Electric Current - Practice Questions & MCQ

Definition: Electric current is defined as the rate of flow of electric charge through any cross-section.

i.e If a charge of $\mathrm{\Delta }{Q}_{\text{flows through an area for time }}\mathrm{\Delta }t$ then
1. average electric current through the area is defined as

$\overline{i}=\frac{\mathrm{\Delta }Q}{\mathrm{\Delta }t}$

2. The instantaneous current at any time $t$ is given by

$i=\underset{\mathrm{\Delta }t\to 0}{lim}\frac{\mathrm{\Delta }Q}{\mathrm{\Delta }t}=\frac{dQ}{dt}$

And If the flow is uniform then Current

$I=\frac{Q}{t}$

If a current I flows through an area for time $t$ then, total charge flowing is

$Q=\int Idt$

• The direction of current:-

As Current is the rate of flow of charge, Charge can be +ve or -Ve.

 The direction of flow of positive charge is considered as the conventional direction of current .

Or the direction opposite to the direction of flow of negative charge is considered as the conventional direction of the current .

• current flows through a circuit only if the circuit is closed

For a closed circuit  

1. Conventional current: Positive terminal to negative terminal

2. Electron current: Negative terminal to Positive terminal

•  Current can be added algebraically.

•  Current is a tensor quantity ( if in options tensor is not given take current as scalar quantity)

• Unit of Electric Current: The unit of electric current is Ampere or A.

The current is said to be 1 ampere when 1 coulomb of charge flows through any cross-section in every second. 

• Calculating current in different situation-

1. When charge is performing translatory motion

• If particles each having charge pass a given area in time

$i=\frac{nq}{t}$

2. When charge is performing rotatory motion

If a point charge q is moving in a circle of radius r

With Velocity V (frequency , angular speed \omega and time period T)

Then $i=\frac{q}{t}\Rightarrow \frac{q}{T}=q\nu =\frac{qV}{2\pi r}=\frac{qw}{2\pi }$

• Types of current

1. Alternating current

• Magnitude and direction both varies with time.

• Shows heating effect only.

• Its  symbol 

2. Direct current

• No change in magnitude and direction with time. 

• Shows heating effect , chemical effect and magnetic effect of current.

• It's a symbol

• Current independent of Area-

Current does not change with change in cross-sectional area

Here i_1=i_2=i_3