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Unit D: Electrical Principles and Technologies Slide 2 1.1 Static Electricity Slide 3 Benjamin Franklin (1706- 1790) introduced the theory of positive and negative charges Slide 4 Atomic Theory and Electricity Protons (+) and neutrons (0) are contained in the nucleus Electrons (-) orbit around the nucleus atoms are neutral because there is a balance between the number of positively charged protons and negatively charged electrons. Slide 5 Three Laws of electrical charges: 1. opposite charges are attracted to each other 2. like charges repel 3. charged objects attract neutral objects Slide 6 (A) Initially, there are equal numbers of protons (+) and electrons (-) on the comb and hair. No net charge. (B) Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair. Slide 7 Try it out with a balloon... Slide 8 The charge on an ion is called an electrostatic charge. An object becomes electrostatically charged by friction, which transfers electrons between two objects in contact Charge separation occurs when a charged object is brought close to a neutral object. Slide 9 (A) The paper normally has a random distribution of (+) and (-) charges. (B) When the charged comb is held close to the paper, there is a reorientation of charges as the comb electrons repel the paper electrons. This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb. Charge Separation The comb has become charged by friction, acquiring an excess of electrons. Slide 10 Static electricity happens when there is an imbalance of electrons between two objects. Electrical Discharge - the movement of charges from one object to another because of this imbalance. Protons of a neutral object attract the charged electrons of the other, and the electrons jump to the neutral object. Eg.When you feel a shock when touching metal Slide 11 Lightning - lightning is a discharge from the static electricity formed by raindrops rubbing against one another Slide 12 Van De Graaff Generators (VDGs) A rubber belt rubs on a piece of metal, acquiring a negative charge by friction building up electrons - the negative charges are then transferred to the sphere. Slide 13 When you touch the sphere, the negative charge builds up on you. Your hair stands up because the like charges in your hair repel. Slide 14 Van Der Graaf Demo P 277 Static Demo Slide 15 Now do Check and reflect P.278 #1-9 Slide 16 1.2 Current Electricity Electric current is a steady flow of charged particles(electrons) in the same way that a water current flows. It is the charge that flows, and the current is defined as the flow of the charge. The difference between static and current electricity is that current is the continuous flow of electrons, while static is simply the rapid discharge of electrons. Slide 17 Amperes (A): Often called amp for short Measures the rate at which electrical current flows Slide 18 A simple electric circuit has a voltage source (generator or battery) that maintains the electrical potential, a device (lamp, motor, etc.) where work is done, and a continuous pathways for the current to follow( wires). Slide 19 Every Electric Circuit Must Contain At Least Three Elements: 1. Energy source: A voltage source of electricity (battery/cell) 2. Conductor: for transmitting electrons (eg. copper wire) 3. a load or form of resistance: resistance can come in many forms. It might be a light bulb, an electric motor or resistor. There is also often a switch to control current flow. Slide 20 Elements of a Circuit When a circuit is closed electrons leave the negative side of the battery and will flow towards the positive side. The flow of electrons past a fixed point is called a current and is measured in amperes or amps Voltage is the force that pushes electrons through the circuit. The greater the voltage and current, the more work that a circuit can do. Slide 21 Current, Resistance and Voltage can be compared to water travelling through a garden hose. Slide 22 Voltage (V): A measure of the potential difference between two places in a circuit. In other words, voltage tells us how much energy each charged particle carries. The higher the voltage, the more energy each particle has. Slide 23 For safety purposes, the voltage of most household appliances is relatively low. The voltage of industrial devices and transmission lines is relatively high. Slide 24 Do p279 Electrical Current Lab Sketch # 4 and 5 Slide 25 1.2 Do page 283 1-9 Slide 26 1.3 Electrical Safety Coming in contact with a power transmission line can prove to be deadly. By touching it, a short circuit can occur, because the electricity is trying to find a path to the ground - you can complete the circuit, but it may be fatal. Slide 27 The Dangers of Electrical Shock High voltage power lines carry 50,000V of electricity. However, amperage is more important to consider. 