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Here's a chance to see if you can combine what you know to add up resistance in series AND in parallel. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

## Want to join the conversation?

• Are the formulas the same as those used for current flowing through a conductor?
• it's just the same as calculating the total resistance of an electric current with serial/parralel circuit
• There's no units for resistance?
• The universal unit of resistance is Ohm, depicted mathematically by the Greek symbol Omega
• when we have 3 tubes Rt = 10% out of sum of these 3 ?
• I'm not sure if you're asking about the three tubes in parallel or in series.

Let's say all three tubes have a resistance of 2 (R=2).
1) If the tubes are in series, then total resistance is the sum of all three resistances.
Rt = R1 + R2 + R3 = 2 + 2 + 2 = 8
2) If the tubes are in parallel, then the total resistance is the inverse of the sum of the inverse of all three resistances
Rt = 1 / (1/R1 + 1/R2 + 1/R3) = 1/ (1/2 + 1/2 + 1/2) = 1/(3/2) = 2/3

In either case, Rt is not 10% of the sum of the three resistances. 10% of the sum would be 10% * 8 = 0.8. Keep in mind that I'm only showing you one example that shows that Rt is not always 1-% of the sum of resistance. There may be a situation with the right numbers where total resistance of the three tubes in series could equal 10% of the sum of resistances. However, this is generally not the case, and you should not use this as a hard and fast rule.

In addition, remember that for tubes in series, the total resistance is always greater than the resistances of each individual component (mentioned in the video "Adding up resistance in series and in parallel").
• is this the easiest way for us to find rt or are there more ways to find rt
• Is current decreases while passing through resistance in series as we know I=Q/t where as t increases continuously as charges lose there energy while passing through resistance
• Isn't it easier to just add up 8+5+5+10+3
• in the beginning why does the smaller one have more resistance?