Current Electricity

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We studied about the charge at rest or the electrostatics.

Here we’ll study the charges in motion.

We divide the motion of charge into two parts:

  1. Constant velocity.

  2. Variable velocity or accelerated motion

The branch of physics which deals with the current flow with a constant velocity is called the current electricity.

Here, the charges are electrons.

So, current electricity is the flow of the current inside the material because of the movement of the charges inside it.

In this article, we will study the charges in motion and various effects, and the phenomenon related to it.


Electric Current Examples

  1. Two Charged Conductors Separated by an Insulator

Here the two conductors are charged to different potentials. they are supported on insulating stands and held at a distance as shown in the figure:

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There is no flow of electric charge between these two conductors through the potential difference existing between them. It is because they are separated by air which is an insulator.


  1. Two Charged Conductors Connected by Metallic Wire

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Now, these two conductors are connected with a metallic wire, and there is a net flow of charge from one conductor at the higher potential to another at a lower potential. This flow continues until their electric potential becomes equal.


Electric Current Definition 

Electric current is defined as the total time taken for the total charge to flow. It is symbolized by I.  

Where  I  = q/t = total charge flowing/total time, and

         Iavg = Δq/Δt

         IInstantaneous  = dq/dt

From Fig.2, if we maintain an unvarying potential difference between the two conductors, we get a constant net flow of steady charge in one direction in the metallic wire connecting the two conductors.

So, the net flow of charge in a definite direction through a metallic wire constitutes an electric current. 


Some Electric Current Examples Are

  1. In the case of lightning, there is a flow of charge from clouds to the earth, resulting in the current called the transient current. This current ceases as the charge flow stops.

  2. A steady flow of charge in a cell-driven clock, a torch, etc.


What is the SI Unit of Electric Current?

The electric is measured in Amperes, symbolized by A.

Where I = q/t = 1 Coulomb/sec = 1 A

Therefore, the current flowing through the wire is said to be one ampere, if one coulomb of a charge flows per second through a section of the wire.

Ampere is one of the seven fundamental units in physics.


Current Definition Physics

The rate of flow of charge through any cross-sectional area of a conductor is the measure of current i.e., I = q/t.

Electric current has both magnitude and direction, so it must be a vector quantity.

Here comes the contradiction that the current doesn’t follow any of the laws of vector addition.

For example,

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The net current comes up just by adding all the currents like:

inet  = i1+ i2 + i3 + i4 + i5 

Here, we get the addition of the current i.e., inet is a scalar quantity and the angle between the wires carrying currents does not affect the total current in the circuit.

We can consider the current as a scalar quantity, in actual it is a ‘tensor’ quantity.


Conventional Current Flow

Conventional current flow is the flow of current from the positive terminal to the negative terminal of an external source like a battery. Here, the direction of an electron is just opposite to the direction of the current as it flows out of the negative terminal and moves towards the positive terminal of the battery.

So, the conventional current flow is opposite to the electron flow whereas the flow of positive charges is in the direction of an electric current.

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Relation Between Resistance and Resistivity

Resistance: To obstruct the flow of electric current is called the resistance.

Let’s say we have a conductor, and we apply a potential across its ends.

Then the resistance of this material is,

                 R = V/I..(1)

So on applying the potential difference, ΔV across the ends of the conductor, if the current I flow through it, then the ratio of V/I is the resistance.

This relationship isn’t for Ohm’s law because this law states that V ∝ I or V = IR 

Where R is constant, but resistance can be variable.

So for the devices having a variable resistance, we can consider the potential difference at an instant and use the above equation (1):

Therefore, Ohm’s law isn’t valid everywhere.

We know that:

  1. R ∝ 1/L

  2. R ∝ A

On removing the constant of proportionality sign, we get:

                         R = ρ L/A

The proportionality constant, ρ is called the specific resistance or the electrical resistivity of a material which is measured in Ohm-m or Ω m.


 Do You Know?

  • The average lightning carries currents in the order of tens of thousands of amperes.

  • The currents in our nerves are in microamperes.

FAQ (Frequently Asked Questions)

 If q = t2+ 5t + 4, Find:

  1.  I Inst at  t = 3 Seconds. 

  2. IAvg from 1 to 3 Seconds

 We are given with q = t2+ 5t + 4…(a)

We know that IInstantaneous = dq/dt

Now differentiating equation (a) with respect to time:

d(q)/dt = d(t2+ 5t + 4)/dt = 2t + 5

So, IInstantaneous (t = 3) = 2 * 3 + 5 = 11 A

We also know that: IAvg = Q2 - Q1/t2  - t1

 = (Q3 seconds - Q1 seconds)/(3 - 1)..(2)

From equation (a):

q3 seconds = 32+ 5 x 3 + 4= 28 C

q1 second = 12+ 5 x 1 + 4= 10 C

Putting values in eq (2), we get:

IAvg = (28 - 10)/2 = 9 A

2. What Does a Variable Resistor Mean?

Variable resistor is an electronic component which is applied in an electronic circuit for adjusting circuit resistance to control voltage or current of that circuit or part of that circuit.

3. What Are the Examples of the Alternating Current?

The devices that run on AC supply are:

  1. Mains

  2. Speakers

  3. Transformers

  4. Car alternators

  5. Ethernet 

4. What Are the Examples of the Direct Current?

The devices that run on DC supply are:

  1. Cell phones

  2. Flashlights

  3. Hybrid and electric vehicles