0.001A will likely not be felt at all, 0.015A to 0.020A will cause a painful shock and loss of muscle control (which means you will not be able to let go of the line). Current as low as 0.1A can be fatal. Electrical Dangers vary, depending on the situation. When the current can flow easily, it is more dangerous. Insulators (such as wood, rubber and air) hamper the flow of electricity, but moisture is a good conductor of electricity, so avoid water when working with electricity. Slide 28 Protecting Yourself From Electrical Shock The Canadian Standards Council issues labels to identify the amount of voltage required to operate electrical devices and the maximum current they use. Slide 29 Electrical Safety Pointers... Never handle electrical devices if you are wet or near water Don't use devices that have a frayed or exposed power cord Always unplug an electrical device before disassembling it Don't put anything into an electrical outlet - except a proper plug for an electrical device Don't overload an electrical circuit, by trying to operate too many devices at once Avoid power lines Don't bypass safety precautions when you are in a hurry Pull on the plug, not the wire Never remove the third prong from a 3 prong plug Slide 30 Plugs, Fuses and Breakers The third prong of a 3 prong plug is a ground wire, connected to the ground wire of the building, in case of a short circuit. Fuses and circuit breakers interrupt a circuit when there is too much current flowing through it. Slide 31 Fuses contain a thin piece of metal, which is designed to melt if the current is too high. Circuit breakers, on the other hand, trip a spring mechanism, which shuts off the flow of electricity through the circuit, when there is too much current. It can be reused over and over (provided the cause of the increased flow is corrected). Slide 32 The Danger of Lightning A lightning strike can have 30,000A - more than enough to kill you. Avoid being the target of a lightning strike by staying low to the ground (horizon) and away from trees. Lightning can also do a lot of damage to a building. Metal lightning rods, which are connected to the ground with a grounding wire, are fixed on the roof of many buildings to prevent damage to the building during an electrical storm. Slide 33 Do Check and Reflect P.287 #1-9 Slide 34 1.4 Cells and Batteries An electrochemical cell supplies a steady current. The electricity comes from chemical reactions within the cell. Slide 35 Dry Cells Electricity-producing cells referred to dry cells are 'dry' because the chemicals used are in a paste. Volatic Cell Animation Slide 36 The chemical reaction in a cell releases free electrons which travel from the negative terminal of the cell, through the device which uses the electricity, and back to the positive terminal of the cell. Dry cells are made up of two different metals, called electrodes in an electrolyte paste. Slide 37 An electrolyte is a paste or liquid that conducts electricity because it contains chemicals that form ions. The electrolyte reacts with the electrodes, making one electrode positive and the other negative. These electrodes are connected to the terminals. Slide 38 Dry Cell Slide 39 Wet Cells These are called 'wet' because the electrolyte is a liquid (usually an acid). In a car battery, the electrolyte is sulfuric acid. Electrons travel from the negative electrode (zinc) through the device and on to the positive electrode (copper). Slide 40 Slide 41 Rechargeable Cells Non-rechargeable cells are called primary cells. The chemical reactions in a secondary cell (rechargeable) can be reversed by using an external electrical source to rejuvenate the cell. Slide 42 The cheapest and most common secondary cells are Nickel Oxide and Cadmium (Ni-Cad). The reactants are restored, but the electrodes wear out over time. Slide 43 Batteries Connecting cells together creates a battery, which is a sealed case with only two terminals. Electrochemistry Allesandro Volta made the first practical battery around 1800, by piling zinc and copper plates on top of each other, separating them with electrolyte- soaked paper discs. Humphrey Davy filled an entire room with 2000 cells to make one massive battery. His work led to a whole new field of science called electrochemistry, the study of chemical reactions involving electricity. Slide 44 Electrolysis:electricity is used to split molecules into their elements. Many industries use electrolysis to separate useful elements from solutions. Slide 45 Electrolysis of Water Slide 46 Electroplating: Silver and Gold plating can make items look expensive. A thin coating is used to cover (and often strengthen) the material. This is often used to protect the metal from corrosion. Slide 47 Slide 48 Other Electrochemical Applications Anodizing and Electrorefining are other examples of electrochemical processes Anodizing is a process that coats aluminum parts with a layer of aluminum oxide, which is much harder than aluminum. It is used in products such as screen doors, airplanes, car parts, kitchenware and jewellery. Electrorefining is used to remove impurities from metal. Another process used by automobile companies bonds special paints onto car parts. Slide 49 Do Check and Reflect P.294 # 1-10 And Section Review P.295 #1-11