Electrostatic Charge Transfer How do objects get charged? Electrons can move Conductors electrons are free and can move throughout the material in good conductors, can move from atom to atom, and object to object Insulators do not allow free movement of electrons throughout the material, or from the object to other things Charges within objects are separated Neutral atoms/objects equal protons and electrons Charged atoms/objects more or less electrons than protons (depending on charge) Law of Conservation of Electric Charge Within and object: If part of an object becomes charged, the other part of an object must become equally and oppositely charged. Example: water molecules Between 2 Objects: If an object becomes charged from interacting with another, the other object must become equally, but oppositely, charged 4 Ways to Charge Objects #1: Polarization Surface charge (object remains neutral, overall) Charges within object shift in response to being near a charged object Only while near the charged object. Lightning Friction causes build-up of electrostatic charges in clouds Bottom of cloud is usually negative Negative buildup repels electrons in the ground, so surface becomes positive. With enough buildup Electrons are attracted to the earth and come down in a streamer of negative charges. Positively charged air particles near the surface of the earth are repelled by the ground and form a streamer upwards. When the streamers connect, a conductive path for an electron surge is formed lightning occurs! Air particles are so excited they emit light Air is heated so intensely in creates sound waves (thunder) Polarization Demo Watch the demo and answer the questions on your worksheet. Experiment #1 Charging by polarization. Rub the round end of the balloon with the fur for a few seconds. Hold the balloon against the wall and release it. Observe what happens. 4 Ways to Charge Objects #2: Friction Electrons transfer between objects by rubbing 2 neutral objects together (objects must have different affinities for electrons) Charging by Friction Experiment #2 Charging by friction. Rub one of the balloons on your hair for a few seconds. Immediately hold it close to the other balloon. Observe what happens. pith ball simulation 4 Ways to Charge Objects The Electroscope made of two thin metal leaves attached to a metal rod with a knob at the top. - when the device is not charged, the leaves hang straight down. - when an electric charge is present, the leaves repel each other, spreading apart. electroscope simulations Electroscope 4 Ways to Charge Objects Teacher Demo Draw a picture of what is happening to the charges in the aluminum cans. Grounding phet - travolta K Extensions Conductors: metals, aqueous solutions of salts (i.e., ionic compounds dissolved in water), graphite, water, and the human body. Insulators: plastics, styrofoam, paper, rubber, glass, and dry air. How a Van de Graaff Generator Works: Van de Graaff video Sharing Charge via Conduction A metal sphere (A) with a charge of +4 units is brought in contact with a metal sphere (B) with -2 units of charge, and the spheres are then separated. What is the charge on each sphere after separation? A B AB A B