Principle of Principle of EngineeringEngineering

Electrical energy and powerElectrical energy and power

Principle of Principle of EngineeringEngineering

Electrical energy and powerElectrical energy and power

Electricity Session 3 (2 Electricity Session 3 (2 hours)hours)

Energy Energy 能量能量• 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 m2 s-2, or 1/4.184 of a calorie.

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

Thermal

Electrical

Mechanical

Sound, light, ….

Joule, James Joule, James (1818-(1818-

1889)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 EnergyHeat Energy = = II22 R t R twhere I is the current, R the resistance, and t the time

Joule’s Law

Joule, James Joule, James (1818-(1818-1889)1889)

• James Joule discovered heat dissipate in resistor with current flow (Joule’s Law: Heat Energy = I2Rt)

• 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 1oF

Law of Energy Law of Energy ConservationConservation

• governs all energy-exchange phenomena of nature

• Input = Output + Losses• Energy may be “consumed” or

“stored”

SystemInputInput OutputOutput

LosseLossess

Power (Power ( 電電 )) 功率功率

• 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 = 4.184 calorie for the

health conscious

YOU!

Power Formulae Power Formulae

• Power = Rate of Energy Usage = Energy Used / Time

• Compare to:– Speed = rate of distance traveled

= distance / time

Power = Energy / TimePower = Energy / Time

or

Energy = Power * TimeEnergy = Power * Time

Turning On a 100W Bulb for One Turning On a 100W Bulb for One HourHour

• 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 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 10-19

Coulomb

Electrical Power Dissipation Electrical Power Dissipation in a Resistorin a Resistor

P = VIP = VI

P = V2 / RP = V2 / R P = I2 RP = I2 R

I = V/R V = IRV = IR

CautionCaution: V is the voltage across the : V is the voltage across the resistor in consideration, not the supplied resistor in consideration, not the supplied voltage!voltage!

Power ExamplesPower 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?

SwitchesSwitches• 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 SwitchSPST Switch• For simple

on-off control.

SPDT SwitchSPDT Switch• Single Pole (Connect

1 wire)• Double Throw (to 2

connections)• For 3-way light circuit

DPDT SwitchDPDT Switch• 2 SPDT

switches with handles attached

• For polarity reversal

3-Way Light Circuit3-Way Light Circuit

Implement this circuit.

Reverse PolarityReverse Polarity• Complete the circuit and

implement it.

motor

RelayRelay• 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)Relay (DPDT)

• Use electromagnet to control DPDT switch

• Need 6+2 = 8 connectors

• Demonstrations/ applications

Bread BoardBread Board•For easy connection of components

•Each hole is connected to others in a row via metal strips underneath