Download Science 9 - Unit D - Lesson 6

Science 9
Sections 3.3 – 3.4
Measuring Energy Input and Output
Power
Power is the Rate at which a device converts
energy
Unit D: Electricity
84 mins
Watt (W) = Joules/second
Formula:
Power = Voltage x Current
P = VI; V = P/I; I = P/V
Hair Dryer
1000W plugged to a 120V outlet
I = 8.33 A
TV draws 1.5 A is connected to a 120V outlet
P = 180W
Energy
Energy = Power x Time
E = Pt; P = E/t; t = P/E
Mircowave
800W
cook roast for 30min
E = 1440000J = 1.4 MJ
Lightbulb
60W
left on for 1hr
E = 216000 = 0.2MJ
Kilowatt Hours
It doesn’t take long for a device to use up a lot of
energy, so we sometimes use kW h to measure
power consumption
E = Joule = Watt x second
E = kW h = 1000Watt x 60 sec = 60000 Joules
Energy Dissipation
Law of Conservation of Energy
Energy is neither created or destroyed just charges
forms.
Most of time some energy is ‘lost’ and not used to
do work for the device
Input Energy > Output Energy
Eg.
Light bulb creates light.. but also a lot of heat
Efficiency
%Efficiency = Joules of Output/Joules of Input x
100
Efficiency = Eout/Ein x 100%
100J into a bulb
5 J out as light… 95J of heat
Efficiency = 5J/100J x 100% = 5%
Chart on pg. 336
Reducing the Energy Wasted by Devices
EnerGuide
Large Appliances have Label that will tell you how
much energy the appliance will use per month.
Look for them when you are buying appliances
Limits to Efficiency
Moving parts always lose energy to friction
Pg. 340 Electric motor vs. Internal combustion
Increasing Efficiency
- Reduce Moving Parts
- Insulate
- Use more efficient technology
Reducing Friction
Fridges stay colder, ovens lose less heat
LED Bulbs vs. Incandescent Bulbs
Questions 1-8 Pg. 343