You and your friends decide to conduct an experiment: you drain your laptops of charge, and they won’t boot. So you bring their adapters and plug it into the socket; you notice that the laptops start now. While you were doing this experiment, it is outside, so you’re sweating all over. You find an AC switch near to the socket where you put your laptop plug, you switch it on, and a cool breeze starts to fill the room thanks to the AC. So what is responsible for making the AC and the laptop function? The answer is electricity. Imagining electricity is easy; it is just the flow of charges from one place to another. A Greek scientist and mathematician, Thales of Miletus, around the BC, noticed that when amber rods were rubbed with animal fur, they produced a spark; this was proof of static electricity (electricity that does not flow). Later around scientist Benjamin Franklin experimented with a flying kite whose end was attached to an electrical wire. The wire acted as a “lightning rod”. He noticed that the rod becomes electrically charged when struck by lightning. This idea of electricity evolved over years thanks to experiments conducted by Faraday, Tesla, and so on. Current density gives an idea about the direction and number of charges in a current carrying conductor. In this article, we will explore current density in detail.
Table of contents
, where is the amount of charge flown in time
indicates the drift velocity of the electrons. It is the average velocity with which electrons get drifted when an external electric field () is applied.
sign indicates that the direction of the electric field and drift velocity are opposite.
is called the conductivity of the material.
Q1. The charge flowing through a conductor as a function of time is given by Calculate the current density of the wire at given that the area of cross section of the wire is ?
(a) (b) (c) (d)
Q2. Which of the parameters has no effect on the drift velocity of electrons flowing through a conductor?
(a) cross sectional area of the wire
(b) length of the wire
(c) The number of free electrons
(d) magnitude of the current
Since current where is the drift velocity.
It is independent of the length of the conductor.
Q3. A silver wire of diameter is connected to an aluminum wire of diameter A current of flows through the wires. Calculate the current density in the silver wire? Given that the number density of electrons in the wire is .
Radius of the silver wire,
Area of the wire,
Now current density
Question 1. Why do we use current density?
Answer. The strength of the current flowing through a given area helps to know the safe current level of the instruments.
Question 2. Can current density be negative?
Answer. Yes, it can be negative. If the direction of flow of current is opposite to that of the area vector for the cross section chosen, then it is negative.
Question 3. Current is a scalar quantity. Why?
Answer. Current is scalar, because it follows algebraic addition or subtraction, but not vector addition or subtraction.
Question 4. What is the meaning of ?
Answer. 1 ohm is the value of resistance of a conductor, when a voltage of applied causes a current of to flow through it.