- Discuss the measurement of power in circuits - Learn how to construct parallel circuits and how they behave differently from series circuits TODAY’S OUTCOMES: ELECTRICITY

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 more power supplied when the switch is closed. 3 light bulbs in series is dimmer than 2 because the voltage must be shared among all devices in the circuit.

CONCEPTUAL EXAMPLE 3 volts What happens to brightness? Fewer bulbs in a series circuit uses more power; battery drains faster 3 volts What if we have zero bulbs? short circuit; VERY HIGH power What happens to current? It increases What happens now? 3 volts It increases (brighter bulb = more current) Which number of light bulbs drains the battery fastest? Brightness, current, power go up again

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? 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? power = voltage ✕ current ⇒ current = power / voltage current = 2,000,000,000 W / 120 V = A ≈ 16.7 million A current = 2,000,000,000 W / V = 20,000 A Which of these 2 currents is easier to transport along a wire?

TRANSMISSION OF ELECTRIC POWER homes, business power generation 16.7 million A in a power line would not work

TRANSMISSION OF ELECTRIC POWER homes, business power generation substation (transformers) 120 V distribution lines transformer lowers the voltage high voltage transmission lines transformer raises the voltage V

What voltages do most household appliances require? What typical currents do most household appliances operate at? 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? Most require ~120 V (some exceptions, like clothes dryers, have special outlets for higher voltages). Current varies but not TOO high - more than A total will trip most circuit breakers. Such devices have special plugs that transform the voltage yet again to a lower value.

- 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 WHAT YOU ARE EXPECTED TO KNOW:

TODAY’S OUTCOMES: - Discuss the measurement of power in circuits ✓ - Learn how to construct parallel circuits and how they behave differently from series circuits ELECTRICITY