Detection of Charges

Some substances, such as metals and salty water, allow charges to move through them easily.  These free electrons can move through the material.  Salty water and other similar conducting materials contain free ions that can move through them.

An ion is an atom or molecule having a positive or negative total charge. In other words, the total number of electrons is not equal to the total number of protons.

Other substances, such as glass, do not allow charges to move through them.  

The gold leaf electroscope

This is an instrument for detecting and measuring static electricity or voltage.

Figure 1: A metal disc is connected to a narrow metal plate and a thin piece of gold leaf is fixed to the plate. The whole of this part of the electroscope is insulated from the body of the instrument. A glass front prevents air draughts but allows you to watch the behaviour of the leaf.

When a charge is put on the disc at the top it spreads down to the plate and leaf. This means that both the leaf and plate will have the same charge. Similar charges repel each other and so the leaf rises away from the plate - the bigger the charge the more the leaf rises.

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Figure 2: (a) A positively charged glass rod is brought near the tip of the electroscope, attracting electrons to the top and leaving a net positive charge on the leaves. Like charges in the light flexible gold leaves repel, separating them. 

(b) When the rod is touched against the ball, electrons are attracted and transferred, reducing the net charge on the glass rod but leaving the electroscope positively charged. 

(c) The excess charges are evenly distributed in the stem and leaves of the electroscope once the glass rod is removed.

The leaf can be made to fall again by touching the disc - you have earthed the electroscope.  An earth terminal prevents the case from becoming live. The electroscope can be charged in two ways:

1. by contact - a charged rod is touched on the surface of the disc and some of the charge is transferred to the electroscope. This is not a very effective method of charging the electroscope.
2. (b) by induction - a charged rod is brought up to the disc and then the electroscope is earthed, the rod is then removed.

The two methods give the gold leaf opposite charges.  Electrostatic repulsion in the leaves of the charged electroscope separates them. 

Since only electrons move in metals, we see that they are attracted to the top of the electroscope. There, some are transferred to the positive rod by touch, leaving the electroscope with a net positive charge.