ELS - Electric Charges

Electric Charges

Introduction

Let's do a demonstration together. Take a sheet of paper and tear it up into very small pieces. Now, find a plastic ruler and touch it to the pile of paper bits. You probably didn't witness anything special just then. Take your ruler and rub it vigorously against your shirt (it works even better if you can rub the ruler against some type of wool garment). Bring your ruler near the pile of paper bits again. If your experience was like mine, you just saw something different and interesting happen. What was the difference between the ruler in the first trial and the second? That's right, you rubbed the ruler against your shirt. This caused an unseen change in the properties of the ruler. You built up static electricity on the ruler and witnessed the effect of bringing a charged object near your small pieces of paper. Static electricity is the build up of electrical charge on an object. Most objects contain two types of electrical charge, positive and negative. At the smallest level, these charges are related to the negatively charged electrons and positively charged protons in the atoms. The overall charge of an object is due to the relative amount of positive charge to negative charge. In most cases, there will be an equal number of positive and negative charges giving the object a net zero charge (neutral charge). When objects do become charged they will exert a force on other charged objects. This is where we get the familiar 'like repels like' and 'opposites attract' statements. These statements refer to the direction of the electrostatic force between two charged objects based on the sign of their charges. Charges can be transferred between objects, changing the net charge. Since protons are held tightly in their nuclei, it is the movement of electrons that allows charge to be transferred. The ruler in our demonstration gained electric charge when it was rubbed with cloth. There are several methods by which an object can have its charge changed.

1. Contact/Conduction: If you bring two objects together, an imbalance of charge will cause charge to move from one object to the other in order to reach equilibrium. This means if you start with an object with zero charge (neutral charge) and bring it into contact with a positively charged object, the charge will redistribute so that the positive object will have a lower, but still positive, charge while the neutral object gains a net positive charge.

2. Friction: When two dissimilar materials are rubbed against each other, charge is pulled from one object to the other. In this way the two objects become charged with opposite charges. Rubbing the plastic ruler with cloth caused the cloth to have one charge and the ruler to have the same magnitude of charge, but opposite sign. You also experience this if you walk across a carpet and shock yourself by touching a metal door handle.

3. Induction: You can induce a charge in an object simply by bringing a charged object near it. Watch this video to see how it's done.

Here's a diagram showing the steps for charging two different objects by induction.

An electroscope is a device that is designed to measure the charge on an object. By bringing a charged object near the electroscope, a charge is induced in the device causing a deflection of the apparatus.

Objects don't tend to stay charged indefinitely. Water molecules in the air will attract to charged objects and eventually relieve them of that charge. This topic will be covered in more detail in the discussion for this unit.

It is important to remember that in any process the net amount of electric charge produced is zero. If charge is lost in one place, that same amount must be gained elsewhere.

Conductors and Insulators

While many objects can be made to carry a charge on their surface, only certain materials allow charge to flow through them. A conductor is a material where the atoms have electrons that are bound very loosely and are free to move from atom to atom, carrying the electrical charge. Examples include most metals. An insulator is a material where the electrons are tightly bound to the nuclei. Wood and rubber fit into this category. Where does water fit in? Is it a better electrical conductor or insulator? Most people believe it is a conductor. Why else would you want to stay away from the water when there is lightening in the area? The truth is that pure water is actually an insulator. In order to conduct electrical charge ions must be introduced to the water so there are charges with the freedom to move. So if you're in the pool and lightening is seen, you should definitely get out, not because the water can conduct the lightening, but because the chlorine ions in the water can.

Electric Charge Self-Assessment

1. When you slide out of a car with cloth seats on a winter's day you notice a small shock when you touch the metal door while closing it. Describe the methods of charge transfer taking place.

2. If you rub a balloon against your hair and then hold it to your clothes, the balloon will try to stick to your clothes. Describe the electrostatic processes that explain this static attraction.

Please click here for solutions to the self-assessments. Links to an external site.

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