 # Electricity Current Electricity Current Electricity Producing Electric Current

• Slides: 28 Electricity Current Electricity Current Electricity Producing Electric Current Conventional current – where charges flow from positive to negative Flow stops when potential difference zero Any closed loop or conducting path of electric flow is electric circuit Current Electricity Producing Electric Current Voltaic or galvanic cell (common dry cell), converts chemical energy to electric energy Battery - several galvanic cells connected together Photovoltaic or solar cell—changes light energy into electric energy Current Electricity Conservation of Charges cannot be created or destroyed, but can be separated. Energy also conserved Energy carried by electric current depends on charge transferred, q, and potential difference across which it moves, V. Thus, E = q. V Power measures rate at which energy transferred in watts, W If generator transfers 1 J kinetic energy to electric energy each second, rate is 1 J/s, or 1 W Current Electricity Rate of Charge Flow Rate of flow of electric charge, q/t, (electric current) measured in coulombs per second I = q/t. So 1 C/s = 1 ampere, A Current Electricity Rates of Charge Flow and Energy Transfer Energy carried by electric current related to voltage, E = q. V Power of electric device found by multiplying voltage (E) and current (I) P=IE Energy equal to product of power and time E = Pt Current Electricity Resistance and Ohm’s Law Resistance determines how much current will flow Current Electricity Resistance and Ohm’s Law Resistance, R = ratio of electric potential difference, V, to current, I Resistance Measured in ohms. One ohm (1 Ω) is resistance permitting electric charge of 1 A to flow when potential difference of 1 V applied Current Electricity Resistance and Ohm’s Law Named for German scientist Georg Simon Ohm, who found ratio of potential difference to current constant for given conductor Resistance for most conductors does not vary Device having constant resistance obeys Ohm’s law Most metallic conductors obey Ohm’s law, over limited range of voltages Wires used to connect electric devices have low resistance Current Electricity Resistance and Ohm’s Law Wires have little resistance Resistor - device having specific resistance. Made of graphite, semiconductors, or thin, long wires Two ways to control current in a circuit: Voltage or resistance I =V/R When V = 6 V and R = 30 Ω, I = 0. 2 A Current Electricity Resistance and Ohm’s Law In first figure, voltage applied across resistor reduced from 6 V to 3 V to reduce current to 0. 1 A. Can also replace 30 -Ω resistor with 60 -Ω resistor Current Electricity Resistance and Ohm’s Law Smooth, continuous variation of current is sometimes desired To achieve control, variable resistor, potentiometer, used Current Electricity Resistance and Ohm’s Law Variable resistors can consist of coil of resistance wire and sliding contact point Moving contact point varies amount of wire in circuit More wire = increased resistance = decreased current Adjusts speed of motor from fast to slow Other examples: volume, brightness, contrast, tone, and hue controls on televisions Current Electricity Diagramming Circuits Standard symbols for circuit elements, used in drawings called circuit schematics Current Electricity Diagramming Circuits When connected across component, called parallel connection because circuit component and voltmeter aligned parallel to each other Current Electricity Diagramming Circuits Ammeter measures current through circuit component Same current going through component must go through ammeter, one current path A connection with only one current path is called a series connection Current Electricity Diagramming Circuits To add an ammeter to a circuit, the wire connected to the circuit component must be removed and connected to the ammeter instead Then, another wire is connected from the second terminal of the ammeter to the circuit component In a series connection, there can be only a single path through the connection Always associate the words current through with a series connection Using Electrical Energy Transfer in Electric Circuits Motor converts electric energy to mechanical energy Lamp changes electric energy into light Some energy converted into thermal energy Some devices designed to convert electrical into thermal energy by acting as resistors in circuit Using Electrical Energy Heating a Resistor Power dissipated in resistor is P = IV Voltage in resistor, V = IR, so Power P = I 2 R Can substitute I = V/R into P = IV to obtain the following equation Power Using Electrical Energy Heating a Resistor Because P = I 2 R and P = V 2/R, total energy converted to thermal energy can be written: Thermal Energy E = Pt E = I 2 Rt E= Using Electrical Energy Superconductors Superconductor is material with zero resistance No restriction of current in superconductors, so no potential difference, V, across them Power dissipated in conductor = IV, superconductor can conduct electricity without loss of energy Almost all superconductors must be kept at temperatures below 100 K Practical uses include MRI magnets and in synchrotrons Using Electrical Energy Transmission of Electric Energy Hydroelectric facilities capable of producing a great deal of energy Energy often transmitted over long distances to reach homes and industries How can the transmission occur with as little loss to thermal energy as possible? Using Electrical Energy Transmission of Electric Energy Thermal energy produced represented by P = I 2 R Unwanted thermal energy called joule heating loss, or I 2 R loss To reduce, either I, or R must be reduced Cables of high conductivity and large diameter available, but such cables expensive and heavy Because loss of energy also proportional to square of the current in the conductors, more important to keep current in transmission lines low Using Electrical Energy Transmission of Electric Energy Cables of high conductivity and large diameter (and therefore low resistance) are available, but such cables are expensive and heavy Because the loss of energy is also proportional to the square of the current in the conductors, it is even more important to keep the current in the transmission lines low Using Electrical Energy Transmission of Electric Energy How can the current in the transmission lines be kept low? The current is reduced without the power being reduced by an increase in the voltage Some long-distance lines use voltages of more than 500, 000 V The resulting lower current reduces the I 2 R loss in the lines by keeping the I 2 factor low Using Electrical Energy Transmission of Electric Energy Electric companies often called power companies, but provide energy rather than power Amount of electric energy used by device is rate of energy consumption, (W), times seconds operated Electric companies measure energy sales in kilowatt-hours, k. Wh Kilowatt-hour equal to 1000 watts delivered continuously for 3600 s (1 h), or 3. 6× 106 J Using Electrical Energy Transmission of Electric Energy A 60 W lightbulb is connected to a 115 V power source. What is the current through the lightbulb? What is the resistance of the lightbulb? A microwave draws a 15. 0 A current from a 120 V power source. How much power is delivered to the microwave? How much energy does the microwave use to heat food for 1 min? A current of 0. 85 A is measured through a 27 Ω resistor for 1. 00 hr. How much heat does the resistor generate? A 60 W bulb powered by a 120. 0 V source has a resistance of 5. 0 Ω at room temperature and 100. 0 Ω at operating temperature. What is the current and power of the bulb at room temperature? What is the current and power of the bulb at operating temperature? Using Electrical Energy Transmission of Electric Energy A 100 W lightbulb is turned on for two-fifths of the time for 30 days. At a price of \$0. 090 per k. Wh, how much does the lightbulb cost to run during the 30 days? (1 k. Wh = 3. 6 x 106 J) A 240 V water heater has a resistance of 15 Ω. What is the power of the heater? What thermal energy is supplied by the heater in 15 minutes? A long distance high-tension wire uses 500, 000 V. What is the power output of these wires in W, k. W, MW if the current is 20 A? What is the power dissipated in the wires if the resistance is 0. 015 Ω?