Chapter 3 Static and Current Electricity 3.1 Static Electricity - the build up of an electronic charge on a body (object) Electroscope - a device for detecting (not measuring) static charge attraction/repulsion Ways To Produce Change Positive ways - acetate rubbed with cotton or paper towel - glass rod rubbed with silk Negative ways - vinyl rubbed with wool - amber rubbed with fur *Neutral - no charge Q: How could you determine that there are only two types of charge? Do Activity 3C - HWK Discussion - HWK Review 3.1 All Charles DuFay Rules of Static Electricity - 1. Objects with like charges repel figure figure Objects with unlike charges attract figure 1
2. Charged objects attract neutral objects figure figure 3.2 Static Electricity and the Atom Atomic structure - heavy, stable, positively charged nucleus. Nuclei are made up of protons (+ charge) and neutrons (0 charge) light, easily movable, negatively charged electrons which form a cloud around the central nucleus. Figure Charge production - When two objects are rubbed together, some of the electrons are transferred: - an object that gains electrons becomes negatively charged 2
- an objects that loses electrons becomes positively charged However, this new net charge will only last until electrons are passively lost (-), or gained (+). Discuss/Review So, now you ve rubbed two objects together and you ve got a transfer of electrons 1)figure 2) Q: Rule 2 says charged (- or +) attract neutral objects. How does that work? A: Induced charge - when a charged object approaches a neutral object. The positively charged object attracts electrons in the neutral object. This creates a temporary shift so that the part of the (overall) neutral object closest the charged rod becomes (-). You then have a (+) (-) attraction. 3
See Figure 3.8 The unit of electrical charge on an object 1 coulomb = 1 C charge of 6.24x10 18 electrons (for (-) charge) 6.24x10 18 (for (+) charge) *Our experiments: 1x10 16 C or 6x10 12 electrons transferred Review 3.2 3.3 Current Electricity potential energy - energy of position electrical potential energy - the energy of position produced by pulling an electron away from the attractive force of the proton no PE figure high PE figure 4
Attractive forces act like a spring trying to pull electrons back to p (+) *In a chemical cell a chemical reaction separates protons and electrons to produce EPE. Figure Measuring EPE Voltage = the amount of EPE supplied per C of charge transferred (volt = V) Introduction to meters Do Activity 3D HWK Discussion Electrical current (what really happens) - the flow of electrons from a negative terminal to a positive terminal Conventional Current (what we used to think happened) 5
figure - the current that flows to a negatively charged object *comes from only belief that a mysterious liquid flowed (+) (-) Measuring Current 1 Ampere = 1C/ 1 second Conductors - materials in which electrons can flow easily Insulators - materials in which electrons do not flow easily Circuit - a completed pathway for electrons to flow. Schematic Diagrams 6
Activity 3E In class/hwk - Discussion Series and Parallel Circuits Series - in a row. Series batteries deliver a higher voltage but most work hard because all the electrons flow through all the cells. Parallel - side by side. Parallel batteries believer same amount of voltage as one cell bit do not work hard because only a small portion of total electrons flows through each cell. Offer alternative electron pathways: therefore, they last longer! Resistance - how hard it is for electrons to flow through a material. Measured in ohms. (figure) Resistors - devices made of materials which offer high resistance *heat and light Rheostats - variable resistors Eg. NiCr wire Resistor codes - Activity 3F in class. 7
Ohms Law- voltage, current and resistance are related by the following equation Resistance ( R ) = Voltage ( V ) Current ( I ) Whereby IF you know any two variables you can calculate the third. Example 1 Suppose your stove is plugged into a 120V outlet and you know that it draws 6 Amps of current. What would the resistance be? R = 120V / 6A = 20 Ohms (?) Example 2 Resistance ( R ) = Voltage ( V ) Current ( I ) A certain light bulb is rated at 96? and needs 1.25A to work. How many volts would be used? V = R X I = 96 X 1.25 = 120V Resistance ( R ) = Voltage ( V ) Current ( I ) Example 3 8
What would the current be in a kettle if it s element was rated at 10? and the voltage was the standard (usual wall socket) 120V? I = V / R I = 120V / 10? I = 12A Resistance ( R ) = Voltage ( V ) Current ( I ) Review 3.3 Chapter Review 9