Principle of Engineering Electrical energy and power Electricity

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 J – 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. • Energy conversion: – Electrical Thermal (class example: … ) – Electrical Mechanical (class example: … ) – Mechanical Thermal (class example: … )

Thermal Mechanical Electrical Sound, light, ….

Joule, James (1818 -1889) • Joule, James (1818 -1889), 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 (1818 -1889) • 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 1840 s in which he found the friction of dropping 772 lbs weight through 1 foot o would cause 1 lb water to rise 1 F

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

Power (電)功率 • • • Power = rate of energy usage Energy = how much Unit: Watt (W) 100 W bulb – uses 100 J/s 40 W bulb – uses 40 J/s Or 1 J = 4. 184 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 100 W 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 100 W * 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 1 V • Power (P) = Rate of Energy = rate of flow of charges * potential difference = I*V 1 e- = 1. 6 x 10 -19 Coulomb

Electrical Power Dissipation in a Resistor P = VI P= I = V/R V = IR V 2 P = I 2 R /R 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 5 A current through a 0. 5 Ohm wire? • P = = 12. 5 W • Does it matter on the magnitude of voltage so long there is 5 A 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 onoff 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 • For easy connection of components • Each hole is connected to others in a row via metal strips underneath