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