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 EMFor Voltage to appear acrossthe 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?

Q

E

Q

WV

Current and Charge The smallest charge is a single electron which

has 1.6x10-19 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?

t

QI

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 VB = 3

what are the voltages VAB and VBC?

A

+

V=10V

-

C

VBC

VC

Figure for Question 1.2 A cell and two resistors in series

I

VB

B

VAB

VB VA

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.

VIt

VQ

t

EP

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

VR

VL

I

+-

+ -

Internal Resistance and Voltage Drop Example: A cell V has an internal voltage 1.8

V and the lost voltage VL 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

VR

VL=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 VBC = 15 V, and the voltage across Cell 2 is VAB = 15 V. If we set VC

= 0, then the voltage across two cells VAC is?

Figure 1.4 Two cells in series

Cell 1VB+- +-

Cell 2

VABVBC

VCVA

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)