Resistivity

• anpanman

  2 Feb 09, 21:27

  Here's a basic sec 2 physics question. But I tried out 2 ways and yielded TWO different answers. One of the methods is wrong I suppose. Please tell me which one is the incorrect method.

  2 identeical resistors each of resistance R are placed in parallel. The effective resistance is

  A 2R

  B 1

  C R^2

  D R/2

  First method: 1/R + 1/R = 2/R

  So 1/R = 2/R

  R = 1 / 2/R

     = R/2

   

  Second method: 1/R + 1/R = 2/R

  1/R = 2/R

  Using cross multiplication

  you'd get R = 2R!!!

  Isn't this weird?

   

  PS: The red refers to the answers and division line. (:

• ChoCoChips

  2 Feb 09, 21:32

  Originally posted by anpanman:

  Here's a basic sec 2 physics question. But I tried out 2 ways and yielded TWO different answers. One of the methods is wrong I suppose. Please tell me which one is the incorrect method.

  2 identeical resistors each of resistance R are placed in parallel. The effective resistance is

  A 2R

  B 1

  C R^2

  D R/2

  First method: 1/R + 1/R = 2/R

  So 1/R = 2/R

  R = 1 / 2/R

     = R/2

   

  Second method: 1/R + 1/R = 2/R

  1/R = 2/R

  Using cross multiplication

  you'd get R = 2R!!!

  Isn't this weird?

   

  PS: The red refers to the answers and division line. (:

  both also wrong

  how can 1/R = 2/R

  u must label them clearly.

  1/Rtot = 2/R

  2Rtot= R           (X multiplication)

  Rtot= R/2

• anpanman

  2 Feb 09, 21:35

  Oh yeah! It's the labelling that caused me to err! Thanks. By the way, does 'tot' refer to total resistance?

   

   *I checked my notes. My teacher wrote that way too! Got me confused.

• ChoCoChips

  2 Feb 09, 21:37

  Originally posted by anpanman:

  Oh yeah! It's the labelling that caused me to err! Thanks. By the way, does 'tot' refer to total resistance?

   

   

  
  yes

  but how u wan to label is up to u la

  1/apple = 2/orange oso can

  1/R = 2/r oso can