Explain how resistance affects current in a circuit.?

There are two ways, two easy ways to look at this, that I can think of immediately. The first is to think of current as water flowing. Your voltage source is like the large tank at the top of a water. tower. There is a valve (which would be like a switch) at the bottom of the tank and a pipe that directs the water down to ground level. In the pipe are spaces for discs of a mesh screen. The more discs of mesh screen that are placed into the pipe, the more resistance there is to the flow of water. It is similar to an electric circuit.

The second way is to look at the equation called Ohm's Law, V = IR.

With this equation, the terms V and I or V and R are said to directly proportional. Take the voltage, V and resistance, R. Let's say that resistance remains the same and the voltage is increased. That means there is a corresponding increase in the current, I. Check this out with the following values:
V = 10 V, R = 10 ohms.
next values:
V = 20 V, R = 10 ohms.

When you manipulate the equation so that you solve for I, you get I = V / R.

You should notice that there is an increase in I from the first set of values to the second set of values. Now, imagine placing those values back in the original equation. Can you imagine how the increase in current, I, must happen?

Now try the same thing with the resistance and hold the current constant while changing voltage. Can you see how resistance must increase with current constant and voltage increasing?

Go back to this equation: R = V / I.

In this equation, I is said to be inversely proportional to R. Play with one value while keeping another constant. You can look up the terms directly proportional and inversely proportional and how they relate to equations.

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think of resistance as friction. Think of water moving through a pipe, a smaller pipe will not allow as much water to pass in the same amount of time as a larger pipe. now substitute amperage as the water and the diameter of the pipe as resisitance. the longer the circuit and the friction involved causes less current to get through it.

Resistance limits the current that will flow through a given circuit at a given voltage.
For simple DC McNuggets advice to think of it as fluid pressure is valid.

Resistors obey Ohm's Law, but there specific effect depends on all the components in a circuit.

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