Static Shock

Static Shock

Pull off a wool jumper in the dark and you hear crackles and see sparks. Rub a balloon on your hair and it sticks to the wall. Both of those need explaining and neither is obvious. The wall is not charged, so why does anything stick to it? And nothing is plugged in, so where does a spark find the energy to jump through air, which does not conduct? This module answers both, and it starts with the one particle that moves.

Why it has to be an insulator

Charging by friction means electrons are transferred from one surface to the other. Nothing is created: the object that loses electrons is left positive, the one that gains them is negative. But whether anything builds up depends on the material:

The metal rod

You hold a metal rod in your bare hand and rub it hard with a cloth. No matter how long you rub, it will not hold a charge. Given the same cloth charges a polythene rod easily, what is going on, and what would make the metal work?

• Electrons are being transferred, and the metal conducts them straight through your hand and body to earth as fast as they arrive. Hold it by an insulating handle instead and the charge has nowhere to go, so it builds up
• Electrons cannot be pulled off a metal surface by friction, because they are held too tightly
• Metals are always electrically neutral and cannot become charged at all
• The cloth and the metal are too similar, so no electrons transfer between them

Why it sticks to the wall

Your balloon is negatively charged. The wall is neutral, with no charge on it at all. Work out why they attract.

• You bring the negative balloon close to the wall. What happens to the electrons in the wall's surface atoms?
• So the wall now has a slightly positive near face and a slightly negative far side, and the two are equal in size. Why is there a net attraction rather than nothing?
• You try the same balloon against a large earthed metal sheet. What do you expect?

Fields and field lines

A charged object changes the space around it, and this is the language for describing that:

Label the electric field

The electric field around a positive charge. Drag each label onto the correct part.

Why do the lines crowd?

On every field diagram the lines are packed tightly near the charge and spread far apart out at the edges. Why does that spacing carry meaning rather than being a drawing habit?

• How closely packed the lines are IS the field strength. The same number of lines leaves the charge in every direction, so further out they are spread over a much larger area and the spacing between them grows, which is precisely how the field weakens
• It is done for clarity, so the lines do not overlap and become unreadable near the charge
• More lines are drawn near the charge and fewer further out
• The lines show how fast a charge would move, so they bunch where movement is quickest

How a spark crosses air

Air is an insulator. That is why a mains cable is safe to stand next to. So how does a spark get through it? As charge accumulates on an insulated object, the potential difference between it and earth rises, and so does the electric field in the gap. Push that field high enough and it becomes strong enough to rip electrons off the air molecules themselves. The air is then ionised: it contains free charges, and a material with free charges is a conductor. At that instant the insulating gap becomes a conducting path, and the accumulated charge flows through it all at once. That sudden rush is the spark: the flash is the ionised air glowing, and the crack is the air heating and expanding faster than the speed of sound. So the air did not conduct until the field made it conduct. Nothing was pushed through an insulator; the insulator stopped being one.

From rubbing to crack

Put the stages of a static discharge into order.

• Friction transfers electrons onto an insulated object
• Because the object is insulated, the charge cannot escape and accumulates
• The potential difference between the object and earth rises, and the field in the gap grows with it
• The field becomes strong enough to tear electrons from the air molecules in the gap
• The ionised air now contains free charges, so it conducts
• The accumulated charge flows across in an instant, producing a flash and a crack

Lightning is the same thing

Inside a thundercloud, ice crystals and hailstones collide in violent updraughts and transfer electrons between one another, exactly as your jumper and your shirt do. The lighter crystals are carried to the top and the heavier hail sinks, so the charge ends up separated: typically negative at the base of the cloud, positive at the top. The base then induces the opposite charge in the ground beneath it, and the potential difference between cloud and ground climbs into the hundreds of millions of volts. When the field is finally strong enough, several kilometres of air ionise and the discharge happens. Every stage is one you have already met. The only differences are the scale, the fact that the rubbing is done by weather rather than by hand, and that the crack is thunder rather than a click.

Before the fuel flows

A tanker refuelling an aircraft must be connected to it by a bonding wire before a drop of fuel moves, and both are usually earthed as well. Why is this a legal requirement rather than a precaution?

• Fuel rushing through a hose charges up by friction. Without a conducting connection, a potential difference builds between tanker and aircraft, and a spark across that gap in fuel vapour would ignite it. The wire keeps both at the same potential, so no difference can build and no spark can jump
• The wire gives the fuel somewhere to send its charge, so the fuel itself stays neutral
• It protects the aircraft from being struck by lightning during the refuelling
• It allows the ground crew to measure how much charge has built up on the aircraft

Static put to work

Static charge is not only a nuisance. Select the THREE processes that genuinely depend on it.

• Spraying a car body: the paint droplets are given the same charge, so they repel one another into a fine even mist, and the earthed panel attracts them
• Cleaning smoke in a chimney: the dust particles are charged as they pass a grid, then attracted onto earthed plates further up
• Laser printing: a drum is given a charge pattern, and toner powder sticks only where the charge is
• Generating mains electricity, which is static charge released in a continuous stream
• Holding a fridge magnet on a fridge door
• Making a battery produce a voltage

Which bit of physics is doing the work?

Static summary

Rubbing transfers _____ between two surfaces, leaving the one that lost them positive. It only builds up on an _____, because on a conductor the charge would spread out and drain to earth. A charged object has an electric field around it, drawn as lines whose _____ shows how strong the field is. When the field across a gap grows large enough, it ionises the air, which then _____, and the charge crosses all at once as a spark.