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Potential Difference (V)

Potential Difference (V). Charge flows from higher potential to lower potential until electric potential equilibrium is reached. When electric potential is zero (equilibrium), current flow stops. Electric Current. Electrical charge in motion is electric current .

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Potential Difference (V)

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  1. Potential Difference (V) • Charge flows from higher potential to lower potential until electric potential equilibrium is reached. • When electric potential is zero (equilibrium), current flow stops.

  2. Electric Current • Electrical charge in motion is electric current. • Current is measured by counting the amount of charges that pass a given point per second Symbol variable Unit I current A (amp)

  3. Durable Comparing a DC circuit to Flow of Water http://faraday.physics.utoronto.ca/IYearLab/Intros/DCI/Flash/WaterAnalogy.html • What component in the electrical circuit is equivalent to the pump in the animation?

  4. Direct Current (DC) • DC stands for direct current. Here current only travels in one direction. DC

  5. Alternating Current (AC) • AC stands for alternating current. Here current switches directions at a set frequency. AC

  6. Alternating Current (AC) • AC stands for alternating current. Here current switches directions at a set frequency. AC

  7. Voltage • Current flows only when there is a potential difference (V) • Voltage sources can sustain a potential difference • Voltage in a circuit is analogous to pressure in a water hose. Voltage Source OFF ON

  8. Zap! Voltage Sources • There are different types of devices that can provide a source of voltage. • Sources that provide DC include: • Batteries • DC Power Supplies • DC Generators • Sources that provide AC include: • Electrical Outlets • Alternators • Power Inverters

  9. Zap! A V Electric Circuits • Electric current flows very well if there is a complete loop for charge to flow. This is called an electrical circuit. • Circuits often contain various elements, giving it practical use. Examples of circuit elements include: • Switches • Sources of Resistance (Such as a Light) • Meters

  10. A V Circuit Symbols • Each circuit element has its own symbol. • Common circuit symbols are shown below. Wire Battery A Conductor of Current Source of DC Charge Flow Switch Ammeter Opens and Closes Circuits Measures Current Resistor Voltmeter Provides Resistance to Current Flow Measures Voltage

  11. More Circuit Symbols Capacitor • Here are some additional circuit symbols that you may see. Diode Stores Charge on Plates Only Allows Current to Flow One Way Potentiometer Junction Variable Resistor All Four Wires Connect AC Source Provides AC Current Crossing Ground Wires Only Cross and do not Connect. Drains Excess Charge Buildup

  12. Electrical Resistance • Every circuit contains some resistance to current flow. • This is due to imperfections in the crystalline lattice structure of the conductor. • Imagine the lattice structure below is the atoms in a wire. • Notice how the moving electrons experience resistance. • The variable for resistance is R. • The unit for resistance is the Ohm (symbol W).

  13. Electrical Resistance R resistance (Ohm) W  resistively (Wm) L Length (m) A area m2

  14. Voltage (V) Resistance (W) Current (A) Ohm’s Law • The current in a circuit is directly proportional to voltage and inversely proportional to resistance. • This relationship is known as Ohm’s Law.

  15. Toasted Sample Problem (Ohm’s Law) • A toaster is connected to a 120V outlet and draws 3A of electrical current. • What is the resistance of the toaster?

  16. Power and Electricity • There are also two useful equations that relate power to electrical quantities. • Notice how current is squared in the second equation. Increasing current in a circuit drastically increases the power consumed. • High current wires generate heat. • This is why electricity transferred over large distances is at high voltage and not high current. Otherwise, the power losses would be very wasteful.

  17. Given Toasted Sample Problem (Electrical Power) • Remember the toaster from a previous problem? • What is the power consumed by the toaster? • If it takes 45s for the toast to be made, how much energy was consumed?

  18. Resistance of power lines is .2 ohms per Km How much power is lost during transmission if a home 3.5Km from the power plant cooks on a stove that draws 41A?

  19. How can energy loss be reduced? Either reduce I or R R is a fix physical property

  20. High voltage lines transmit power between 500,000V-750,000V to reduce energy loss.

  21. Time (h) Power (kW) Energy (kWh) Cost ($) Rate The Kilowatt Hour • Did you ever read an electric bill? • You are charged for the number of kilowatt hours used during the month. • This is energy, not power, because: • The amount of kilowatt-hours gets multiplied times a rate to find the overall energy cost. 1kW =1000W

  22. How Much Energy is a kWh? • We all know that the SI unit for energy is the Joule (J). • How many Joules are there in one kilowatt hour? or

  23. Sample Problem (KWh) • During the winter, an electric heater runs 8 hours every day over the course of a month (30 days). • The power consumed by the heater is 1200W. • How many kWh of energy are consumed? • If the rate is $0.11 per kWh, then what is the cost to operate the heater?

  24. Zap! A V

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