Principle of Engineering Electrical energy and power Electricity Session 3 (2 hours)

Energy 能量 Energy is required to: –Heat up an object (thermal energy) –Move an object (mechanical energy) –Light up a lamp (electrical energy) Unit: Joule or JJoule –The fundamental unit of energy (as well as heat and work) in the MKS system of units, equal to 1 kg m 2 s -2, or 1/4.184 of a calorie.energyheat workMKScalorie Energy conversion: –Electrical  Thermal (class example: … ) –Electrical  Mechanical (class example: … ) –Mechanical  Thermal (class example: … )

Thermal Electrical Mechanical Sound, light, ….

Joule, James ( ) Joule, James ( ), English physicist who was initially interested in the efficiency of electric motors. He discovered the heat dissipated by a resistor is given by: Heat Energy = I 2 R t where I is the current, R the resistance, and t the time Joule ’ s Law

Joule, James ( ) James Joule discovered heat dissipate in resistor with current flow (Joule ’ s Law: Heat Energy = I 2 Rt) James Joule also discovered the mechanical equivalent of heat in the 1840s in which he found the friction of dropping 772 lbs weight through 1 foot would cause 1 lb water to rise 1 o F

Law of Energy Conservation governs all energy-exchange phenomena of nature Input = Output + Losses Energy may be “ consumed ” or “ stored ” System Input Output Losses

Power ( 電 ) 功率 Power = rate of energy usage Energy = how much Unit: Watt (W) 100W bulb – uses 100 J/s 40W bulb – uses 40 J/s Or 1 J = calorie for the health conscious YOU!

Power Formulae Power = Rate of Energy Usage = Energy Used / Time Compare to: – Speed = rate of distance traveled = distance / time Power = Energy / Time or Energy = Power * Time

Turning On a 100W Bulb for One Hour Energy = ? Energy = 100 W * 3600 s = 360,000 J Cost = ? CLP charges by KW-hr: $0.90 per KWhr The bulb uses 100W * 1 hr = 100 Whr or 0.1 KWhr. It costs $0.09

Electrical Power Energy is the “ Work Done ” to move charges (Q) across a potential difference (V) Energy (E) = Q V It takes 1 J to move 1 Coulomb (C) of electrical charge across a potential difference of 1V Power (P) = Rate of Energy = rate of flow of charges * potential difference = I*V 1 e - = 1.6 x Coulomb

Electrical Power Dissipation in a Resistor P = VI P = V 2 / R P = I 2 R I = V/R V = IR Caution: V is the voltage across the resistor in consideration, not the supplied voltage!

Power Examples What is the Power rating of a Hair Dryer for use in Hong Kong that takes 5 A? P = VI = = 1100 W What is the Ohmic value of the heating element in the hair dryer? R = = 44 Ohm What is Power (heat) consumed in passing a 5A current through a 0.5 Ohm wire? P = = 12.5 W Does it matter on the magnitude of voltage so long there is 5A flowing?

Switches Single Pole Single Throw (SPST) 單刀單擲 Single Pole Double Throw (SPDT) 單刀雙擲 Double Pole Double Throw (DPDT) 雙刀雙擲 Single Pole – controls one wire Single throw – makes one connection (throw) Double throw – makes two connections Double Pole – controls two wires

SPST Switch For simple on- off control.

SPDT Switch Single Pole (Connect 1 wire) Double Throw (to 2 connections) For 3-way light circuit

DPDT Switch 2 SPDT switches with handles attached For polarity reversal

3-Way Light Circuit Implement this circuit.

Reverse Polarity Complete the circuit and implement it. motor

Relay Switch is activated electrically by solenoid. Use a small current to control a large current (IC controls relay and relay controls power consuming devices) Use DC to control AC SPST

Relay (DPDT) Use electromagnet to control DPDT switch Need 6+2 = 8 connectors Demonstrations/ applications

Bread Board Bread Board For easy connection of components Each hole is connected to others in a row via metal strips underneath