Electric Current and Current Density

- Electric current is the motion of electrically charged particles from one region to another. This motion of electric charges takes place within electric circuit which is a conducting path forming a closed loop.
- In a conductor electric charge will flow from its one end to another if and only if both the ends of the conductor are at different electric potentials.
- Continous flow of electric current in a conductor for a relatively long period of time can be attained using battries which could supply continous flow of charge at low potentials.

Electric current
- Here in this section we would discuss about the electric current in conductors.
- When electric field inside the conductor is zero then there would be no net flow of current in the conductor because in this case electrons in the conductors moves about randomly leaving no net flow of charge in any direction and without the flow of charge there would be no net electric current.
-To maintain a constant current in a conductor we would have to ensure that a constant and steady electric field is established inside the conductor in order to maintain a force on the mobile charges in the conductor.
-Once the electric field is maintained inside the conductor charged particles in the conductors are now under the influence of driving force F = qE.
- In an conductor charged particles undergoes frequent inelastic collision with fixed massive ions in the conductor and undergoes random change in the direction of motion.
- Hence on an average charged particle moves in the direction of driving force with an average velocity known ad drift velocity.
- Electric current is defined as the quantity of charge ΔQ flowing through cross-sectional area A in time interval Δt . Thus,
Iav = ΔQ/Δt
which is the average current flow in time Δt.
- If dQ is the amount of charge flowing in infinitesimally time interval dt through a cross-sectional area of the conductor then instantaneous current I is defined as
I = dQ/dt
- Electric current is a scalar quantity.
- SI unit of current is Ampere (A) where 1A is one coulomb per second.

Drift velocity and Current density - Consider a portion of a conductor of cross-sectional area A. Also consider a small section of conductor of length Δx. Now volume of conductor under consideration is AΔx.
- If there are n number of mobile charge carriers per unit volume then total charge in the section under consideration is
ΔQ = (number of charge carriers in the section) x (charge per carrier)
= (nAΔx)q .................1
where q is the amout of charge on each carrier.
- If charge carriers are moving with speed vd, then distance travelled by the charge carriers is Δx = vdΔt. Putting this in equation 1 we have,
ΔQ = (nAvdΔt)q
- Now current in the conductor is
I = ΔQ/Δt
I = nqvdA ....................2
where vd is average velocity known as drift velocity as defined earlier.
- Current density j is defined as current per unit cross-sectional area. Thus from 2
j = I/A = nqvd
Current is a scalar quantity but current density can also be defined too include both magnitude and direction. Thus vector current density is
j = nqvd
Direction of current density is same as the direction of electric field.
- Unit of current density is A.m-2.
- Current density tells us about how charges flow at a certain point and also about the direction of the flow at that point but current describes how charges flow throughout an extended object.

Solved example
In this solved example we will lern to apply the principle of current to the problems.

Question:-Amount of charge that passes through a certain conducting wire in 4 sec is 6.5 C. Find (a) the current in the wire and (b) number of electrons that passes through the wire in 8 sec.
Solution : -
Problem solving strategy
1. From the lesson learned about electric current identify the equation for calculating current when charge and time are given.
2. Put the values of charge and time at respective place and calculate the answer.

In this case Q = 6.5 C and t = 4 sec.
now I = q/t = 6.5C/4 sec = 1.625 A
which is the required answer.

Problem solving strategy
1. Here we have to find total charge through the wire in a given time.
2. Total charge through the wire would be equal to number of electrons through the circuit multiplied by the charge on each electron.

Here we have to find the number of electrons passing through the wire in 8 sec. If qe is the amount of charge on a single electron and total n number of electrons passes through the wire then
I = nqe/t
or, N = It/qe
putting values of I , t and qe = 1.6 x 10-19 and calculating we get
N = 8.125 x 1019

Same way problems related to current density and drift velocity can be solved.

Vector Algebra 1

 Here in this post we will go through a quick recap of vector algebra keeping in mind that reader already had detail knowledge and problem solving skills related to the topic being discussed. Here we are briefing Vector Algebra because concepts of electrostatics , electromagnetism and many more physical phenomenon can best be conveniently expressed using this tool.

A vector is a quantity that requires both a magnitude (= 0) and a direction in space it can be represented by an arrow in space for example electrostatic force, electrostatic field etc. In symbolic form we will represent vectors by bold letters. In component form vector A is written as
A = Axi+ Ayj+Azk

Two vectors A and B can be added together to give another resultant vector C.
C = A + B

Two vectors A and B can be subtracted to give another resultant vector D.
D = A - B = A + (-B)

When we multiply any vector A with any scalar quantity 'n' then it's direction remains unchanged and magnitude gets multiplied by 'n'. Thus,
n(A) = nA
Scalar multiplication of vectors is distributive i.e.,
n(A + B) = nA +nB

Dot product of two vectors A and B is defined as the product of the magnitudes of vectors A and B and the cosine of the angle between them when both te vectors are placed tail to tail. Dot product is represented as A.B thus,
A.B = |A| |B| cosθ
where θ is the angle between two vectors.
Result of dot product of two vectors is a scalar quantity.
Dot product is commutative : A.B = B.A
Dot product is distributive : A . (B+C) = A.B + A.C also A.A = |A|2

Cross product or vector product of two vectors A and B is defined as
A x B = |A| |B| sinθ nˆ
where nˆ is the unit vector pointing in the direction perpandicular to the plane of both A and B. Result of vector product is also a vector quantity.
Cross product is distributive i.e., A x (B + C) = (A x B) + (A x C) but not commutative and the cross product of two parallel vectors is zero.

