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TODAY’S OUTCOMES: ELECTRICITY - Discuss the measurement of power in circuits - Learn how to construct parallel circuits and how they behave differently from series circuits GRADING OF VOLTAGE, CURRENT AND POWER 5 points: 2 points: 1 - Complete expression of ideas 1 - Correct circuit chosen 3 points: 1 - Method used to get answer shown 2 - Correct currents given 13. In the circuit of Q12, there are more light bulbs lit when the switch is open, but the light bulbs are brighter when it is closed. For which case is the power supplied by the battery greater? (Write out an explanation of your answer). In both cases, the voltage supplied is the same, (it is the 4 V supplied by the battery). When the light bulbs are brighter, there is more current, so the current is greater when the switch is closed. Because power = voltage ✕ current, there is 3 light bulbs in series is dimmer more power supplied when the switch is closed. than 2 because the voltage must be shared among all devices in the circuit. CONCEPTUAL EXAMPLE 3 volts 3 volts What happens to brightness? It increases What happens to current? It increases (brighter bulb = more current) Which number of 3 volts 3 volts light bulbs drains the battery fastest? Fewer bulbs in a series circuit uses more power; What if we have battery drains What happens now? zero bulbs? faster Brightness, current, short circuit; power go up again VERY HIGH power REVISIT LED + BULB EXAMPLE + LED – + – light bulb In this case, the LED was brighter than the bulb, but both have the same current.... The LED requires less current (and less power) than the bulb to glow! CURRENT AT ALL POINTS IN A GIVEN SERIES CIRCUIT IS THE SAME. CURRENT AND POWER DECREASE IN A SERIES CIRCUIT WHEN MORE DEVICES ARE ADDED, IF THE VOLTAGE IS KEPT THE SAME. 14. When all the generators are running at Hoover Dam, it produces 2,000,000,000 Watts. This power is then sent elsewhere by power lines. The diagram at right is a simplified version of the situation, where the battery represents the generators and the light bulb represents the city that uses the power. If the “battery” is driving the circuit at 120 V (household voltage), what is the current in the power line? power = voltage ✕ current current = power / voltage current = 2,000,000,000 W / 120 V = 16666666.6666.... A ≈ 16.7 million A They use transformers to change the voltage, so that the Hoover Dam power station acts like a 100,000 Volt battery. Now what is the current in the power lines? current = 2,000,000,000 W / 100000 V = 20,000 A Which of these 2 currents is easier to transport along a wire? TRANSMISSION OF ELECTRIC POWER power generation 16.7 million A in a power line would not work homes, business TRANSMISSION OF ELECTRIC POWER transformer raises the voltage power generation high voltage transmission lines substation (transformers) 100000 V 120 V homes, business distribution lines transformer lowers the voltage What voltages do most household appliances require? Most require ~120 V (some exceptions, like clothes dryers, have special outlets for higher voltages). What typical currents do most household appliances operate at? Current varies but not TOO high - more than A total will trip most circuit breakers. Some devices, like cell phone chargers and electronics (especially computers) can be damaged by more than about 10 Volts. How can these be plugged into 120 V outlets? Such devices have special plugs that transform the voltage yet again to a lower value. WHAT YOU ARE EXPECTED TO KNOW: - Power = current × voltage; how to solve for any of these 3 quantities - What happens to the current and power in a series circuit when elements are added and removed - The purpose of transformers between power generation and household electrical usage TODAY’S OUTCOMES: ELECTRICITY - Discuss the measurement of power in circuits✓ - Learn how to construct parallel circuits and how they behave differently from series circuits