Engineering Science EAB_S_127 Electricity Chapter 1

Electrical Energy  Energy cannot be created or destroyed, however, it may be converted from one form into another.  In the next four lectures we are going to investigate electrical energy and its application, from basic concepts to electric circuits.  A cell is a device that can generate electricity, more precisely, it is a device that converts stored chemical energy into electrical energy  An electrolyte causes EMF or Voltage to appear across the terminals of the cell

Conventional Current  In reality the flow of current relates to the movement of charged particles (i.e. electrons) which are in fact negatively charged through conductive material (e.g. metal wires)  However, historically scientists have considered the flow of current from high to low potential (voltage)  This is considered “Conventional Current” and most scientists and engineers use this and not “electron flow”.

Charge and Voltage  Cells have two principle parameters, the Charge stored, Q and the terminal Voltage, V.  Charge is measured in Coulombs [C]  Voltage is measured in Volts [V]  Voltage is the Energy Stored per Coulomb of Charge  Where W = Energy Stored in Joules [ J ] and Q = Charge  Example: A cell uses 1500 Joules of energy to generate 1000 Coulombs of charge, what is its voltage?

Current and Charge  The smallest charge is a single electron which has 1.6x Coulombs  The rate of flow of electrical charge is termed ‘Current’  Where Q = Charge [C] and t = time [s]  Current is measured in Amps [A]  Example: If 1000C of electrons travel through a wire in 100s, what is the current in A and mA?

Resistance  Resistance is the property of a material to “resist” the flow of current  Conductors have low resistivity per unit area  Insulators have high resistivity per unit area  The flow of current through a resistive material causes a potential difference (or voltage) to develop across it  Fixed external resistors are very useful circuit components and are made from materials with a known resistivity per unit area

Voltage Divider Example  Given that V = 10 and the voltage at V B = 3 what are the voltages V AB and V BC ? A + V=10V - C V BC VCVC Figure for Question 1.2 A cell and two resistors in series I VBVB B V AB VBVB VAVA

Electrical Power  Electrical power, P, is given by the amount of electrical energy converted (or absorbed) per unit time in Watts [W]  Hence  Where E = Energy [ J] absorbed, t = time [s], Q = Charge [C] and I = Current [A]  Example: A DC motor consumes 2000J of electrical energy per second when it is in use. Find:  a) the power consumed by the motor  b) given that the motor requires 200V to operate deduce the electric current flowing through the motor.

Internal Resistance of Cells  All the materials inside cells have some resistance  The resistance inside a cell is called its “internal resistance”, this is denoted by r, and causes a voltage loss when loaded by an external resistance + V - External resistor, R r VRVR VLVL I +- +-

Internal Resistance and Voltage Drop  Example: A cell V has an internal voltage 1.8 V and the lost voltage V L is 0.3 V (dropped across its internal resistance). What is the terminal voltage V across an external resistor? + V =1.8V - External resistor, R r VRVR V L =0.3V I +- +-

Connecting Cells in Series  In order for cells to be connected in series the positive terminal of Cell 1 must connect to the negative terminal of Cell 2  The voltage across Cell 1 V BC = 15 V, and the voltage across Cell 2 is V AB = 15 V. If we set V C = 0, then the voltage across two cells V AC is? Figure 1.4 Two cells in series Cell 1 VBVB Cell 2 V AB V BC VCVC VAVA

Connecting Cells in Parallel  In practice two (or more) cells with the same voltage can be connected in parallel  The voltage across the connected cells is the same as that across any single one of them  The current available from the connected cells is then multiplied by the number of cells (as the internal resistance is effectively reduced)