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ELECTRIC CURRENT
Electric current:
Electric current is
- the rate of flow of electric charges.
- These electric charges can be electrons, holes or ions based on the material we use. In circuit theory, we mainly deal with conductors (wires), hence here electric charges = electrons.
Hence,
Electric current is the rate of flow of electrons in a conductor
$$i(t)=\frac{dq(t)}{dt}$$
Unit: ampere (A) or coulombs/sec
Practice Question - TRB 2006
Which of the following equation is correct?
a) $I=Q×t$
b) $I=Q/t$
c) $Q=I/t$
d) none of these
Correct Answer: Option B
Current is defined as rate of flow of electric charges $$i(t)=\frac{dq(t)}{dt}$$
If the amount of flow of charges is steady, we can rewrite the above equation as $$I=\frac{Q}{t}$$
Conventional current:
- As we know, charge can be positive or negative. Also positively charged particles flow in opposite direction to that of negatively charged particles. Hence we have two choices to define the direction of current flow (as the rate of flow of charges constitutes current). To avoid such confusions, we need a convention to define the direction of current flow.
- Benjamin Franklin’s One-fluid theory provides the basis for conventional current. According to One-fluid theory, a positively charged object would contain too much fluid, while a negatively charged object would contain too little fluid. Electricity was thought of flow of fluid from ‘too much fluid’ to ‘too little fluid’.
- This convention was adapted as current flow direction, even after the discovery of electrons.
- Hence, conventional current flow = opposite to the direction of flow of electrons or as same as the direction of flow of positively charged particles.
Practice Question
Unit of current is
a) ampere
b) coulombs/sec
c) ampere – sec
d) both A and B
Correct Answer: Option D
Unit of current is ampere
Since current is nothing but rate of flow of electric charges $$i(t)=\frac{dq(t)}{dt}$$
Hence unit of current is also coulombs per second
Types of Current:
Direct Current (DC):
- Steady flow of electric charges in one direction constitutes direct current.
- DC current always flows from positive terminal to negative terminal.
- DC is always constant over time.
- DC has zero frequency.
DC source:
- Batteries
- Solar Panels
- DC generators
- Rectifiers, etc.,
Alternating Current (AC):
- Bi-directional flow of electrons constitute alternating current.
- AC is alternating in nature over time.
- AC voltage changes its polarity continuously on its own.
- AC has frequency ranges from $1$ Hz to infinity.
AC source:
- Alternators
- AC generators
- Inverters, etc.,