#### Transcript Electrical energy and power

```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 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
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 = I2 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 = 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
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)
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 = 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 10-19 Coulomb
Electrical Power Dissipation in
a Resistor
P = VI
P=
I = V/R
V = IR
V2
P = I2 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 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 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