|Electricity - Resistance||Teacher's Notes|
Resistance is the friction in an electrical circuit that controls the flow of current.
As previously mentioned, voltage causes current. When a voltage is present and there is a path (circuit) for electron flow, then there will be a current.
Question: How much current will flow? Answer: As much as can be produced by the pressure (Voltage) working against the friction (Resistance) of the circuit.
If you open all of the faucets in your house, a lot of water will flow, but not enough to instantly empty the water tank on the hill above your house. The faucet valves that the water must flow through tend to limit the flow rate of the water. In a sense, they have friction that works against the water pressure to limit the flow. You can adjust the flow by how far you open each faucet. In electrical terms, the faucet is an adjustable resistor.
Wires, like water pipes, have very low resistance. Other circuit elements are intentionally designed to have specific amounts of resistance in order to accomplish the operating goals of the circuit. Light bulb filaments, stove heating coils, and motor windings all have resistance that regulates the flow of current through them.
Why do we need Resistance?
Every real-world circuit must include an appropriate resistance so that there is an appropriate current flow. Recall the battery and bulb circuit on the voltage/current page. In that circuit the bulb is the primary circuit resistance. Here are three examples of voltage, resistance, and resulting current in that circuit:
1. Not Enough Resistance
If we use a 6 Volt battery and a bulb that is intended for 1 1/2 Volts, the pressure is too high and the filament resistance allows too much current to flow. We get a single bright flash as the bulb burns out!
2. Too Much Resistance
If we use a 1 1/2 Volt battery and a bulb that is intended for 6 Volts, the pressure is too low and the filament resistance allows very little current to flow. The battery lasts for a very long time but the bulb is too dim to be of much use.
3. Exactly Enough Resistance
When we use a 1 1/2 Volt battery and a bulb that is also intended for 1 1/2 Volts, the current is just right. The bulb is bright, but not doesn't burn out, and the battery lasts for quite a while.
What is the exact relationship between Voltage, Current, and Resistance?
Here are two statements that describe the relationship:
1. Given a fixed Resistance, more Voltage causes more Current.
2. Given a fixed Voltage, more Resistance causes less Current.
(Before proceeding I need to remind you that we use Volts for electrical pressure, Amps for electrical current, and Ohms for resistance.)
Ohm's Law is a set of three related equations that encapsulate the two statements above:
E = I * R I = E / R R = E / I
(For historical reasons, we use "E" to represent Voltage, "I" to represent Current, and "R" to represent Resistance.)
Some Example Calculations
Using the Ohm's Law equations above we can analyze real-world circuits. Yes, it is math, but it is really fairly simple. Please don't give up until you have tried to understand the examples. Get out your calculator and follow along.
There are just three steps: First, choose the equation form where the facts you already know are on the right side and the answer you want is on the left side. Second, plug in your known values. Third, do the arithmetic.
Here are some examples using the same flashlight circuit we have already looked at:
1. Suppose that our bulb has a resistance of 50 ohms and we know that it should be used with a current of 0.03 Amps. What battery voltage is needed?
We know the current and resistance and want to calculate the voltage, so we should choose the first equation ( E = I * R ) then we calculate 50 * 0.03 which equals 1.5 so we need a 1.5V battery.
2. Now suppose that we have a 3 Volt battery and that our bulb has a resistance of 100 ohms. How much current is flowing when the bulb is lit?
We know the voltage and resistance and want to calculate the current, so we should choose the second equation ( I = E / R ) we calculate 3 / 100 which equals 0.03 so 0.03 Amps of current is flowing.
3. Now suppose that we have a 4.5 V battery and that there is 0.03 Amps of current flowing. What is the bulb resistance?
We know the voltage and the current and want to calculate the resistance, so we should choose the third equation ( R = E / I ) and we calculate 4.5 / 0.03 which equals 150 so the bulb resistance is 150 ohms.
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