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Understanding Electric Current and Circuits

Explore the concepts of electric current, circuits, and potential difference. Learn about charge flow, circuit components, and power calculations. Engage in hands-on activities to reinforce learning.

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Understanding Electric Current and Circuits

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  1. Early Work – May 5 • With a partner, get a battery, light bulb, and paper clip. • Find the two ways to light up the light bulb using just these three items. • Draw pictures of what the set up looked like if you notes

  2. Ch 22 Electric Current

  3. 22.1 Current and Circuits

  4. Producing Electric Current • Charge flows from an item with a higher potential difference to something with a lower potential difference until the two have equal charges • The reason there is static discharge • Electric Current: the flow of charged particles

  5. Electric Potential Difference Briefly Revisited • If moving a charge against the electric field, work is done on the charge • The work done changes the charges potential energy to a higher value • The work is equal to the change in the potential energy • There is a difference in electric potential between the two locations • This difference is represented by ΔV and called electric potential difference

  6. Electric Potential Difference Briefly Revisited • By definition, electric potential difference is the difference in electric potential (V) between the final and initial location of the charge • And we drop the Δ as a historical convention (not a correct one) and just use V • Standard unit for electric potential difference is the volt (V), named in honor of Alessandra Volta

  7. Electric Potential Difference Briefly Revisited • An electric potential difference between two locations of 12 volts means that one coulomb of charge will gain 12 joules of potential energy when moved between those two locations • Because it is expressed in volts it is sometimes referred to as voltage

  8. Electric Current • Two models • Conventional current: assumes positive charges flow out of a positive terminal and travel to the negative terminal. • Electron Flow: what actually happens, is that electrons flow out of negative terminals and travel to the positive terminal

  9. Electric Current • Charge will always flow from one more charged body to another, but what makes it continue to flow? • You need a generator of some sort • Most common generator is several galvanic cells (or dry cells) connected together – they form a battery • Chemical energy generator • Other include hydro, steam, and wind generators • Photovoltaic cell: solar cell

  10. Electric Circuit • Electric Circuit: a closed loop in which current flows • Includes a charge pump (battery) which increases potential energy and a device to reduce potential energy (light bulb) • Think of a charge pump as the work done by a roller coaster to get the cars to the top of the hill, they go down the other side naturally • Once a positive charge gets from inside the battery to the positive terminal, it flows naturally to the negative terminal

  11. Charge is conserved • In a 9V battery, there is 9 volts of potential difference between the two terminals. That means that the battery must do 9 joules of work moving a positive charge from the negative wire to the positive wire. The positive charge then gains 9 joules of potential energy in which it can deliver to the light bulb and then come back with no energy and do it all again.

  12. Electric Energy • When a charge moves through a circuit, the amount of potential energy it loses is qV (charge times potential difference) • So the generator/charge pump/battery needs to increase the charges potential and the energy required to do that is qV • The change in electric energy, E, is equal to qV • E = qV

  13. Rates of Charge Flow • Power is the measure of the rate at which energy is transferred (P = E/t) • Transferring 1 joule per second is 1 watt • The rate of flow of electric charge, or electric current, I, is measured in coulombs per second • 1 coulomb per 1 second is 1 ampere, A • Ammeters measure amperes

  14. Energy Transfer • Suppose current is flowing at 3C/s (3A) and the potential difference is 120V, which means that each charge supplies the motor/light bulb/etc with 120J • To find the power delivered we multiply the current and the potential difference • P = IV • The power delivered in this situation would be 360 W

  15. Example • A 6.0 V battery delivers a 0.50 A current to an electric motor that is connected across its terminals. • What power is consumed by the motor? • If the motor runs for 5 minutes, how much electric energy is delivered?

  16. Example • The current though a lightbulb connected across the terminals of a 120-V outlet is 0.50 A. At what rate does the bulb convert electric energy to light?

  17. Example • A car battery causes a current of 2.0 A through a lamp while 12 V is across it. What is the power used by the lamp?

  18. Example • The current through the starter motor of a car is 210 A. If the battery keeps 12 V across the motor, what electric energy is delivered to the starter in 10.0 s?

  19. Resistance • Resistance, R, is the ratio of the potential difference, V, to the current, I • R = V/I • Measured in volts per ampere, which is ohms • Named after Georg Simon Ohm • 1 Ω (ohm) is equal to 1 A of flow when 1 volt is applied • Ohm discovered that a devices resistance stays the same no matter the potential difference that is applied (Ohm’s Law)

  20. Resistors • Most wires used in circuits have a very small resistance (they don’t reduce the potential difference much) • Factors of resistance – think garden hose • Small diameter v. large diameter • Short v. long • Resistors are devices use to have a specific resistance

  21. Example • A 30.0 V batter is connected to a 10.0 Ω resistor. What is the current in the circuit?

  22. Example • A lamp draws a current of 0.50 A when it is connected to a 120 V source. • What is the resistance of the lamp? • What is the power consumption of the lamp?

  23. Symbols for drawing circuits • P515 • Redraw the circuits you made at the beginning of the hour with proper circuit diagrams

  24. Types of Connections • Parallel • When two devices are connected so they are parallel to each other • Voltmeters need to be connected this way • Series • When two devices are connected so that the current that flows through one also flows through the other • Ammeters need to be connected this way

  25. 22.2 Using Electric Energy

  26. Power • Substituting new equations learned

  27. Example • A heater has a resistance of 10.0 Ω. It operates on 120.0 V. • What is the current through the resistance? • What thermal energy is supplied by the heater in 10.0 s?

  28. Example • A 30.0 Ω resistor is connected across a 60 V battery. • What is the current in the circuit? • How much energy is used by the resistor in 5 min?

  29. Kilowatt-Hour • 1 kWh is equal to 1000 watts delivered continuously for 3600 seconds (1 hour) • A television set draws 2.0 A when operated on 120 V. • How much power does the set use? • If the set is operated for an average of 7.0 h/day, what energy in kWh does it consume per month (30 days)? • At $0.11 per kWh, what is the cost of operating the set per month?

  30. Bkwk • P527: 21, 23, 29, 32, 41, 44

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