Static electricity and electric forces
Physics Narrative for 11-14
Electric forces between electrically charged objects
Most pupils will have experienced, and be aware of, some of the everyday effects of static electricity. These might include getting a shock from walking on a synthetic fibre carpet and then touching a metal door handle, or hair
sticking to a comb or a sweater. However, that's very different from making a connection between these experiences and the explanations involving the transferring of charged particles from one place to another.
The origin of this force is not obvious – the mobile particles involved (electrons) are much too small to see, as are the immobile charged particles, the protons. At the same time, it is the case that electric forces are responsible for holding most of the perceived world together. Atoms and molecules stick close together in solids, liquids and gases because of electric forces. When you tear a piece of paper, you are overcoming the electrical interactions that hold the fibres together. When you chop a piece of wood you are doing the same.
You can explain the electric forces using a simple model: everything is made of atoms and each atom has a number of smaller parts, each carrying an electric charge, which can be either positive or negative. Atoms are neutral – their positive and negative charges add to give no overall charge. It's the negative parts of atoms that can readily depart the atoms, to leave a positive residue. So every time you charge and object by rubbing, you are transferring electrons from one thing to another, leaving one positive and the other negative.
When the negatively charged part (the electrons) is separated from the positively charged part the parts attract each other, since oppositely charged objects attract. Two positively charged objects repel each other, as do two negatively charged objects. Rubbing a plastic material is one way to separate electrons from the atoms of the plastic.
The electric force of attraction reaches out through space without the need for contact. Again we can use the idea of a field to moderate this action – connecting the two interacting charges.
This idea of
neutral as a balance of positive and negative charge turns out be really important: when you
charge objects, you're upsetting the balance, by shifting electrons to (resulting in negatively charged objects) or from (resulting in positively charged objects) one of a pair of objects.