Electric Current

Charges   
passing through any cross-section of a conductor in time deltat is called Electric Current.

Unit: The SI unit of current is Ampere

The current will be 1 ampere if 1 coulomb charge flows through any cross-section of a conductor in 1 second.

Charge Carriers: Electric current is due to flow of charge particles. These charge particles are called charge carriers.

• Metals:  In metals charge carriers are negatively charged  e.g electrons
• Electrolytes: In electrolytes charge carriers are negative and positive ions e.g
• Gases: In gases charge carriers are electrons and ions

Current direction:

Conventional current:

The current which passes from a point at higher potential to a point at lower potential as if it shows the movement of positive charges.

Explanation: 

Early scientists regarded an electric current as flow of positive charges from positive to negative terminal of a battery through an external circuit as shown in fig.3.1 below

Fig.3.1 (conventional current direction)

Later on it was found that a current in metallic conductors is actually due to the flow of negative charge carriers called electrons moving in the opposite direction i.e from negative to positive terminal of the battery. But it is convention to take the direction in which positive charges flow.This current is called Conventional Current.

Reason:

The reason is that it has been found experimentally that positive charges moving in one direction is equivalent in all external effects to the negative charges moving in opposite direction. As the current is measured by the external effects, so the current due to the motion of negative charges, after reversing its direction of flow, can be considered as the current flowing due to the positive charges.
Source of current:
A source which produces a constant potential difference across the ends of a conductors is called Source of Current.
Explanation:
 when two conductors at different potential are connected by a wire , current starts flowing through the wire from high potential to the low potential. This current flows until both the conductors are at the same level of  potential. Then the current stops flowing. So current through the wire decreases from maximum value to zero. In order to get a constant current the potential difference across the ends of the wire should remain constant. This is done by connecting the ends of the wire to the terminals of a device called Source of Current.

fig.3.2

Types of Current Source: 

Every source of current converts some non-electrical energy into electrical energy. A few example are mentioned.

• Cell : It converts chemical energy into electrical energy.
• Electric generator: it converts heat energy into electrical energy.
• Thermo couples: It converts heat energy into electrical energy
• Solar cell: It converts light energy into electrical energy.

Fig.3.3(cell source)

Effects of Current:

The presence of current can be detected by the various effects which it produces, e.g

1. Heating effect
2. Magnetic effect
3. Chemical effect

Heating effect:

   Current flows through a metallic wire due to motion of free electrons. These electrons collide with the atoms of the metal and give some of their kinetic energy to these atoms. As a result, it increases the kinetic energy of the vibrations of the atoms of the metal, i.e it generates heat in  the wire. The heat 'H' produced by a current 'I' in the wire of the resistance 'R' during time 't'

Equation

Uses: 

Heating effect of current is used in:

• Electric heater
• Kettles
• Toaster
• Electric iron

Magnetic effect:
When current flows in a straight wire, coil or solenoid, a magnetic field is produced around them as shown in fig.3.4. The strength of field depends on the value of current and distance from the wire.

Uses:
Magnetic effect is used in galvanometers to detect the current. Motors, fans,drill machines and grinders etc use this effect.

Fig.3.5(magnetic effect)