Chapter 13
  The Current

 

 

By Keiji Oenoki and Hector Judez [physics@amersol.edu.pe]

1. Flow of Charge: The Current
2. Electric Resistance and Ohm's Law
3. Voltage Law and Current Law
4. Chapter Quiz

 

  Section
Section 1. Flow of Charge: The Current
  Check!
The flow of charge is called the current and it is the rate at which electric charges pass though a conductor. The charged particle can be either positive or negative.

In order for a charge to flow, it needs a push (a force) and it is supplied by voltage, or potential difference. The charge flows from high potential energy to low potential energy.
   
  Suppose A has a potential of 12 V and B has a potential of 2 V. There is a potential difference. A has higher potential energy than B, and it means there is voltage. The potential difference is

VA - VB = 12 - 2 = 10 V.

If there is a potential difference between two regions and if we join them together, the charge will flow. The charge will always move until the force acting on it is reduced to a minimum or until the voltage becomes the same.

Potential difference between A and B
  Suppose C and D has a potential of 7 V. There is no potential difference between two plates. The potential difference is VA - VB = 7 - 7 = 0 V. Therefore, it has no voltage and it means no flow of charge. Potential difference between C and D
     
  Battery and potential energy What will happen if something pushes the charge from the bottom plate to the upper plate E in the diagram? This will generate a potential difference, and hence, there will be continuous flow of charge. This is how a battery works; it takes "+" charge from bottom and push it to top.
     
  The system like E, a closed loop of current, is called an electric circuit.

 


  Check!
The current [I] measures the amount of charge that passes a given point every second. The unit for current is Ampere [A]. 1 A means that 1 C of charge passes every second.

1 C/s = 1 A

 

  Why can a bird sit on the power line ?
  Why do many cows die during a thunder storm even though the lightning does not hit them directly?

 

  Section
Section 2. Electric Resistance and Ohm's Law.





  Check!
Resistance is a feature of a material that determines the flow of electric charge. The unit of resistance is ohm. The resistance varies in different materials. For example, gold, silver, and copper have low resistance, which means that current can flow easily through these materials. Glass, plastics, and wood have very high resistance, which means that current can not pass throught these materials easily.

A German scientist Georg Simon Ohm experimented with circuits and found out the relationships between current, voltage, and resistance. It became known as Ohm's law and can be written in an equation V=IR, where V is voltage, I is current, and R is resistance.

   
 
Practice problem.
What is the current of a circuit that has 3 V and 0.5 ohm of resistance?

Answer:
You first have to modify the equation V = IR to I = V / R. Then you just have to input the numbers into this equation.
I = 3 / 0.5 = 6 A [ampere].

 

 
Example Problem 1.

a). What is the voltage if current is 0.5 A [ampere] and resistance is 0.8 ohm?

(e.g. "0.1") V V=I*R

b). What is the resistance if voltage is 3.0 V and current is 1.5 A?

(e.g. "1") A R=V/I

 




  Check!
Resistance of a piece of wire (or any other conductor) is proportional to the length of the material, and is inversely proportional to its thickness or sectional area.

R = k * (L / A)

where:

  • k is resistivity constant that depends on the types of material,
  • L is the length (m), and
  • A is the area (m2).

A potentiometer uses this characteristic. It can change its resistance by controlling its length. This apparature is used in T.V. volume, light controller, and everything else dealing with changing its electricity gradually.

Symbol for potentiometer is Symbol of potentiometer

and its inside looks like this Inside of potentiomenter.

The gray bar inside is connected to the black wire of its around, and it rotates (see above). Let's say an electrical line is connected to the left and the middle. If the gray bar is at the left, then the circuit will have small resistance because the length of the black wire outside is shorter. When the gray bar trun to the right, then it will have larger resistance.

   
  Check!
Learn how to read a resistor Resistor

 

  Section
Section 3. Voltage Law and Current Law
  Voltage Law
  Check!
"In any circuit, the sum of decrease in voltage equals the sum of its increase."

For example, imagine a circuit with a battery and a bulb. If the battery generates 3 V, then the bulb must consume 3 V, because the sum of decrease in voltage (-3 V consumed by the bulb) must be equal to the sum of increase in voltage (+3 V generated by the battery).
     
  Current Law
  Check!
"At any junction point in an electric circuit, the total electric current into the junction is equal to the total electric current out."

A junction is any point where wires are splits into two or more.

In the left diagram, incoming current (2 A) is being split into half at a junction point (1 A), and then is combined back to the original current (2 A).

In other words, current coming in is equal to the current going out.

 Current law

 

 
Example Problem 2.

What is the current on X, Y, and Z? (resistance on X, Y, and Z are same)

Example Problem 2 (e.g. "10") A

 

  Section
Section 4: Chapter Quiz

Try Chapter 13 Quiz and see how much you learned.

[Ch 11] - [Ch 12] - [Ch 13] - [Ch 14] - [Ch 15]

Colegio Franklin Delano Roosevelt
July 1997

Comments or Questions to physics@amersol.edu.pe