Circuit Concepts
Current conservation, junctions, voltage changes, and Kirchhoff's rules
|
Temperature and Current - MIT TV
Show Me
The Physics
II. Circuit Concepts
Conservation of charge and energy for electric current
Gustav Robert Kirchhoff This image is in the public domain because its copyright has expired.
1) Kirchhoff's 1st Law
“Amps In = Amps Out”
Ex 1)
Amps In = Amps Out 5 Amps = 3 Amps + x
X = 2 amps
Remember
Ex 2) Current in bottom branch?
Amps In = Amps Out
Ex 3)
Amps In = Amps Out
Ex 4)
Amps In = Amps Out 5 A + x = 15 A
x = 10 A IN
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
( + )
0 = 12 V + -2 V + x + -6 V
0 = 12 V + -8 V + x
0 = 4 V + x
x = - 4 volts
|
||||||||||||||
Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. Kirchhoff's 1st Law ("Amps In = Amps Out") is based on the conservation of:
2. 12 A of current flows into a junction. One wire leaving it carries 2 A. How much does the other leaving wire carry?
3. Kirchhoff's 2nd Law (voltages around a circuit add to zero) is based on the conservation of:
4. A 12 V battery is connected to three resistors in a loop. Two have drops of 2 V and 6 V. What is the third drop?
5. A wire in a bulb circuit is dipped in liquid nitrogen and the bulb gets brighter. Why?
Current at a Junction
In a kitchen circuit, a current of 15 A enters a junction box. From there, the circuit splits into three branches that power a microwave (9.0 A), a toaster, and a light (0.50 A).
What is the current in the toaster branch?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. Which principle explains why current into a junction equals current out?
C2. A coffee maker is plugged in as a fourth branch, and the current entering the junction box increases from 15 A to 18.5 A. The other three branch currents do not change. Substitute numerical values into a mathematical representation that can be used to determine the current in the coffee maker. Then calculate the current.
C3. Place a check mark in three boxes to indicate the statements that are true for the kitchen circuit described.
C4. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
C5. The circuit breaker trips at 20. A. Which additional device could be turned on safely?
C6. The homeowner keeps tripping the 20.-A breaker and asks an electrician to install a 30.-A breaker on the same kitchen wiring. Identify one benefit and one drawback of this change.