Lee O'Neill of James Hargest College asks :-

If an electrical current increases, do more charges leave the battery or do they just travel faster?

John Campbell, a physicist at the University of Canterbury, responded.

An electrical current is the rate at which electrical charge is transferred, so there are various possibilities. For example, we can increase the current by causing more electrical charges to travel at the same speed, or have the same number of electrical charges transferred in shorter time.

It may be easier to first think of a similar situation, the flow of water through a pipe. For a given pipe, the rate of flow of water (in litres/sec or kg/sec), which depends on the cross-sectional area of the pipe (eg diameter) and a few other fixed quantities, is then mainly dependent on the pressure difference along the pipe. The higher the pressure difference the faster the water flows. So if we increase the pressure then in any given time a larger mass (or volume) of water will exit the pipe. Or we can keep the pressure the same but switch in a parallel pipe through which the water can also flow, thus increasing the total amount flowing in a given time without changing its speed in either pipe.

So the answer is that it depends on what change you make to cause the current to change.

When we are dealing with electrical charges flowing in a vacuum, as in a glass-vacuum TV tube or X-ray tube, we can increase the current by increasing the speed of arrival of the electrons emitted from the heated filament. We can do this by increasing the accelerating voltage. For a given voltage (pressure) we can increase the current by raising the temperature of the electron emitter (filament), so more electrons are emitted per second but they arrive at the original speed.

The speed of charge flow is more complicated in a metal. Metals conduct electricity because a small piece of the atom, an electron or two, is available to move through the wire leaving the atom behind. When a current is switched into a wire a metre long the first electron comes out the far end about fifty millionths of a second later.

At any instant one of these electrons can be travelling at about one hundred thousand metres every second. Thats pretty fast. However the electron doesn't travel very far before rebounding after smacking into some sort of impurity in the copper wire, such as a vibrating atom or an oxygen atom. The conducting electron therefore drifts only very slowly down the wire. If we look at a particular electron entering a copper wire that is one meter long, one millimetre in diameter and carrying one ampere of electrical current then it will take that particular electron nearly three hours before it emerges from the other end.

If there is just one wire connecting the battery terminals, we can increase the current by reducing the resistance to the flow. This can be done in several ways, such as by changing to a bigger diameter wire of the same material (more electrons travelling at the original speed), by cooling the conductor (higher drift speed), or by changing to a better conductor (a combination of both methods). Semiconductors and gas discharge tubes are even more complicated.