Transcript Ch 22 PPT
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 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 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 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 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 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 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 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 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. 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 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 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 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? The current though a lightbulb connected across the terminals of a 120V outlet is 0.50 A. At what rate does the bulb convert electric energy to light? 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? 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? 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) 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 A 30.0 V batter is connected to a 10.0 Ω resistor. What is the current in the circuit? 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? P515 Redraw the circuits you made at the beginning of the hour with proper circuit diagrams 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 Substituting new equations learned PI R 2 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? 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? 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? P527: 21, 23, 29, 32, 41, 44