = 10 A. A 50 A arc welding process carries 5 EMU. The unit is encountered primarily in pre-1960 scientific literature.
Watt per volt – Frequently Asked Questions
Why would an electrician think in watts per volt?
When sizing circuits, electricians know the appliance power (watts from the nameplate) and the supply voltage (120 V or 230 V). Dividing watts by volts gives the current in amps — which is what determines wire gauge and breaker size. "1,800 W ÷ 120 V = 15 A, so I need a 20 A circuit" is daily electrician math.
Is watts per volt ever written on any product label?
No — product labels list watts, volts, and amps separately. The W/V expression lives in textbooks and engineering calculations. But every time you read "1,500 W, 120 V" on a space heater and mentally divide to get 12.5 A, you are computing watts per volt without calling it that.
Does the watts-per-volt calculation work for AC power?
Only approximately. For AC, real power (watts) = V × I × power factor. So I = W / (V × PF). A motor rated at 1,000 W with a power factor of 0.85 on 230 V actually draws 1,000 / (230 × 0.85) = 5.1 A, not the 4.35 A that simple W/V would suggest. Always account for power factor in AC circuits.
How does watts per volt help with USB power delivery calculations?
USB PD negotiates voltage levels (5 V, 9 V, 15 V, 20 V) and maximum power (up to 240 W). Dividing the negotiated power by voltage gives the cable current: 100 W at 20 V = 5 A, requiring a 5 A rated cable. At 5 V the same 100 W would need 20 A — which is why PD uses higher voltages.
What is the relationship between watts per volt and Ohm's law?
From P = IV and V = IR, you get I = P/V = V/R = P^(1/2)/R^(1/2). The W/V form is just one of many equivalent expressions for current. Which one you use depends on what you know: power and voltage gives W/V, voltage and resistance gives V/R (Ohm's law directly).
EMU of current – Frequently Asked Questions
What does EMU stand for and why was it created?
EMU stands for "electromagnetic unit." In the 1860s–1870s, physicists needed separate unit systems for electrostatic and electromagnetic phenomena because they had not yet unified them. The EMU system was built around magnetic force between currents, while the ESU system was built around Coulomb's electrostatic force. The ratio between them turned out to be the speed of light — a clue that led to Maxwell's equations.
Is the EMU of current the same as a biot?
Yes, exactly. Both equal 10 amperes. The biot is the named unit; "EMU of current" is the generic label. It is like saying "SI unit of force" versus "newton" — same thing, different label. The CGS-EMU system also has named units for other quantities: the gauss (magnetic field), the oersted (magnetising field), and the maxwell (magnetic flux).
Why did physics abandon the EMU system?
The EMU system was awkward for practical electrical engineering — 1 EMU of resistance (the abohm) equals 10⁻⁹ ohms, making everyday values absurdly large numbers. The SI system, adopted in 1960, unified mechanical and electrical units into one coherent framework with human-scale values. Practicality won over tradition.
Where might I encounter EMU of current in old scientific papers?
Pre-1960 physics journals, particularly in geomagnetism, plasma physics, and early electrical standards work, routinely use EMU. Geophysicists measuring Earth's magnetic field historically reported results in CGS-EMU units (gauss, oersted, EMU). Some geophysics reference data still has not been converted to SI.
How did the speed of light connect the EMU and ESU systems?
Weber and Kohlrausch discovered in 1856 that the ratio of the ESU to EMU charge was approximately 3×10¹⁰ cm/s — the speed of light. This was no coincidence: Maxwell showed that light is an electromagnetic wave, and the unit ratio reflects the fundamental coupling between electric and magnetic fields. One of the greatest insights in physics history, hidden in a unit conversion.