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Chapter 8

Chapter 8. Ohm’s law describes the relationship of current, voltage, and resistance. Terminology. Battery: is a combination of electrochemical cells connected together

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Chapter 8

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  1. Chapter 8 Ohm’s law describes the relationship of current, voltage, and resistance

  2. Terminology • Battery: is a combination of electrochemical cells connected together • Electrochemical cells (or just cells): converts chemical energy into electrical energy stored in charges. There is a separation of charges. • Energy: the ability to do work • Kinetic Energy: energy that a moving object has because of its motion • Potential energy: the energy stored in an object • Electric potential energy: the electrical energy stored in a battery is called this because the electrons have stored energy and the ability to do work after they leave the battery

  3. Electric Potential Difference (Voltage): The amount of electric potential energy per one coulomb of charge • The unit for measuring potential difference is the volt(V) • Voltmeter: is a device that measures the amount of potential difference (voltage) between two locations of charge separation.

  4. Comparing Potential Energy and Potential Difference • When you climb a set of stairs your body has done work. The work done is called potential energy • The more mass moved up the stairs the more potential energy. • Think of potential difference in a battery as being like the height of the stairs. The amount of charge separated in a battery is like the mass moved up the stairs.

  5. Measuring potential difference • Activity 8-1B , page 255

  6. Electric Circuit • Electric Circuit: a complete pathway that allows electrons to flow. • Electrical Charge: described as building up at the negative terminal of the cell/battery which then flows from this terminal as negative charges repel one another. • Battery has two terminals (ends) called electrodes. Usually two different metals • Electrolyte: the electrodes are placed in this which can conduct electricity.

  7. A reaction occurs between the electrodes and the electrolytes, leaving electrons on one electrode and taking electrons from another. One electrode is negative and the other is left positive. The opposite charge on each electrode means that there is a potential difference (voltage) between the two electrodes.

  8. Parts of an electric circuit • Electrical load: any device that changes electrical energy into other forms of energy. Ex: light bulb • Control/Switch: a device that can turn the circuit on or off by closing or opening the circuit • Source: the source of electrical energy • Conductor: the wire through which electric current flows. • The energy is provided at the source and then “used” by the different parts of the circuit (a potential or voltage drop occurs)

  9. Energy around a circuit

  10. Current Electricity: the continuous flow of charge in a complete circuit. • Electrons are pushed or repelled from the negative end of the battery along a circuit and end up on the positive terminal of the battery • Electric Current: the amount of charge passing a point in a conductor every second • Ampere (A): the SI unit used to measure current. The rate of flow of electric charge. • Ammeter: deviceused to measure current

  11. Voltage vs. Current • Analogy: Two identical busses, identical routes leave a school (the battery). One has more students than the other. The speed of the busses can be the same (current) but when they reach the park the bus with more students will have more energy (voltage)

  12. Static vs. Current Electricity • Cannot use static electricity to run an electrical device • Static electricity is a charge that remains stationary • Current electricity is the continuous flow of charge

  13. Circuit Symbols ammeter resistor

  14. Create circuit diagrams • Website: www.andythelwell.com/blobz/guide.html • Activity 8-3C: Circuit Diagrams with Resistors

  15. Resistance: the property of any material that slows down the flow of electrons and converts electrical energy into other forms of energy. • Ex: In high resistance wire in light bulbs, the energy is converted to heat and light • Electrical Resistance: the ratio of the voltage to the current. • Ohm(Ω): the SI unit for electrical resistance • Analogy: running on a sandy beach versus running in knee high water. Easier to run on beach just as it is easier for electricity to flow through some materials than others

  16. Task • Activity 8-3A page 271 (factors affecting resistance)

  17. Factors affecting Resistance in a wire • Factors that affect Resistance • 1. Type of Wire‑ Copper offers less resistance than nichrome • 2. Thickness of the Wire‑ The thinner the wire, the greater the resistance • 3. Length of the Wire‑ The longer the wire, the greater the resistance (coiled tungsten wire) • 4. Temperature‑ The greater the temperature, the greater the resistance

  18. Relationship between voltage, current and resistance • Ohm’s Law: I = V R • V = voltage, I = current, R = resistance • The more resistance in a circuit the less current flowing • The more voltage in a circuit, the more current • For example: more batteries or volts the greater the current

  19. Ohm’s Law calculations • Page 273, practice problems. Use the triangle method. V I R

  20. Potential difference and Current • Interactive websites to demonstrate how one affects the other • www.article19.com/shockwave/oz.htm • Phet.colorado.edu/simulations/sims.php?sim=Circuit_Construction_Kit_DC_Only

  21. Core Lab • Resistors and Ohm’s Law

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