 | Join Karen as she shares her enthusiasm for teaching STEM subjects, gives you what you need to know to get started on electronics projects, and more. | The Learning Circuit |
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Karen illustrates Ohms law by making a simple circuit on a breadboard. She uses two sources of power, a 2 AA battery pack that supplies 3 volts and a 4 AA battery pack that supplies 6 volts to power a red LED. By applying the concept of series and parallel circuits, she'll show you how to combine a pair of resistors, using Ohms law to calculate the resistance needed to supply the proper voltage to a diode. | ||||||||||||||||||||||
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Karen illustrates Ohm’s Law by making a simple circuit on a breadboard. To do this she uses two sources of power: a 2- AA battery pack that supplies 3 Volts and a 4 - AA battery pack that supplies 6 Volts to create a circuit that powers a red LED. Karen supplies 6 volts to the LED and demonstrates how it’s not a good voltage for it. If you want to use a 6 volt battery pack but don’t want to damage or burn out your LEDs you’ll need to add resistors to your circuit. In order to figure out how, you need to look at Ohm’s law. Ohm’s Law describes the relationship between voltage current and resistance. You use V for voltage (measured in VOLTS), I for current (measured in AMPS), and R for resistance, measured in OHMS). Current is measured in AMPS, named for a French mathematician Andre-Marie Ampere, the I originates from the French phrase for current intensity, intensite de courant. While current intensity has been shortened to just current, it is still represented by I for intensity.
To make things easier to understand, Karen compares relationship between voltage, current, and resistance to water flowing through a pipe. Using this analogy, voltage is like water pressure, pushing the electricity through the circuit. Current is the amount of water flowing through the pipe. Resistance is the size of the pipe, which determines how much the flow is restricted. If the pressure (the voltage) stays the same and the resistance increases, making it more difficult for the water to flow, then the flow rate (or the current) must decrease. According to Ohm’s law, you need three variables: voltage, resistance, and current. To solve for resistance, you’ll need to find the voltage and current. The red LED is rated for 2.1 volts and 20 milliamps. In order to make the math a little easier, she rounds that to 2 volts. The battery pack supplies 6 volts, but they need to deprecate that to 2 volts and no more than 20 milliamps (equivalent to 0.02A) of current to power the red LED. Plugging this into Ohms law, resistance equals voltage divided by current so you would apply that by dividing 0.02A by 4V to give you 200 ohms.
This means you’ll have to add 200 ohms of resistance to your circuit to supply your LED with the correct voltage. If you were to use a 2 AA battery pack, that supplies 3 volts, that would mean a difference of 1 volt between your supply voltage and their LED voltage. Plugging this into your previous formula tells you that you need to add 50 OHMS of resistance. Because there is not a resistor for every value, you should calculate and come up with a value before looking for the resistor nearest to that value. Looking around the shop, Karen finds a 47 OHM resistor and a 220 OHM resistor. This would work but so would combining two 100 OHM resistors. To demonstrate, Karen puts the concept of parallel and series circuits to practice. If you put 2 100 OHM resistors in series, the two values added together equal 200 OHMs of resistance.
She can use this for her 6 volt circuit. If you put 2 100 OHM resistors in parallel, it gives the electricity two paths with which can flow, cutting the resistance in half, and giving you 50 OHMs of resistance. Combining resistors is a good way to create values that might not otherwise exist or you just don’t otherwise have in hand. Be careful when combining more than two resistors as the match can get complicated, particularly with putting resistors in parallel. Karen goes over a diagram of a 6 volt circuit. Taking a look at Ohm’s law, you need three variables: voltage, resistance, and current. Since she’s trying to find the resistance that she needs, she’ll need to find the voltage and current to solve for it.
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