1. Models of Electricity Scientists use observations about nature to construct models and theories to explain how nature works.

2.  These models can change over time when new observations are made.  This is what happened to the theory that explains how electricity works.

3.  In the 1600’s, the theory to explain how electricity was called the one-fluid model.  William Gilbert suggested that electricity was a fluid that could flow between objects.

4.  Benjamin Franklin explained that positive and negative charges were due to the electric fluid.  When an object lost fluid it became negatively charged, and when it gained fluid it became positively charged.

5.  Over time the theory changed because off new observations.  The two-fluid model of electricity was suggested. This model used two types of fluid instead of just one.

6.  One of the fluids was thought to be positively charged while the other was negatively charged.  An object could gain a positive charge by either gaining positive fluid or losing negative fluid.  An object could gain negative charge similarly.

7.  Then, in 1897 J.J. Thomson discovered the electron (which you discovered, if you recall, in the chemistry unit).

8.  This meant that the model of electricity had to be changed again, to pretty much what we use now.  This is called the particle theory of electricity.

9.  Remember an atom is made up of a nucleus containing protons and neutrons.

10.  The nucleus has an overall positive charge.  Spinning around the positive nucleus are electrons, which are negatively charged.

11.  The particle model of electricity states that when two objects are rubbed together, electrons are removed from the atoms of one object and move to the second object, it becomes negatively charged.

12.  The first object is positively charged because it is missing some electrons.  Nothing in the nucleus moves.  Negatively charged electrons move, while positive charges are stuck where they are.

13.  Some atoms hold their electrons loosely while others hold on tight.