Chapter 5: Electricity to the Rescue

static electricity | AC vs DC | electrolysis | electroplating | ancient battery

Static Electricity

Electricity is a flow of electrons. Because the word "static" means "not moving", static electricity is an electric charge that is built up, but is not an electric current. The zap/spark it gives is the sudden discharge of that charge.

Static electricity is normally quite safe because even though the voltage may be several thousand volts the current is very low. One way to work out how much voltage is generated is to measure the length of the spark created. Air breaks down at about 1,000 to 3,000 V/mm, depending on humidity, temperature, etc, so if a spark between two objects is 2 mm long there's 2,000 to 6,000 volts difference between them.

Static electricity is generated when two objects are rubbed together. The further apart the objects are on the triboelectric series the larger the charge is likely to be (as a generalisation). "Tribos" is Greek for "rubbing" while "electron" is Greek for amber, a material that is good for generating a charge when rubbed with rabbit fur (for example). The further apart on the list the objects are, the more easily a large charge can be built up - for example, dry skin rubbed with a PVC pipe. They're at opposite ends of the series, so the pipe should be easily charged for some good hair-raising experiences.

See the Van de Graaff picture page for pictures of a Van de Graaff generator in action.

Mythbusters investigated how static electricity can cause car fires at petrol stations. The worst thing to do is to return to your car while filling it, then get out without touching the door (or anything else), then grab the pump nozzle (handle). This is quite likely to cause a large spark between you hand and the pump nozzle, possibly lighting petrol fumes. Mobile phones do not cause petrol station fires.

AC vs DC

AC stands for alternating current. In a circuit the electricity keeps changing direction. Our mains power is 230 volts, 50 hertz AC (230 V, 50 Hz), which means that the current changes direction 100 times every second, flowing in one direction for ¹/₁₀₀ second then in the other direction for ¹/₁₀₀ second, and so takes ¹/₅₀ second to complete one full cycle. 50 hertz means 50 cycles per second.

Mains power hurts (hertz) because the voltage is high, and so any current will also be high for any particular object like a human.

DC stands for direct current. In a circuit the electrons are always moving around the circuit in the same direction.

Electrolysis

Electricity can be used to break molecules apart, and is an efficient and inexpensive way of making aluminium, hydrogen, etc. Sodium can be made by electrolysis of table salt, sodium chloride (NaCl).

Nuclear submarines are able to generate breathing oxygen from the water around them, so can remain underwater for as long as their fuel lasts (months).

The reaction to break water (H₂0) into hydrogen and oxygen gas is the opposite to that when hydrogen burns. Instead of energy being given out in the combustion reaction, energy is put in by the electricity.

2H₂ + O₂ → 2H₂O + heat

2H₂O + energy → 2H₂ + O₂

Direct current is used, so one electrode in a solution will always be positive (anode) and the other will be negative (cathode). Each electrode attracts ions which are of the opposite charge. If we rewrite the equation to show the ions (charged atoms) involved we get:

4H_⁺ + 2O^- + energy→ 2H₂ + O₂

So the hydrogen ions will be attracted to the negatively charged electrode - the cathode. If we did electrolysis in a cup of water the hydrogen could be collected by holding a test tube over the cathode, since hydrogen is lighter than air and any air will be pushed out the bottom of the test tube by the "floating" hydrogen. To test that we do have hydrogen we would use the pop test.

In the Hoffman apparatus pictured, while the taps are turned off the gases collect in the top of the apparatus, pushing the water levels down. Note that twice as much hydrogen is formed.

Electroplating