Understanding EMF

Understanding EMF

By Erik Johnson, BSEE, MBA

 

About the Author:

Erik Johnson is the CEO of High Tech Health.  He holds a bachelor’s degree in electrical engineering from the University of California at Davis and a master’s degree in business administration from the University of Colorado at Boulder.  He is a member of Beta Gamma Sigma.  Prior to joining High Tech Health, he worked at several semiconductor companies in Silicon Valley as a member of the development teams working on analog, digital, and mixed-signal semiconductor products.

 

What is EMF? Electric Fields, Magnetic Fields, and RF waves.

EMF (electromagnetic fields) describe how charged particles (such as electrons) affect each other from a distance.  EMF is present in 100% of electronic or electrical devices.

 

EMF (electromagnetic fields) can best be thought of as three things: low frequency electric fields, low frequency magnetic fields, and higher frequency electromagnetic radiation or “radio frequency” (RF) waves.  Any product with the goal of being low EMF must address each of these 3 things.

 

The frequency of these fields and waves is an important distinction.  If a field is not changing with time, it is said to be “static”, or have zero frequency.  Fields or waves that change with time have a frequency that is described by the number of cycles per second, measured in Hertz (Hz).

 

Electric and magnetic fields below approximately 100 kHz (100,000 Hz) exist independently of each other.  Sometimes the acronym ELF (Extremely Low Frequency) is used to talk about electric fields and magnetic fields that exist independently of each other.  Above approximately 100 kHz, electric and magnetic fields couple together and behave like a single thing.  Engineers call those RF waves, or electromagnetic waves, but they are more easily understood as “light”.

 

While EMF “radiates” out from its source (such as light coming out of a flashlight), it is NOT the same thing as “radiation”.  Radiation is how we describe the decay of radioactive materials, such as those used in nuclear power.  Radiation has nothing to do with this discussion of EMF.

 

EMF: Electric Fields

Electric voltage (measured in volts: V) produces electric fields (measured in volts per meter: V/m).  A typical electrical power outlet in the United States operates at 120 V with a frequency of 60 Hz.  Whether you are using your power outlet or not, electric fields will radiate from the presence of voltage.  Electric fields related to this voltage will mostly be at 60 Hz, but also at several multiples of this value up to approximately 300 Hz (this is due to non-linear mathematics, which I will not discuss here).

 

Example: Air does not conduct electricity.  BUT, if an electric field is large enough (very large), then the insulating properties of air breakdown and it conducts electricity.  This is what is happening when we see lightning.

 

EMF: Magnetic Fields

Electric current (measured in amperes: A) produces magnetic fields (measured in gauss: G, or in tesla: T).  1 milligauss (0.001 G) equals 0.1 microtesla (0.0000001 T).  I will be using milligauss (mG) for future discussion.  Most things we will be interested in will measure between 0 and a few hundred mG.  Any time electricity is flowing, that electric current produces magnetic fields.

 

Example: Magnets with the same polarity repel each other and opposite magnetic fields attract each other.  A refrigerator magnet has a static magnetic field (a magnetic field with no frequency). The earth itself has a static magnetic field, and that is what moves the needle of a compass.

 

(Side Note: What is Power?)

Power is the rate of energy transfer with respect to time and is measured in watts (W), which can be calculated by multiplying volts times amperes (“amps”).  P = V x I.  Because our wall outlets are always at the same voltage (120V in the United States), current is proportional to power – the more power something consumes, the more current it will consume.  The more current it consumes, the higher the magnetic fields present will be.  The power we use from our wall outlets are responsible for the electric and magnetic fields we all are commonly exposed to.

 

Question: If I have two 300 watt heaters, one is running at 120V, the other is running at 240V, how will the fields coming from these heaters be different?

Answer: In order to be of equivalent power, the 120V heater will consume twice the current as the 240V heater.  The 120V heater will radiate ½ the electric field, but twice the magnetic field.  The 240V heater, on the other hand, will have twice the electric field, but half the magnetic field.

 

EMF: RF (“Radio Frequency”)

emwave
RF or radio frequency, is the generic term engineers use to refer to the coupled magnetic and electric fields that are EMF above roughly 100kHz.  These are more easily understood as “light”.  We give different names to different ranges of frequencies.  Visible light is simply RF in a particular range.  As you can guess from the phrase “radio frequencies”, it covers the range of frequencies that are used for radio and communications equipment.  Examples in order of increasing frequency (decreasing wavelength):

  • Radiowaves
  • The frequencies used in cell phone communications
  • The frequencies used in Wifi and Bluetooth electronics
  • Microwaves
  • Infrared light (from far infrared, to mid infrared, to near infrared)
  • Visible light (all the colors from red to violet)
  • Ultraviolet light
  • X-rays
  • Gamma rays

When electric current (such as inside electronic devices) is oscillating at radio frequencies, it can radiate off its wires into space as RF electromagnetic waves.