# Electric Current (SS)

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The electric charges in motion is called electric current and it forms the basis of current electricity. Static electricity, or electrostatics, on the other hand involves charges at rest.

Electric current (I) is the rate of flow of charges (Q).

• SI unit: Ampere (A)
• Can be measured by an ammeter (must be connected in SERIES to the circuit)

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

A current of one ampere is a flow of charge at the rate of one coulomb per second.

For electric current in a metal conductor (a solid), the charge carriers are electrons. For historical reasons, the direction of the conventional current is always treated as the opposite direction in which electron effectively moves.

• Current in gases and liquid generally consists of a flow of positive ions in one direction together with a flow of negative ions in the opposite direction.

Electric current generates a magnetic field. The strength of the magnetic field depends on the magnitude of the electric current.

Current electricity consists of any movement of electric charge carriers, such as subatomic charged particles (e.g. electrons having negative charge, protons having positive charge), ions (atoms that have lost or gained one or more electrons), or holes (electron deficiencies that may be thought of as positive particles)

• If the direction of the current (charge flow) is fixed, it is known as a direct current. If the motion of the electric charges is periodically reversed; it is called an alternating current.

## Analogy to river:

In order to help you understand the concept of current better, you can think of a river. Current in an electric circuit is similar to water flowing through the river.

## Examples:

An electric current in a wire involves the movement of

1. electrons
2. atoms
3. molecules
4. protons

A. Electric current in a wire (solid conductor) involves the movement of electrons.

The lower part of a cloud has a positive charge. The cloud discharges in a flash of lightning. In which direction do electrons and conventional current flow?

As the cloud is positively charged, negative charges (electrons) flow upwards. Therefore, the conventional current flow is in the opposite direction (downwards).

A battery moves a charge of 60 C around a circuit at a constant rate in a time of 20 s. What is the current in the circuit?

Current is the rate of movement of charge.

\begin{aligned} I &= \frac{Q}{t} \\ &= \frac{60}{20} \\ &= 3.0 \, C/s \\ &= 3.0 \, A \end{aligned}