K323 / 2027
Static electricity overview

Lesson 5 of 5

Discharge hazards and particle collection

  • I can give examples of hazards caused by electrostatic charging.Syllabus

    Syllabus K323, 13(h). Give examples of hazards caused by electrostatic charging.

  • I can explain the electrostatic precipitator and apply its principles to a new situation.Syllabus

    Syllabus K323, 13(i). Explain the electrostatic precipitator and apply its principles to a new situation.

Make a guess. You are not marked.

Why must a fuel tanker be connected to earth before its fuel is pumped out?

  1. So charge built up by friction flows away instead of making a spark.
  2. So the fuel flows faster through the hose by electrical attraction.
  3. So the tanker is protected from lightning strikes while it is parked.
Show the answer

So charge built up by friction flows away instead of making a spark.

Fuel rubbing through the hose can build up charge. A spark from that charge could ignite fuel vapour. An earth wire lets the charge flow away safely.

Accumulated charge can lead to an unwanted spark. Controlled charging can also give suspended particles an electric force that removes them from a gas stream.

Electrons carry negative charge. A conducting Earth connection can accept or supply electrons, and an electric-field arrow gives force direction for a positive test charge.

Explain how the hazard develops

Contact and separation between materials can separate charge. If the charge cannot flow away, it can accumulate and produce a strong electric field across an air gap. If the air becomes conducting, a rapid discharge through it can produce a spark.

Flammable vapour during liquid handling
Flow and contact between liquid and surfaces can separate charge. A discharge spark can ignite a suitable flammable vapour-air mixture. The dangerous combination is an ignition source and an ignitable mixture, not merely the existence of any charged object.
Combustible dust in air
Particle movement and contact can lead to charge accumulation. A spark can ignite a suspended cloud when the combustible dust and air form an ignitable mixture.

A suitable conducting path to Earth allows excess charge to flow away and can reduce accumulation. Connecting conductors together, called bonding, allows charge to redistribute between them and reduces the conditions for a spark between them. These connections must actually conduct; an insulating coating or broken connection can leave the intended path unavailable.

This reasoning is different from induction's deliberate isolation step. Induction disconnects a path at a chosen stage to retain charge; reducing a static hazard often requires a path that prevents charge from building up.

Collect particles with an electrostatic precipitator

A dry electrostatic precipitator removes suspended dust or smoke particles from a gas stream. The model here uses negative discharge wires between earthed collecting plates.

A negative dust particle is forced towards a collecting plate

This is a cross-sectional force model of a dry wire-and-plate precipitator. A representative negative discharge wire lies between two earthed collecting surfaces.

Gas transport, electric field and force on negative dust have different directionsHorizontal collecting plates above and below are connected to Earth at the right. A negative discharge wire is shown in cross-section between them. The gas moves right, as indicated by blue arrows. A selected negatively charged dust grain above the wire has a downward electric field arrow pointing towards the negative wire, and an upward electric force arrow pointing towards the upper collecting plate. The grain's force is opposite the field because it is negative. Deposits are marked on the collecting surfaces. Charged gas particles transfer charge to dust; a grain need not touch the wire. Later shaking and the hopper are outside this cross-sectional force view.Earthed collecting platesEarth-Negativewire-Force on dustFieldSelectednegative dustGas moves rightDeposits collect on the plates

Charged gas particles transfer negative charge to suspended dust. The electric force drives it towards a collecting surface while gas continues through. A dust grain does not have to touch a wire to become charged.

Later, mechanical shaking removes the deposited dust into a hopper. This is particle removal; it does not remove every gaseous pollutant.

Gas flows to the right. For the selected negative dust particle above a wire, the local field points down towards the negative wire, while the particle's electric force points up towards the collecting plate. Field, force and gas-flow arrows describe different things.
  1. Charge the suspended particles. The strong field near the discharge wires ionises gas, producing charged particles. Electrons can attach to gas molecules, forming negative ions that transfer negative charge to dust as they collide with it. Dust grains need not touch a wire to become charged.
  2. Exert a sideways electric force. In the space between the negative wires and earthed collectors, negatively charged dust experiences force towards the collecting plates, opposite the local field direction.
  3. Deposit the particles. Dust collects on the plates while gas continues through the device with fewer suspended particles.
  4. Remove the deposit. Periodic mechanical shaking dislodges the collected material into a hopper, allowing collection to continue.

Earthing a plate does not make it electrically inactive. The connection allows charge to redistribute between the conducting collector and Earth. Its facing surface can attract negative dust in the field produced by the negative wires. This model does not require a separate positive supply attached to the collecting plates.

The collected particles can move across the gas flow because an electric force acts on them. Removing particles does not mean that every gaseous pollutant has been removed, or that every particle is captured.

Use the same reasoning in a different separator

Application to a supplied design

Particles enter upwards through a leftward field

For the supplied field in the separator example, positive particles have force left and negative particles force right. Side collectors can therefore receive different charge signs.

The design reasoning is: identify the charge sign, read the field direction, predict the force, then consider how it changes the existing motion.

Explain charging and collection separately. The gas first transfers charge to the dust; the field then exerts a force on that charged dust. Neither step is explained by saying only that the particles are attracted to a plate.

Check your understanding

In a separator, positive and negative particles enter moving upwards. The electric field points left. Ignore gravity and air resistance. How does the electric force begin to deflect the two kinds?

  1. Both are forced left, because the field points left and pushes every charge that way.
  2. Neither is deflected because their initial motion is upwards.
  3. Positive particles are forced left and negative particles right, while both initially continue moving upwards.
Show the answer

Positive particles are forced left and negative particles right, while both initially continue moving upwards.

Use the positive-test-charge convention: positive follows the field-force direction; negative has force opposite it. The force changes the motion progressively, so the initial upward velocity does not instantly disappear.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

An electrostatic precipitator removes smoke particles from the waste gases of a power station chimney. Explain how it works. [3]

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