Chapter II

Introduction to the Electricity

Voltage

Voltage is the energy per unit charge or the difference in potential between two points. The correct name for voltage is the potential difference or p.d.. The S.I. unit of voltage is the Volt [V], but from the definition, it can also be expressed as [N.m.C^-1].

Voltage in Electronic Circuits

· Voltage is supplied by the battery (or power supply).

· Voltage is used up in components, but not in wires.

· We say voltage across a component.

· Voltage is measured with a voltmeter, connected in parallel.

The Link between Current and Voltage

The link between current and voltage is as follows, it is not possible to have a current (a flow of charge) without a difference in potential.

Secondly, in order for the current to flow there has to be a closed path or circuit around which the charge can flow. A disconnected battery has a voltage but there is no current because there is no closed path for the electrons to flow around.

Hydrodynamic Model of Voltage

Often, an analogy between electricity and water is made. The voltage is like the pressure, and the current is analogous to the flow of the water through the pipes.

It is easy to imagine, that when we increase the pressure of the water the flow rate will increase and it is the same with electricity. If we increase the voltage, then the rate of flow of charge willl increase, which is the current.

There is another factor which is important in the link between current and voltage and that is the resistance. The resistance is the coefficient of proportionality which determines the current for a given voltage.

For example if we have a 6 V battery and we set up a circuit which allows a current to pass through a resistor. The current flowing through the resistor can be measured using an ammeter. Let’s say the ammeter gives a reading of 0.02 A. The resistance of the resistor would be 6 V/(0.02 A) = 300 Ω

This leads us to the famous equation V =IR, which is also known as Ohm’s Law.

Current and Voltage in Circuit Components

Series

For components in series, the current I is the same in all the components but the voltage is different.

Parallel

For components in parallel, it is the voltage that is the same in all components but the currents are different.

Electric Current

Electric current is the flow of charge. The SI unit of current is the Amp. [A].

In mathematical terms we can describe the current as the rate of change of charge with time.

I = dQ/dt.

In order for the charge to flow there has to be a potential difference V.