Transcript Power Point
Current Electricity Producing Current • Current: Flow of charged particles • Cell: Source of conversion of chemicals into electric energy. Types of Cells: Voltaic - Common everyday ‘battery’. Photovoltaic – Solar Panel. • Battery: A collection of cells. Circuits • Circuit: A closed loop of current. • A Circuit Must Include: – Voltage Source (i.e. Battery) – Conductive Pathway (i.e. wire) – A Load (i.e. motor, bulb, resistor) Electrical Terms • Coulomb: The unit of measure of electrical charge. Abbreviated: C Seen in equations as q 1 Coulomb is the charge of 6.25 E 18 electrons. 1 electron has 1.6 E -19 Coulombs of charge. • Ampere: Rate of charge transfer. Transferring 1 Coulomb of charge in 1 sec. Abbreviated: A or Amp Seen in equations as I • Voltage: The potential difference between two charged bodies. Also known as the ‘Electromotive Force’, EMF. Abbreviated: V Seen in equations as V Water Similie • Voltage is like pressure. (Voltage is often referred to as ‘Electrical Pressure’) • Current is like the amount of water. Low Voltage and Low Current. Low Voltage and High Current. High Voltage and Low Current. High Voltage and High Current. Rate of Charge Flow • Power (Watts): Product of applied voltage and current. Abbreviated: W Seen in equations as P P IV Example 1 A 6.0 V battery delivers a 0.50 A current to an electric motor that is connected across its terminals. A) What is the power consumed by the motor? B) If the motor runs for 5.0 minutes, how much electric energy is delivered? Extra Problems 5. How many electrons does it take to produce 50 Coulombs of charge? 6. If you have one mole of electrons, how much charge do they possess, in Coulombs? 7 If 5 E 20 electrons pass from point A to point B in 30 seconds, how many amps is that equivalent to? • Resistance (Ohms Ω): The opposition to the flow of current. All materials have a natural resistance. Glass / Plastic have high resistances. Known as Insulators Copper / Gold have low resistances. Known as Conductors Some materials have close to zero resistance, Superconductors Potentiometer: A variable resistor. (The volume knob on your stereo) Resistors • Resistors are devices that limit current. • Click here for a walk through of the color code for resistors. How to read Resistor Color Codes Black Brown Red Orange Yellow Green Blue Violet Gray White 0 1 2 3 5 6 7 8 9 4 Click here for more online practice Example 2 V IR • Ohm’s Law: Voltage equals current times resistance A 30.0 V battery is connected to a 10.0 Ω resistor. What is the current in the circuit and the power consumption of the resistor? Schematic Drawings 1. Draw the battery. Label the pos. and neg. side. 2. Draw the wire coming from the pos. When you come to an electronic device, draw the appropriate symbol. 3. If you reach a point where there is a split in the path, draw If two wires cross without joining, draw Follow one path until the two current paths join. Then draw the second path. 4. Follow the current path until it reaches the negative side. 5. Draw an arrow to indicate current flow. (From + to -) Schematic Symbols Voltmeters and Ammeters • Ammeters measure current (Amps) The current has to go through them. Known as a Series Connection. • Voltmeters measure voltage (Volts) They have to be placed across the device to measure Known as a Parallel Connection. Example 3 • Draw a schematic diagram showin a 4.5 V battery, a resistor, and an ammeter reading 90 mA. Label the size of the resistor Heating a Resistor • To find Energy (J) consumed and given off by any appliance, simply find the power (Watts a.k.a. J/s) and multiply it by the time it was on (Sec.) E I Rt Pt 2 Energy Current Resistance Time Power Joules Amps Ohms Sec Watts Example 4 A heater has a resistance of 10.0 Ω. It operates on 120.0 V. A) What is the current flowing through the heater. B) How much thermal energy is supplied by the heater in 10.0 s? The Kilowatt - Hour • Energy is sold to us by the ‘power’ company by the Kilowatt – Hour. When we multiply the power consumption rate (W or J/s) by the time used (s) we calculate the energy consumed. Hence the Watt – Second (Ws). This is too small for commercial use so the unit of Kilowatt – Hour (kWh) took over. 1 kWh = Example 5 A television draws 2.0 A when operated on 120 V. A) B) C) How much power does the set use? If the set is operated for an average of 7.0 hr / day, what energy in kWh does it consume per month (30 days) At 11 cents per kWh, what is the cost of operating the set per month?