Current Electricity Concept Page - 1

Definition

Steady current

An electric current that does not change with time is known as steady current. An example would be closing or opening a switch to add or remove a battery, current may initially change, but then reach a final ``steady-state'' value after a long time, or go to zero, depending on the situation.

Definition

Flow of charges between 2 bodies

The process of supplying the electric charge (electrons) to an object or losing the electric charge (electrons) from an object is called charging. An uncharged object can be charged in different ways.

1. Charging by friction
2. Charging by conduction
3. Charging by induction

Result

Direction of current flow

Current flow occurs due to flow of electrons. Electrons flow from lower potential to higher potential whereas current flows from higher potential to lower potential. Thus, the direction of current flow is opposite to the flow of electrons.

Definition

Amount of charge flowing through a wire

Amount of charge flowing through a wire in time t$t$,
q$q$ =It$It$. where I$I$ is current through the wire.

Example

Example

Example: A 60 W$60W$ bulb carries a current of 0.5 A$0.5A$. Find the total charge passing through it in one hour.
Solution:Total charge flown q=It=0.5×3600=1800C$q=It=0.5\times 3600=1800C$

Definition

Current through a wire

Current through a wire, I=qt$I={\displaystyle \frac{q}{t}}$, which is charge passing through a cross-section per unit time.

Example

Example

Example:If 240$240$ coulomb of charge passes through a cross section of a conductor in 4$4$ minute, find the average current.Solution:Current I=dqdt=2404×60=1A$I={\displaystyle \frac{dq}{dt}}={\displaystyle \frac{240}{4\times 60}}=1A$

Definition

Potential difference

The electric potential difference between two points in an electric circuit is the work done in moving a unit positive charge from one point to the other.
Potential difference, V=Work doneCharge=WQ$V={\displaystyle \frac{Workdone}{Charge}}={\displaystyle \frac{W}{Q}}$

Definition

Unit of potential difference

The S.I Unit of potential difference is volt (V).
1 volt =One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one point to the other.
1 Volt=1 Joule1 Coulomb$1Volt={\displaystyle \frac{1Joule}{1Coulomb}}$
The potential difference is measured by an instrument called voltmeter.

Example

Calculate Current Density

Q. A current of 6 mA is flowing through a copper wire that has an area of 4 mm2. What is the current density?
Sol : 
Step 1 : The current through the conductor is I = 6 mA=0.006 amperes(6×10−3 amps)$6mA=0.006amperes(6\times {10}^{-3}amps)$
              The area of the wire is A = 4 mm2=0.004 m2(4×10−3m2)$4m{m}^2=0.004m^2(4\times {10}^{-3}{m}^2)$
              As we know that current density, J=IA$J={\displaystyle \frac{I}{A}}$
Step 2 : J=0.006 amps0.004 m2$J={\displaystyle \frac{0.006amps}{0.004m^2}}$
              J=1.5 amps/m2$J=1.5amps/m^2$

Definition

Current density

Current density is the current flowing per unit area normal to the flow. The current density j$j$ at a point is given by
j=limΔS→0ΔiΔS=didS$j=\underset{\mathrm{\Delta }S\to 0}{lim}{\displaystyle \frac{\mathrm{\Delta }i}{\mathrm{\Delta }S}}={\displaystyle \frac{di}{dS}}$
where i$i$ is the current flowing through the surface S$S$.

Definition

Drift of electrons

In the absence of an electric field, the electrons inside a conductor will be moving randomly due to thermal agitation during which they will collide with the fixed ions. After collision, they will emerge with the same speed as before but in random directions.When the same conductor  is in an electric field, the electrons are subjected to a steady force given by F=qE$F=qE$  where F$F$ is the force, E$E$ is the applied electric field and q$q$ is the charge of an electron.
The net effect is that in addition to the random motion of the electrons, there is also a very slow net motion or drift of the electrons. This drift or flow with a constant velocity (in a direction opposite to ) is called the drift velocity VD$V_D$.

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