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Chapter 2; Lesson 2.1 T.O.C: Charge Needs a Continuous Path to Flow Electric Charge Flows in a Loop • *Remember* Electric current is electric charge that flows from one place to another • Charge does NOT flow continuously through material unless the material forms a closed path, or loop. • Circuit(w.w)- closed path thru which a continuous charge can flow. – Path provided by low-resistant material (usually wire) – Designed for specific jobs (like light a bulb) Parts of a Circuit • Typically have: 1. Voltage Source- Battery or power plant 2. Conductor- forms connection from voltage source to electrical device & back (usually wire) 3. Switch- designed to break the closed path of charge. When switch is open, it produces a gap so the charge cannot flow. 4. Electrical Device- any part of circuit that changes electrical energy in another form of energy. – Resistor(w.w)- electrical device that slows flow of charge in a circuit. This allows energy to be converted into light or heat. Ex: light bulb is a resistor. Converts energy into light. Identify Circuit Parts Open & Closed Circuits • Current in circuit is similar to water flowing thru hose. However, if you cut hose in half the water keeps flowing out. Not so if you cut a wire in half. • Most cords on appliances have 2 wires. One leads to device from source, the other goes from device back to source to complete the circuit. • Switches work by opening/closing the circuit. • Most circuits have standard symbols used to represent parts of a circuit. Paperclip/nails/switch Lab! Current Follows the Path of Least Resistance • Although current follows a closed path, the path does not have to be made of wire. • All materials w/ low resistance are good conductors. This includes you. – This is why you should never use electronics w/ bare wires! – Water is a good conductor, especially when mixed w/ salt from a person’s skin. This is why you don’t use electronics near sinks/tubs full of water! Dangers in Electricity Short Circuit • Short circuit(w.w)- an unintended path connecting one part of a circuit with another. • The current follows a path a closed path, but not the right one. 1. Functioning circuit- charge flows thru one wire to device and back thru other wire to outlet. 2. Short circuit- cord is damaged& 2 wires inside have formed a connection. w/out resistance from lamp there more current in the wire. This causes wires to overheat & start a fire. *Create Illustration Grounding Circuits • Recall; when lightning strikes a rod, charge flows thru the least resistant path. This is why it hits the lightning rod instead of a house or person. • The 3rd prong (the ground) on some plug-ins acts in a similar fashion as the lightning rod. • A circuit that connects stray current safely to the ground is a grounded circuit. • Charge usually flow thru 1 prong to device & back thru 2nd prong to outlet. If there is a short circuit; charge might flow dangerously to the outside of the shell of the appliance. • If there is a ground wire; the current will flow along 3rd wire & safely into ground, along buried rod or cold water pipe. • Imagine: you are sitting at home in the living room. The circuit for the room is currently supplying charge to your t.v. & several lights. You turn the A/C on & suddenly the circuit wires are providing more current than before. The lights dim. What has happened? Safety Devices Control Current • Imagine: you are sitting at home in the living room. The circuit for the room is currently supplying charge to your t.v. & several lights. You turn the A/C on & suddenly the circuit wires are providing more current than before. The lights dim. What has happened? • Too much current is dangerous! There are safety devices built into circuits to prevent dangerous situations from occurring. How Fuses Work • Fuses automatically shut off current before circuits overheat. • Fuses open circuits when there is too much current flowing thru. • Found in cars, some appliances, and older houses • Consist of thin strip of metal inserted into the circuit. The charge then also flows thru the fuse. Too much charge melts the metal strip causing the circuit to open. • Fuses are measured in amps, because fuses monitor electric current. Build your own Fuse Lab! Other Safety Devices • Most modern homes use Circuit Breakers. (breaker boxes) • They do not have to be replaced every time they open a circuit like fuses. • How they work: when too much current is moving thru the circuit; a piece of wire in the breaker begins to heat up & expand which presses against a switch which flips off & opens the circuit stopping the flow of charge. Ground Fault Control Interrupter • (GFCI)- Sometimes a little current leaks out of an outlet or an appliance. Often it is so small you don’t notice it. However, if you have wet hands, even a small amount of current is dangerous. • GFCI- has tiny circuit inside which monitors incoming & outgoing current. If it detects a change in the current it will flip a switch to open the circuit Circuit Breaker 2.1 Review Q’s 16 points 1. Describe the 4 parts of a circuit & explain what each one does. (8 pts) 2. Explain why short circuits are dangerous. (2 pts) 3. What is the purpose of a ground wire? (2 pts) 4. How do switches control the flow of electric current? (2 pts) 5. What do fuses & circuit breakers have in common? (2 pts)