Aerosols and pressure. Electrostatic effects

5. Electrostatic effects

Sparks

P.21

Picture 5.1. When the charge builds up on a Van der Graaf generator, a spark will jump across the air gap.

		Aerosols often contain flammable liquids – most usually the LPG that is used as a propellant in many aerosols. Aerosol manufacturers take a number of precautions to avoid fires or even explosions. They include: electronic gas ‘sniffers’, which will sound an alarm if they detect any gas injecting the propellant in a separate gassing room (see page 6) preventing sparks by using special motors, pneumatic drivers and special conveyer belts.

Sparks

There are two main ways that sparks might be caused in industrial equipment:

static sparking due to a build up of charge (page 22)
inductive sparking when a circuit breaks (page 26)

In each case, there is a spark because a large voltage has broken down the air and made charge jump across the gap. Let’s see how this happens.

Graphic of effect of electric field on charges

Picture 5.2. Air contains neutral helium atoms (simple atoms). In an external electric field, the nucleus and electrons will feel forces in opposite directions.

	Breakdown of air
		Air is a mixture that includes nitrogen, oxygen and carbon dioxide molecules (there are also trace amounts of the inert gases). These gases are all insulators – under normal circumstances, air does not conduct electricity. However, if there is a big enough electric force on the charged particles in these molecules, then they will be ripped apart. The negative electrons will be pulled one way, leaving behind a positive ion, which will be pulled the other way. The electric force on charged particles is called the electric field strength, E. It is a measure of how much force there is on unit charge and is measured in N C^-1 (newtons per coulomb). We can show that the electric field strength is also equal to the potential gradient, or voltage per metre. So it is also measured in V m^-1 (volts per metre) (see question 16).

Picture 5.3. Parallel plates with field between them

	What field is required?
		We can make a small electric field by setting two plates 1 cm apart and attaching a voltage of 100 volts. The field strength will be 100 V cm^–1 (equal to 10,000 V m^-1). Any charge between these plates will feel a force (of 10,000 N C^-1). However, although this force might distort some molecules, pulling the negative electrons towards the positive plate, it won’t actually pull any electrons off the molecules. However, once the field strength gets close to being a hundred times bigger (just less than 10,000 V.cm^-1), the force on the electrons will start to pull some of them off. This will produce a positive ion and a free electron. The electron will accelerate towards the positive plate and the ion will accelerate towards the negative plate. The ions will collide with other air molecules and lose their energy, eventually picking up free electrons, making them neutral again.

	Cascade of ions
		If the ions had been really fast by the time they hit the first air molecule, they could have knocked off an electron, producing another ion. Both of the ions would now be accelerated again and might produce two more ions in their next collisions. Now there are four ions, which will double up on the next set of collisions and so on. We have a cascade of ions – and this is a spark (it glows because the ions recombine with electrons to form excited atoms and these give out light as they return to their ground state). The electric field strength that produces this spark is 10,000 V cm^-1 (or 1 MV m^-1). At this field strength, we say the air has broken down.

Question 16
In this question, you will use dimensions to show that the two units for electric field strength are the same.
a) The definition of the volt is joules per coulomb. Express this in terms of newtons, metres and coulombs.
b) Electric field strength can be measured in volts/metre. Show that this is equivalent to newtons per coulomb. (Note this isn’t a full proof that force/charge=voltage/separation because there could be a dimensionless constant in the equation).
Click shift/return to get a line break in your answer
c) The charge on an electron is 1.6 x 10^-19 C. What will the force be on an electron in a field of 10,000 V cm^-1?
d) The average ionisation energy of an air particle is about 13 electron volts (air is a mixture of elements and compounds). How far will the electron have to travel in order to gain this energy in a field of 10,000 V cm^–1?

Summary Close

sparks occur when air breaks down
a spark is a cascade of ions
air breaks down at a about 100 V cm^–1