Temperature and Current

 

"Why does the bulb

get brighter when

the rod is put in the

liquid nitrogen?"

 

 

 

 

"Lowering the

temperature of a wire

 reduces resistance

and increases current"

 

 

II. Circuit Concepts

 

Conservation of charge and energy for electric current

Gustav Robert Kirchhoff
(March 12, 1824 – October 17, 1887) was a German physicist

This image is in the public domain because its copyright has expired.

 


1) Kirchhoff's 1st Law

Total current arriving
at a point in a circuit
=
  Total current leaving
the  point.

(Conservation of Charge)

 

 

“Amps In = Amps Out”

 

 

 

 

 

 

 

 

Ex 1)

 

 

 

 

 

 

 

 

 


 Amps In = Amps Out

 

5 Amps = 3 Amps + x


 

X = 2 amps

 

 

 

 

 

 




 

 

 

 

 


Remember

 “Amps In =
 Amps Out”






 

 

 

Ex 2) Current in bottom branch?



Amps In = Amps Out

2 Amps + X = 12 Amps


X = 10 amps

 

 

 

 

 

 

Ex 3)




 


Amps In = Amps Out

22 Amps  =  x Amps + 6 Amps


X = 16 amps

 

 






 

Ex 4)

 

 

Amps In = Amps Out

5 A + x = 15 A

 

x = 10 A IN

 

 

 

 


 What's Wrong With This Picture?

 

 

 

 

 

Current In (10 A),
does not equal current out (4 A)

 

 

 

 

 

 

 

 

2) Kirchhoff's 2nd Law - the algebraic sum of all the voltage drops and applied voltages around a circuit = 0

(conservation of energy)

 

Amount of PE gained when you go up a ski slope = energy released going down a ski slope 

(KE, Work against friction and snow)

 

 

 

 

Applied Voltage ( + )
Voltage Drop ( - )

 

0 =  12 V + -2 V + x + -6 V

 

0 = 12 V + -8 V + x

 

0 = 4 V + x

 

x = - 4 volts

 

 

VT = V1  + V2 + ...

 

 

 

 

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