[Physics Class Notes] on Electric Current Pdf for Exam

An electric current is a rate of flowing electric charge past a point or region. An electric current exists when there is a net flow of electric charge through a region. The moving particles are called charge carriers, and there may be different types of particles in different conductors. In electric circuits, the charge carriers are electrons moving through a wire. In an electrolyte, the charge carriers are ions, and in an ionized gas, they are ions and electrons. Electric current is measured through a device called an ammeter. Electric current causes Joule heating, which creates light in incandescent light bulbs. 

Types of Electric Current

1.  Alternating current (AC) — In its systems, the movement of electric charge reverses direction periodically. AC is the form of electric power that is most commonly delivered to businesses and residences. Audio and radio signals that are carried on electrical wires are some examples of AC. 

2. Direct current (DC) — It is the unidirectional flow of electric charge and a system in which electric charge moves in one direction only. Sources of direct current are batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. DC flows in a conductor like wire, but can also flow through semiconductors, insulators, or a vacuum as in electron or ion beams. 

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Ohm’s Law

A law regarding the flow of currents was discovered by G.S. Ohm in 1828 before the physical mechanism responsible for the flow of currents was discovered. Let’s imagine a conductor through which a current I is flowing and let V be the potential difference between the ends of the conductor. Then Ohm’s law states that

V ∝ I

or, V = RI

Here, the constant of proportionality R is called the resistance of the conductor. SI unit of resistance is the ohm, denoted by the symbol Ω. Resistance R depends on both the material and the dimensions of the conductor. 

Conventions

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In a conductive object, the moving charged particles that constitute the electric current are known as charge carriers. In metals, the positively charged atomic nuclei of the atoms are held in a fixed position whereas the negatively charged electrons that are charge carriers that are free to move in the metal. In semiconductors, the charge carriers can be positive or negative, depending on the dopant used. A flow of positive charge gives the electric current and has an effect in a circuit the same as an equal flow of negative charges in the opposite direction. 

Measurement of Current 

Current can be measured using an ammeter. It’s also measured by detecting the magnetic field associated with the current. 

Different Techniques to Measure Current are:

Resistive Heating

Joule heating or resistive heating is a process of power dissipation through which the passage of an electric current through a conductor increases the internal energy of the conductor, converting thermodynamic work into heat. James Prescott Joule, the inventor of this phenomenon, immersed a length of wire in a fixed mass of water and measured the temperature rise due to a known current through the wire for 30 minutes. By varying the current and length of the wire, he deduced that the heat produced was proportional to the square of the current multiplied by the electrical resistance of the wire.

P ∝ I²R

This relationship is known as Joule’s Law. The SI unit of energy was named joule and given the symbol J. The SI unit of power, watt (W), is equivalent to one joule per second.

Fun Facts

• Up to 3,000 volts can be measured by a spark of static electricity.

• Lightning bolts travel up to 130,000 miles/hour and its temperature can reach around 54,000° F. Lightning is also a discharge of electricity in the atmosphere.

• Electric eels can produce strong electric shocks of around 500 volts for self-defense and hunting.

• Birds sitting on a power line don’t get electrocuted because when a bird sits just on one power line then it is safe but if the bird touches another line with a wing or by its feet, then this creates a circuit which causes the electricity to flow through the bird’s body which results in electrocution.

• Positive charges repel each other and negative charges also do the same. On the contrary, the two opposite charges (positive and negative) attract each other.

• When coal is burned in furnaces that boil water the steam coming from the boiling water helps generate electricity by spinning the turbines that are attached to generators, coal is also the world’s largest source of energy for the production of electricity.