Electric Charge and Current

Card 1: Electric Charge

Electric Charge

  1. There are only two kind of electric charge, namely the positive charge and the negative charge.
  2. Like charge repel each other.
  3. Unlike charge attract each other.
  4. The SI unit of electric charge is Coulomb (C).
Card 2: Unit of Charge

Unit of Charge

The SI unit of electric charge is Coulomb (C)

1Coulomb (C) = 1 Ampere Second (As)

Example

  • Charge of 1 electron = -1.6 × 10-19 C
  • Charge of 1 proton = +1.6 × 10-19 C
Card 3: Formula - Total Charge

Formula - Total Charge

Example 1
1.25×1019 electrons are added into an object. Find the nett charge of the object in the unit of Coulomb. [Charge of 1 electron = -1.6×10-19]

Answer:

Number of electrons, n = 1.25×1019
Charge of 1 electron, e = -1.6×10-19
Total charge, Q = ?

Q = ne
Q = (1.25×1019)(-1.6×10-19) = -2C

 

Card 4: What is current?

Current

An electric current I is a measure of the rate of flow of electric charge (Q) through a given cross-section of a conductor.

Card 5: Direction of Current

Direction of Current

direction of current

Conventionally, the direction of the electric current is taken to be the flow of positive charge.

The electron flow is in the opposite direction to that of the conventional current.

Card 6: Unit of Current

Unit of Current

The SI unit for current is the ampere (A).

Therefore, we can say that a current of one ampere is a flow of charge at the rate of one coulomb per second. (Note: This is not a definition of ampere.)

 

Card 7: Formula of Current

Formula of Current

Example 2
If 30 C of electric charge flows past a point in a wire in 2 minutes, what is the current in the wire?

Answer:

Charge, Q = 30C
Time taken for the charge flow, t = 2 minutes = 120s
(Since the unit of current, Ampere (A) is also equal to Coulomb per second (Cs-1), the unit of time must be changed to second)
Current, I = ?

\begin{gathered}
I = \frac{Q}
{t} \hfill \\
I = \frac{{30}}
{{120}} = 0.25A \hfill \\ 
\end{gathered} 
            

 

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