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Chemistry and Physics

Use Ohm's law and parallel resistors

Find the combined resistance of two resistors in parallel and then the current from V = IR.

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Ohm's law links voltage V, current I and resistance R: V = I × R, so I = V ÷ R.

Resistors in series simply add. For two in parallel, add their reciprocals and take the reciprocal again: 1/R = 1/R₁ + 1/R₂. For 100 Ω and 220 Ω that gives R = 68.75 Ω.

A 12 V supply across 68.75 Ω then drives a current of 12 ÷ 68.75 ≈ 0.1745 A.

Classic Graphing 84

Start on the main screen: Press 2nd then mode (QUIT) to get back to the home screen. Pressing clear on a menu also backs out of it.

  1. Type the bracket (100⁻¹ + 220⁻¹) with the x⁻¹ key, close it, and press x⁻¹ again.

    (100x⁻¹+220x⁻¹)x⁻¹enter

  2. The resistance is 68.75 Ω. Now divide the voltage by it: type 12 ÷ and the last answer (2nd, then the (-) key).

    12÷2ndANS(−)enter

You should seeThe current is about 0.1745 A.

Classic Graphing 84 key reference · Classic Graphing 84 manual

Natural Scientific 991

Start on the main screen: Press MENU, then 1 (Calculate) to return to the main calculation screen. AC clears the line you are on.

  1. Type (100⁻¹ + 220⁻¹)⁻¹ using the x⁻¹ key. Press =.

    (100x⁻¹+220x⁻¹)x⁻¹=

  2. Type 12 ÷ then the Ans key and press = . Press S⇔D to turn the fraction into a decimal.

    12÷Ans=S⇔D

You should see0.1745454545

Natural Scientific 991 key reference · Natural Scientific 991 manual

RPN Scientific 15

Start on the main screen: There is no menu to leave. Press the back arrow to clear a number you are typing, or CLx (g then back arrow) to clear the X register.

  1. Key in 100 and press 1/x. Key in 220 and press 1/x. Press + and then 1/x again.

    1001/x2201/x+1/x

  2. The resistance 68.75 is in X. Key in 12, swap the two numbers with x⇄y, and press ÷.

    12x⇄y÷

You should see0.1745

RPN Scientific 15 key reference · RPN Scientific 15 manual

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