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ELECTRIC CURRENT

ELECTRIC CURRENT. Electric Current is the rate of flow of charge. current = charge I = q/t time Unit of current = ampere = 1 coulomb 1 second. AMMETER.

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ELECTRIC CURRENT

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  1. ELECTRIC CURRENT Electric Current is the rate of flow of charge. current = chargeI = q/t time Unit of current = ampere = 1 coulomb 1 second

  2. AMMETER An ammeter measures the flow of current in the circuit, so the circuit is broken (X) and the ammeter is placed in the path. ________X_____ ______X_______ A

  3. ELECTRIC CIRCUIT An electric circuit is a closed electrical path that contains a source of electrical potential energy where work can be done or energy is used/transformed by a resistance (load.) The electrical potential energy (work) per unit charge is the electric potential. V = (Work)/charge Unit (V = J/C) The unit is (V) volts = Joule/coulomb

  4. VOLTMETER A voltmeter measures the voltage drop across an element (usually a resistor) _____________________________ __________ ____________ Thus the voltmeter makes a parallel path with the element to be measured. ELEMENT MEASURED V

  5. Examples of Electrical Circuits Power Source Load Battery Flashlight 120 Volt AC Source Light bulb 240 Volt AC Source Stove (heating Two part element) a) 120 Volt Source Power Amplifier b) Power Amplifier Speaker

  6. OHM’S LAW All linear circuits obey Ohm’s Law Elements to construct a circuit: Power Source (V) Resistance (R) Wires to connect power source to the resistance. When connected current flows through the circuit.

  7. OHM’S LAW (continued) Current is not used in a circuit, rather energy is used (work is done that heats the resistor) or if observing the (energy/time), power is used by the resistor. The current in the resistor is ratio of the voltage dropped across the resistance divided by the value of the resistance. I (A) = V (V) R ()Where  = Ohm

  8. Types of Circuits Series circuit – One path for all the resistors. (Each resistor has the same current) Parallel circuit – A separate path for each resistor. ( Each resistor has the same voltage)

  9. POWER & ENERGY IN AN ELECTRICAL CIRCUIT Each resistor uses power (energy/time) Work = energy used/transformed Work = Charge * Voltage Power = Work = Current * Voltage time P = I * V Unit => W = A* V

  10. The Power Equation Since there are three variables in a circuit V, I & R the power equation can be expressed in three forms. These are derived from the power definition by using Ohm’s Law. Basic Definition: P = I * V Eliminate I = V/R,P =(V/R)*V = V2/R Eliminate V= IR,P = I * IR = I2 R

  11. OHM’S LAW LAB There is an Ohm’s Law Lab this week It is composed of a single resistor in series with a battery. In order to measure the current (I) one uses an ammeter. To measure the potential (V) one uses a voltmeter. Both Ohm’s Law and the Power Law are used in the laboratory.

  12. ENERGY USED – UNITS Since energy = power * time then we can express energy as: joule = watt* seconds For large amounts of energy then Watts kilowatts seconds  hours 3600 * 1000 Joules = 1 kilowatt-hour 3 x 106 J = 1 kWh

  13. Energy Used and Purchased • Power companies do not actually charge you for power. • They charge you for energy. • So a kilowatt-hour is really energy.

  14. ENERGY USED - COST COST = Rate * NUMBER (rate units) Rate = $ 0.11/ kWh (local rate) Number = # kWh used Cost = $0.11 * Number (kWh)

  15. Electric Bill– Example Suppose one uses a hair dryer for 10 min The power is 1200 W = 1.2 kW Total number of units of energy is: 1.2 kW * 1/6 h = 0.2 kWh Cost ($) = rate * Number (kWh) = $ 0.11/kWh * 0.2 kWh = $ 0.022 or 2.2 cents

  16. DC Direct CurrentAC Alternating Current • A power adapter converts the AC from the outlet to DC to charge a battery or power some device. • There are two types of current flow: • direct current (DC) represents current flow that is always in the same direction. • Batteries provide DC. • alternating current (AC) represents current flow that alters direction periodically. • Wall outlets provide AC.

  17. DC Direct Current • Here is an example of DC provided by a battery to a light bulb. • The current always passes from the positive battery terminal, through the bulb, and then to the negative terminal. • The current is constant in time.

  18. AC Alternating Current • Here is an example of AC provided by a wall outlet to a light bulb. • The current always passes from the positive terminal, through the bulb, and then to the negative terminal. • But the terminals swap position over time.

  19. AC Alternating Current • So the current passing through the bulb changes direction. • Our wall outlets operate at 60 Hz. • So the current changes direction 120 times each second.

  20. AC Current Versus DC Current • Here is one advantage of AC over DC. • A transformer is a device that can increase or decrease AC voltage. • If the transformer increases the voltage, it is called a “step up” transformer. • If the transformer decreases the voltage, it is called a “step down” transformer. • But there is an important consideration.

  21. AC Current Versus DC Current • The power into the transformer must (ideally) equal the power out. • Recall that electrical power is P = IV. • If the voltage is increased (stepped-up), the current must be decreased by the same factor. • If the voltage is decreased (stepped-down), the current must be increased by the same factor. • Basically, energy must be conserved.

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