Episode 103: Currents and charge carriers

There are two main aims for this episode: to present a range of examples involving different types of charge carrier, and to measure currents and link the measured current to rates of flow of charge.

The episode consists of a series of demonstrations which could be set up as a ‘circus’ before the lesson. The students can then be taken around as each one is discussed.

Summary

  • Demonstration: Identifying charge carriers (20 minutes)
  • Demonstration: An electron beam (15 minutes)
  • Demonstration: Students conducting electricity (15 minutes)

Demonstration: Identifying charge carriers
Students are used to thinking of metals as good conductors. However, they should appreciate that there are other situations, more or less familiar, in which current flows.

In a filament lamp: Conductor: metal (tungsten). Charge carriers: electrons. Remind them of the free-electron model (i.e. in a metal, there are ‘free’ electrons which can move about within the metal). Discuss the behaviour of the charge carriers as the supply voltage is increased. (They move faster to make a bigger current.)

A spark through air

A spark through air: The level here is variable. The essential idea involves ionisation. You could ask why air is usually a good insulator and what must happen in order for it to ‘break down’ and conduct. The charge carriers are positive ions and electrons. These move in opposite directions. Link this to lightning.

A fluorescent tube: Conductor: Plasma. Charge carriers: ions and electrons. Plasma is the ‘4th state of matter’ and is the most common phase of matter in the universe (e.g. in stars).

Electrolysing copper sulphate solution with copper electrodes: Conductor: Electrolyte. Charge carriers: positive (copper) and negative (sulphate) ions.

So both electrons and ions are ‘charge carriers’; when they move, a current is flowing.

Episode 103-1: Identifying charge carriers (Word, 39 KB)

Demonstration: An electron beam
Show the path of beam of electrons in a vacuum tube. You will need to practice setting this up; follow the manufacturer’s instructions.

Demonstration: an electron beam

Conductor: charged beam in a vacuum. Charge carriers: electrons. The high speed and low density of charge in the beam can be contrasted with the low speed and high density of charge carriers in a metal (this helps to lead into the derivation of I = n A v q if your specification requires it).

Episode 103-2: Current and charge in electron beams (Word, 53 KB)

Demonstration: Students conducting electricity
This can be used to show the effect of series and parallel circuits. It can also lead to a discussion of electric shock and electrical safety. It takes a few tens of mA to kill a person. A car battery can supply hundreds of A if it is shorted, but 12V is not sufficient to push a tangible current through a person. The amount of current depends on the contact resistance and path of the current through the body. We conduct because much of our body is effectively an ionic electrolyte (like salty water).

Episode 103-3: Conduction by students (Word, 122 KB)

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Episode 103: Currents and charge carriers (Word, 198 KB)