In our next post we'll study vector algebra in component form and also lern about vector triple products.

Overview of electrostatics and electricity

Electrostatics involves electric charges namely positive and negative charges, the forces between them which is known as electric force , the field that surrounds them, and their behavior in materials. Coulumb's law is the simple relation that governs electrostatic interactions and the field around the charges is described using the concept of electric field. Coulumb's law is a inverse square law which gives the force between two charges kept at some distance (say r ) apart from each other. Like Coulumb's law, law of gravitation is also a inverse square law but gravitational interactions are only attractive in nature and electrical interactions are attractve as well as repulsive depending on the nature if interacting charges. Charges of same kind repel each other and charges different kinds , i.e. one charge positive and other negative , attract each other. One more thing electric interactions are much more stronger then gravitational interactions and gravitational force are almost negligible in comparison to the forces of electric origin. This is always true when we study the interactions of atomic and subatomic particles. But when we study objects very large in size say a person ,a planet or satellites, the net governing force in this case is gravitational force not electric.
Now coming to the properties of electric charges we know that electric charge is quantized and it also obeys law of conservation means total charge remains conserved. In what we say electrostatic interactions electric charges are at rest in our frame of refrence. Now think what happens when we are at rest in our frame of reference and charge under consideration is moving with velocity v with respect to us such a miving charge leads to the origin of magnetism which we will discuss in later section.
Again a question what is electricity. Electricity, deals with stationary and moving electric charges, the actions of force between these charges, and the electric and magnetic fields generated by them. Electrostatics is simply the electricity at rest. Electricity is the backbone of the modern society in which we use various instruments which depends on electric current for their functioning and without it we would not have telephones, television, household appliances and many more gadgets which are now part of our daily life. In electricity we study the motion of electric charges, or electric currents and also the voltages that produce currents and the ways to control currents.
We have learned about what electrostatics is and what we study in it. Now we will discuss why we study electrostatics and where it finds its applications.Electric interactions are of immence importance in chemistry and biology and have many technological applications. Concepts of electricity proved to be of basic importance for studying atomic physics, nuclear physics and solid state physics. It also find importance in studying advanced level physic.

How to prepare for Physics for IITJEE

IITJEE is one of the toughest examination in India.SO it requires lots of systematic efforts to pass the examination.
Here i would to try to explain what should be the strategy and plan for physics for IITJEE

  • First things is the right selection of books for the IITJEE as good books play an important role in clearing the concept in Physics.
    You can check at the following links for the list of books for the IITJEE.

    IITJEE Physics Books

  • Second things is to plan your syllabus .Decide about the time for the each units.I will be soon giving u the IITJEE physics planner for theb whole year which will greatly help you in planning.
  • The next steps to stick to the schedule and start the study.First of all clear your all concepts.Read the chapter many times so that concept becomes crystal clear and you can connect the concept with our daily life.For example.Lets take Projectile Motion.A baseball in motion describes an Projectile motion.You can think of baseball example in learning the concepts.It will be good if you can prepare some notes of the chapter for fast revision.
    Check out your concept by attempting the conceptual questions.You can lots of comceptual test on the my blog.

    You can get good study Material at the following link
    IITJEE Study Material

  • Once you have cleared the concept,you are ready take the questions.First study some examples to get the feel of the concept.While going through examples,you can see and analyze the way a particulr physical concept is applied towards solving the problem. This will help you a great deal while you begin to solve problems.Try to find the best possible way to tackle the problem.Lets take the example for Projectile motion

    1.Gather all the information from the question
    2.Select a coordinate system and resolve the initial velocity vector into x and y components.
    3.Follow the techniques for solving constant-velocity problems to analyze the horizontal motion. Follow the techniques for solving constant-acceleration
    problems to analyze the vertical motion. The x and y motions share the same time of flight t.

    You can find lots of such ways and hint from the following link
    IITJEE Tips
  • Revise your material and practice questions on a systematic basis.
  • Attempts the question in previous year IITJEE questions papers.This will give you a feel of the IITJEE questions.You can find the papars and solutions in the below link
    IITJEE Previous Year Papers

  • Take Test on regular basis.It give you the feel of the examination.Give you full attempt to the test.Take online Test on the below links.
    IITJEE Test Series
  • Take some simulated IITJEE test series (not online) by the some renowned institues.This will help you in preparing for the Main Examination

Best of luck
Physics Expert

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