Resistor help?

Question

ok i need to make a series circuit with 9v and less than 300mA.

but i don't know how to fill out the chart.

Voltage Current Resistance

R1 | . . . . . . . . | . . . . . . . . | . . . . . . . . . . . |

R2 | . . . . . . . . | . . . . . . . . | . . . . . . . . . . . |

R3 | . . . . . . . . | . . . . . . . . | . . . . . . . . . . . |

Total | . . . 9V . . . | . . . . . . . . .| . . . . . . . . . . . |

Answer 1

Well, a good place to start with is determining what the total circuit resistance would be for 9 volts at less than 300 mA. Let's let total circuit current be 300 mA.  Thus, total circuit resistance is simply Rt = E / It    Rt = 9.0 volts / 300 mA or

9.0 volts / 0.300 amps. = 30 ohms.  So, we know that when you add R1 + R2 + R3 it must add up to 30 ohms.

Now, we know that in a series circuit, the current is the same in all parts of the circuit, no matter what resistor values are in it.  The circuit current is 300 mA.

Now, if you are to meet the requirement that total circuit current be less than 300 mA, you will have to make total circuit resistance somewhat greater than 30 ohms.  As a result, just about any combination of three resistors when connected in series adding up to something greater than 30 ohms will meet the requirements.

For example, if you let R1 = R2 = R3 = 12 ohms each or 36 ohms all together, circuit current will be 9.0 volts / 36 ohms or 250 mA.

Answer 2

This is a little silly as an exercise unless some values are being forced on you.   In any case, E=IR > R = E/I = 9/0.300

Therefore  total resistance must be 30 ohms or greater to make 300 mA.  Simplicity is to make R1=R2=R3 = R/3

So the first column could be 3V all the way down, second 300 ma all the way down and last column 10 ohms all the way down with 30 ohms at bottom.