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You are here: Home / Research / Mechanisms / Total field and AC field

Total field and AC field

How do AC and DC fields relate to each other?

We live in a DC magnetic field, the earth's geomagnetic field, and any AC field is superimposed on it.

 ac-dc-explanation

The AC field is often less than the DC field. In fact it's often very much less. This example, not drawn to scale, shows the 0.4 µT AC field implicated in the epidemiology of childhood leukaemia with the typical 48 µT geomagnetic field in the UK. (0.4 µT is the rms value, the peak is slightly higher, see a tutorial on ways of characterising AC fields.)

When the AC field is less than the DC field, it doesn't change the average value of the total field.  The AC fluctuations average to zero, and the average field is still 48 µT in this example.

What does this mean for mechanisms?

Let's consider proposed mechanisms for how an AC magnetic field might produce effects in living systems.  Clearly, if a mechanism depends just on the AC component, the DC field and the total field are irrelevant.  Induced currents, for example, are produced only by AC fields.

But some mechanisms depend on the total field.  The free radical mechanism is one example.  It operate on such a fast timescale - tens of nanoseconds - that it just "sees" the instantaneous total field without knowing whether that is an AC field or a DC field.

If these mechanisms that depend on the total field were strictly linear - the effects they produced was exactly proportional to the field - any effects of the AC field produced would also average to zero.  On the positive half cycle there would be a positive effect, on the negative half cycle an equal and opposite negative effect, and the two cancel.  For free radicals, it might be an increase in the concentration on the positive half cycle and an equal and opposite decrease on the negative half cycle.  The average concentration of free radicals is unchanged.

The only way these mechanisms can produce an overall effect from an AC field is if they are not strictly linear.  Then the effect on the positive half cycle might be slightly larger than the effect on the negative half cycle, and the two would not exactly cancel, leaving a small overall effect.

This has two consequences.

  • because the effect arises from the imperfect cancellation of two opposite effects, it is expected to be a smaller effect; and
  • mathematically, we are saying that the linear component still averages to zero, and the net effect comes from the non-linearity, which is expressed as a quadratic or square term.  So any effect of AC fields from these mechanisms will dependent not on the field but on the field squared (or, in principle, an even higher power).

We also predict, for mechanisms that depend on the total field, that if the AC field produces effects, then so would changes in the DC field, for example as we move from one lattitude to another on the earth.  See one test of the implications of this for epidemiology.

See also:

  • What are DC fields?
  • Are there health effects associated with DC?

Latest news

  • New publication on cancer incidence from the UK electricity industry Cohort Study August 27, 2019
  • How has the reported risk for childhood leukaemia changed over time? February 11, 2019
  • Media stories about microshocks in children’s playground September 10, 2018
  • New studies on leukaemia and distance from power lines June 1, 2018
older news

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Navigation
  • What are EMFs
    • Terminology – an introduction
    • Electric fields
    • Magnetic fields
    • Units for measuring EMFs
    • Measuring and calculating EMFs
      • “EMF Commercial”
    • Adding fields together
    • Radiofrequencies
    • Screening EMFs
  • Sources
    • Overhead power lines
      • Fields from specific power lines
        • 400 kV
        • 400 kV – specific cases
        • 275 kV
        • 132 kV
        • 66 kV
        • 33 kV
        • 11 kV
        • 400 V/230 V
        • Replacing a 132 kV line with a 400 kV line
      • Summaries of fields from all power lines
      • Factors affecting the field from a power line
        • Voltage
        • Current
        • Clearance
        • Height above ground
        • Conductor bundle
        • Phasing
        • Balance between circuits
        • Balance within circuit
        • Ground resistivity
        • Two parallel lines
      • Calculating and measuring fields from power lines
        • Geometries of power lines
        • Raw data
        • On-line calculator
      • Fields from power lines – more detail on the physics
        • Field lines from a power line
        • The direction of the field from a power line
        • Power law variations in the field from a power line
      • Statistics of power line fields
    • Underground power cables
      • Different types of underground cable
      • Fields from cables in tunnels
      • Gas Insulated Lines (GIL)
      • Underground cables with multiple conductors
      • Effect of height on fields from underground cables
      • Screening fields from underground cables
    • Low-voltage distribution
      • UK distribution wiring
      • USA distribution wiring
    • House wiring
    • Substations
      • National Grid substations
        • Static Var Compensators
      • Sealing-end compounds
      • Distribution substations
      • Final distribution substations
        • Indoor substations
    • Transport
      • EMFs from electric trains (UK)
      • EMFs from cars
    • Appliances
    • Electricity meters
      • Smart meters
      • Traditional meters
    • Occupational exposures
      • Live-line work
      • Static Var Compensators
      • Occupational exposures on pylons
    • Field levels and exposures
      • Personal exposure
      • Other factors that vary with magnetic fields
      • Fields greater than 0.2 or 0.4 µT
    • Screening EMFs
      • Screening fields from underground cables
      • EMF Reduction Devices
  • Known effects
    • Induced currents and fields
    • Microshocks
      • Control of microshocks in the UK
      • Microshocks from bicycles
      • Bees and microshocks
    • EMFs and medical devices
      • Standards relating to pacemakers and other AIMDs
    • Effects of EMFs on equipment
  • Research
    • Types of research
    • Epidemiology
    • Animal and laboratory experiments
    • Mechanisms
    • Specific studies
      • UKCCS
      • CCRG
      • French Geocap study
      • CEGB cohort
      • Imperial College study
  • Current evidence on health
    • Childhood leukaemia
      • Survival from childhood leukaemia
      • Childhood leukaemia and Downs
      • Childhood leukaemia and night-time exposure
      • The “contact current” hypothesis
    • Other health effects
    • Scientific review bodies
      • WHO
      • IARC
    • Electric fields and ions
    • Comparing EMFs to other issues
  • Exposure limits for people
    • Limits in the UK
    • Limits in the EU
    • Limits in the USA
    • Limits in the rest of the world
    • Limits from specific organisations
      • ICNIRP 1998
      • ICNIRP 2010
      • NRPB 1993
      • NRPB 2004
      • EU 2004
      • EU 2013
  • Policy
    • UK policy
      • Power lines and property – UK
    • Compliance with exposure limits
    • European EMF policy
    • Precaution
    • SAGE
      • SAGE First Interim Assessment
        • Government response to SAGE First Interim Assessment
      • SAGE Second Interim Assessment
        • Government response to SAGE Second Interim Assessment
        • SAGE Second Interim Assessment – the full list of recommendations
  • Finding out more
    • EMF measurement and commercial services
    • Links
    • Literature
    • Contact us
  • Static fields
    • Static fields – the expert view