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Principles of Electromagnetic Fields

PRINCIPLES OF ELECTRIC FIELDS

Electric fields are created by differences in voltage. The higher the voltage, the stronger will be the resultant field. The units for electric voltage is Volts [V] and an electric field strength is expressed as Volts per Meter (V/m). Take for example a battery of 2 V connected to 2 plates that is 1 m apart, the electric field strength would be 2 V/m.

 

PRINCIPLES OF MAGNETIC FIELDS

Magnetic fields are created when electric current flows. The greater the current, the stronger the magnetic field. The magnetic field strength [H] is expressed in amps per meter [A/m] where amperes is co-related to the amount of current flow within the circuit. Take for example if we connect a battery to a blub in a closed circuit with 1 Ampere of current flowing within and the circumference of the circle is 1m, the magnetic field [H] will be equals to 1 A/m.

 

Apart from Ampere per meter, another 2 acceptable units of expression of magnetic fields are Magnetic Flux [B] and Magnetic Gauss [G].

Magnetic Flux [B] is measured in Newton-meters per ampere (Nm/A), also called Tesla (T) whereas Magnetic Gauss [G] is the cgs unit of measurement of magnetic flux density. The correlation between Magnetic Gauss and Magnetic Flux is that 1 T = 10,000 G.

So, what is the context of measurement in terms of Gauss and Flux? The former is handier when used to measure weaker magnetic fields (i.e. the Earth's magnetic field which is on the order of 1 G) whereas Tesla is more useful for measuring very large magnetic fields. However, when do we measure the Magnetic Field Strength [H] and when do we measure in terms of Tesla [T] and Gauss [G]? A typical common usage of this will depend on the frequency of the alternating current sources if it is low or high. Thus is further elaborated in the sub paragraph titled "Alternating Electromagnetic Fields" below.

 

ELECTRIC FIELDS VS MAGNETIC FIELDS

A magnetic field is only produced when current flows in a circuit (i.e. when the equipment is switched on. However, electric fields are produced in cables even when the equipment they are connected to are not turned on.

 

Electric Fields

Magnetic Fields

Causes

Raises due to Voltage

Raises due to Current

Units

Measured in Volts per Meter (V/m)

Measured in Ampere per Meter (A/m), Tesla (T) or Gauss (G)

Existence

(Natural)

Could exist in our Earth natural environment

Existence (Man-made)

Field exists with presence of voltage even when equipment is switched off.

Field exist only in close loop when equipment is turned on and when current flows.

Shielding

Electric fields may be shield off by Materials (i.e. building materials)

Magnetic Fields are not attenuated by most materials.

 

STATIC ELECTRIC FIELDS

Static fields have a polarity that remains constant over time. It could occur naturally in our environment or due to human activities.

Electric fields are oriented from a positive charge to a negative charge.  Our earth consists of Natural Static Electric field which has an electric field strength of 0.1 to 0.5 kV/m during clear weather. This fields can increase up to 20kV/m during thunder storms.

As for Man-made Static Electric fields, it could occur and emit due to electronic operation such as usage of power coating machines, galvanization or metal refining which typically requires large amount of voltages. Note that the electric field can exist even when the equipment is not in operation.

 

STATIC MAGNETIC FIELDS

Even though Magnetic Field only arises due to a direct current flowing in equipment due to human activities, there are also Naturally Occurring Static Magnetic field which is the earth’s Magnetic field which has a magnitude of approx. 40uT (microTesla) in central Europe.

Man-made Static magnetic fields are generated wherever electricity is used in the form of direct current (DC), such as in some rail and subway systems, in industrial processes such as aluminum production and also in nuclear spin tomography. One prominent application of strong static magnetic fields is Magnetic Resonance Imaging (MRI) that provides three-dimensional images of soft body tissue such as the liver, brain, lung and the spinal cord. This medical imaging technique uses very powerful permanent magnets, which can lead to high exposure levels and could pose certain health hazards to both patients and operators.

 

ALTERNATING ELECTROMAGNETIC FIELDS

The battery example above illustrates a generation of static fields. However, if we continuously rotate the battery (tuning its poles), this would produce an electromagnetic field with a continuously changing direction. This is known as an “alternating field” and the number of oscillations it happens per second is also known as the frequency (which is expressed in Hertz [Hz] of the emitted fields.

 

LOW FREQUENCY (LF) VS HIGH FREQUENCY (HF)

Alternating fields are divided into low frequencies fields (up to about 100 kHz) and high frequency fields (from 100 kHz to 300 GHz). Typically, it is traditional to specify the magnetic flux in terms of Tesla [T] or Gauss [G] at lower frequencies and magnetic field strength in terms of amperes per meter [A/m] in higher frequencies.

 

APPLICATION AND HAZARDS

As it can be seen by now, a lot of our daily appliances and work-related machinery would result in the emission of electromagnetic radiation. Below table detailed out some example of applications which runs based on alternating electromagnetic fields.

Example of Applications of Alternating Electromagnetic Fields

Low Frequency

High Frequency

In Power Systems

Cellular radio

Electric Railways

Broadcasting

Industrial process such as melting, heating, welding

Industrial process such as melting, heating, welding

 

Satellite Communications

 

Microwave System

 

In all of these application areas, radiation exposure is possible, so it is important to pay attention to the relevant limits and ensure proper monitoring and protective measures are put in place for workers safety. Click here to find out more about the potential health hazards electromagnetic radiation poses to us and what can we do to protect ourselves.