III. What is electricity? (cont.)
A. Electricity is a form of energy that happens when electrons (e-) are lost by some atoms and acquired by others
B. When e- move from a "normal" (or neutrally charged atom), the atom becomes positively charged 1. A positively charged atom attracts a negatively charged atom C. There are two main forms of electricity 1. Static electricity: the imbalance of positive & negative charges a. When you rub two objects together that are good insulators (such as a balloon with hair or wool) the wool gives its electrons to the balloon, causing the balloon to become negatively charged b. Negatively charged objects attract to the positive charges in an object, even if the object is neutrally charged (see the interactive below) i. Balloons rubbed on hair or wool sticks to a wall ii. Laundry sticking together after being in the dryer c. Opposite charges attract, like charges repel d. Excess electrons discharge (or burst out) in an uncontrolled way i. Lightning ii. "Zapping" your friend after going down a plastic slide 2. Current electricity a. Electricity flowing in streams of electrons, usually along wires b. This is the electricity we use when we plug something into the wall electric socket |
Click here to watch an interactive about how we use electricity.
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IV. Current Electricity & Circuits
A. The simplest safe circuits have 3 basic parts
1. Power source a. Battery (direct current DC) b. Generator (alternating current AC) 2. Conductor a. Materials that readily share their electrons are good conductors of electricity i. Metal wires (especially copper) ii. Impure water (has minerals in it) iii. Humans (made up of mostly impure water) 3. Load (or resistor) a. light bulb b. computer c. anything you plug into a wall electrical socket |
4. For the circuit to work, electrons must be able to flow through the circuit uninterrupted (called a closed circuit).
a. The flow of electrons is called current (see below) b. The (-) terminal of the battery, when connected to a wire, produces e- that flow towards the (+) terminal c. For a circuit to work, all parts must be connected for the e- to flow i. Open circuit - gap or break in e- flow ii. Closed circuit = no break in e- flow NOTE: If a power source is connected by a wire without a load, you are creating what is called a short circuit. A short circuit allows the electrons to flow so quickly that the battery will overheat and even explode. By adding a load, the electrons flow through a load (or resistor), which slows the current (flow of e-) and reduces the chances of the battery overheating. So, the circuit becomes a safer circuit when a load is added. |
AC vs. DC - What's the Difference?
PBS Learning has a nice webpage that illustrates the difference between direct current (DC) and alternating current (AC). The only thing is that the image may be a bit confusing unless you look carefully where the switch is placed. When you go to the webpage, note that the image illustrating alternating current shows the electrons moving back and forth. Also note that the switch is placed so that the AC circuit is closed and the DC circuit is open. The direct current image on the right shows that the switch is positioned so that the DC circuit is closed and the AC circuit is open.
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Comparing electron flow to a river
A nice analogy for flowing electrons comes from the website, Physics4Kids "Electric current is very similar to a flowing river. The river flows from one spot to another and the speed it moves is the speed of the current. The size of the current flow is related more to the size of the river than it is to the speed of the river. A river carries more water each second than a stream, even if both flow at the same speed. With electricity, current is a measure of the amount of charge transferred over a period of time. Current is a flow of electrons, or individual negative charges. When charge flows, it carries energy that can be used to do work [like light a bulb]."
If you've ever been to a lazy river water park, you know that water flows around a "circuit". Because their is no slope, the water would not move due to gravitational force like a regular river does. So water park designers build in a pump that forces the water to "circulate" around the lazy river. The water is like the electrons and the river is like the wire. The pump is like the battery. Sometimes the lazy river splits and takes two paths. This is much like a parallel circuit (see below.) If one of the paths was blocked (or closed) the water would simply flow down the other path. |
B. Two basic types of circuits: Series and Parallel
1. Series Circuits a. A series circuit has a single circuit or pathway b. A series circuit may have one or more power sources, and one or more loads, but only ONE pathway i. ex. Christmas lights c. Each bulb is the same brightness i. Adding more light bulbs (load) increase the resistance (less current) & dims all the bulbs equally d. With each power source added in series, the amount of power (or voltage) increases i. Ex. Two 1.5 Volt (V) batteries will provide 3 Volts of power ii. Adding voltage increases the current, which increases the bulbs' brightness d. For this circuit to work, all parts must be connected at the same time for the electrons to flow. i. If there is a broken filament in one light bulb, the other light bulb will not light. 2. Parallel Circuits
a. Parallel circuits have more than one pathway. b. Current leaving the power source may follow 2 or more pathways before returning to the source c. Bulbs in parallel bur more brightly than the same number of bulbs in series i. The more pathways, the less the resistance, the greater the current (This may not make sense at first, but read about the Tollbooth Analogy to understand why) d. Parallel circuits provide the same voltage for each load, no matter how many power sources are used (see Visualizing how batteries are connected in series & parallel below) i. Two 1.5 Volt batteries provide only 1.5 Volts of power e. Only power sources with the same voltage can be connected in parallel i. Two 1.5 V batteries may be connected, but one 1.5 V & one 9 V battery canNOT be connected f. If there is a broken filament in one light bulb, the other bulb will remain lit f. Parallel circuits are useful in homes or other places where one load needs to be turned on while another load is turned off PLEASE NOTE: Power sources (batteries) can be arranged in parallel and series. So can loads (i.e. light bulbs). In fact, you can arrange your batteries in series and your loads in parallel on the same circuit. You can also arrange your batteries in parallel while your loads are in series in the same circuit.
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Comparing Series & Parallel Circuits
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Imagine that the delivery trucks pick up electrons at the warehouse (the battery) and then drive to the "store" (the light bulb) to make their delivery. In a series battery, the same truck picks up electrons in both warehouses before making their delivery, so twice as many electrons are delivered to the store at one time. In a parallel circuit, different trucks go to different warehouses that load the same number of electrons. The trucks must wait their turn to enter the highway and must also wait their turn to deliver to the store.
Because current is the number of electrons flowing, you can see that twice as many electrons are flowing down the highway in a series circuit than in the parallel circuit. This gives you twice the "pumping" power and twice the current, which then makes the bulbs burn more brightly.
Because current is the number of electrons flowing, you can see that twice as many electrons are flowing down the highway in a series circuit than in the parallel circuit. This gives you twice the "pumping" power and twice the current, which then makes the bulbs burn more brightly.
Visualizing how LOADS are connected in series and in parallel:
The Tollbooth Analogy
From the Physics Classroom comes a great analogy for understanding how adding light bulbs in parallel actually decreases the resistance compared to adding the same number of light bulbs in parallel. At first it seems counterintuitive - especially based on how we noticed that adding lights in series dimmed the bulbs. Why does adding them in parallel not dim the lights similarly?
Imagine a 3-lane highway (conducting wire) crowded with trucks and cars (electrons). Adding 1 tollbooth (light bulb) to the road increases the resistance and slows down the traffic (current). Now imagine if 3 tollbooths were set up so that the cars had to pass from one tollbooth to the next (in series). This really slows down the traffic! What if those same 3 tollbooths were set side by side so that the cars could pass through any one tollbooth? The traffic (current) would flow through the tollbooths (light bulbs) more quickly if they were set up in parallel than if they went through the tollbooths one after the other as they do in series. |
In summary...
-Adding 1 tollbooth (light bulbs) increases the resistance and slows the current, thus dimming the bulbs -Adding extra tollbooths in series slows the flow of traffic (current) even more, dimming the bulbs even more. -Adding extra tollbooths in parallel increases the flow of current and decreases the overall resistance, so the bulbs remain bright. Therefore, because the current flows better through the lights in parallel than in series, the light bulbs in parallel will glow more brightly. |