Understanding Electric Charge in Electrical Engineering

Electric Charge

The International System Unit of charge: C (Coulomb)

Electricity is based upon the existence of electric charges, which are positive and negative. A force exists between electric charges, which is described by Coulomb’s law. Like charges repel each other and unlike charges attract each other.

From the physical point of view, every charge is a multiple of the elementary charge e.

Elementary charge e ­ ±1.602 × 10^-19 Coulomb

Electrons carry a negative charge and Protons carry a positive charge. A lack of electrons in a body means the body is positively charged. Similarly, an excess of electrons means it is negatively charged.

Electric Current

The international Systems Unit of Current: A (Ampere)

The directed motion of electric charge carriers is called an electric current.

I = dQ/dt

The electric current I in a conductor is the charge dQ passing through the conductor cross-sectional area during the time interval dt. The current is a direct current if the charge passing the conductor per time interval is constant.

DC current: I = dQ/dt = constant

Technical direction of current:

The positive current direction is the motion of the positive charge carriers. This is equivalent to the opposite motion of negative charge carriers. In metal conductors electrons are the charge carriers. From the physical point of view, the electrons therefore move opposite the positive current flow.

Electric charges always move in a closed loop. This means:

The electric current always flows in a closed circuit.

Voltage and Potential

The International Systems unit of Voltage: V (Volt)

The electric voltage is the force that causes the movement of the charge carriers.

The electric current always flows from the positive terminal to the negative terminal of the voltage source. Since the current flows in a closed loop, inside the voltage source the current flows from the negative to the positive terminal.

The potential φ is a scalar quantity. Given that one point in space has the potential φ = 0, then all the other points in space can be assigned an absolute potential. This potential is obtained from the energy that has to be provided to move the elementary charge from the point with φ = 0 to the given point. In this physical model, the voltage V is the difference between two potentials. For this reason voltage is often referred to as potential difference.

V₂₁ =φ₂ – φ₁

Ohm’s Law

The current flowing through a load is dependant on the driving voltage. Provided the properties of the load are independent of the current flowing through it and the voltage applied to it.

The current changes proportionately with the voltage. The constant R relating current and voltage is called electric resistance.

Resistance and Conductance

SI unit of resistance is Ω (Ohm), 1Ω = 1 V/A

SI Unit of conductance: S (Siemens), 1S = 1 A/V

The relationship between current and voltage is described by the quantities resistance R and conductance G.

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