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Chapter 9: circuits are designed to control the transfer of electrical energy
2 types of circuits
1. Series circuits
–only one path for current to travel
– bulbs connected in series when one goes out they all do.
–add more bulbs then the brightness decreases (voltage is affected)
– increasing the resistance (adding more bulbs) decreases the current
– ex Christmas lights
2. Parallel Circuits
- a circuit with several different paths
- bulbs connected in parallel when one goes out the others remain lit
- add more bulbs the brightness is unaffected (voltage does not change)
- the resistance does not increase
- no current is created or destroyed only split up
- Adding resistors in parallel will decrease the total resistance. This will increase
the total current leaving the battery.
Fill in the following table for series and parallel Circuits
Series circuit
Parallel Circuit
# of
one
Two or more
pathways
Effect of
removing
a load
No electrons move circuit Electrons continue to flow through
is broken
remaining paths
Voltage
drop
Add up all the voltage
lost and it will be the
same as the voltage
supplied
Add up all the voltage lost in one
pathway and it will be the same as
the voltage supplied
Current
Current is the same
everywhere in the circuit
Total current entering or leaving a
junction point is equal to the sum of
the current in individual paths
Current depends on total
resistance in the circuit
Resistance increase amt of resistors
then you increase total
resistance
increased total
resistance means overall
current decreases
Connecting Cells connected series
cells to
-sum of the voltages is
form a
effective voltage
battery
-overall life span same as
individual cell
-ex flashlight (increased
voltage =increased
current= more bright
Current in each path depends on the
total resistance in that path
Place resistors in parallel then total
resistance decreases
decreased total resistance means
overall current increases
Cells connected parallel
- Effective voltage same as the
voltage of single cell
- overall lifespan is the sum of life
spans for all the cells
Ex lighthouse increased
lifespan=decreased maintenance
Technologies for Safe Use of Electricity
1. fuses (contain metallic conductors that melts when excessive current heats it up.
2. circuit breakers (act as a switch to cut power coming into your house)
3. grounding terminals (allows excess current to flow in to the ground instead of
giving you a shock.
Electrical Energy
- The ability to do work
- Measured in joules (J)
Electrical Power
- The rate of change in electrical energy
- The rate at which work is done or energy is transformed
- Measured in Watts (W)
Power is the rate at which work is done, for example electrical power (how much heat an
electrical hair dryer outputs per a given time interval) or mechanical power (how much an engine
can move per a given amount of time.)
Electrical Energy Costs
Depends on three factors
1. voltage drop
2. electrical current
3. time
Power = Voltage x Current P=VI
For example oven, baseboard heaters and clothes driers all operate on 220V verses
110V so the operating cost will be greater.
A toaster and a radio both operate at 110V but it cost more to operate the toaster
for the same amount of time because the current to the toaster is higher.
Power Rating
A measurement of how much electrical energy an electrical device consumes for
every second it is in use Households use kW•h.
Energuide Labels – details how much an appliance uses in a year compared to other
appliances. See page 320
Calculating Energy Consumption
E = energy transferred (J) or
kW•h
P = power (W) or (kW)
T = time (s) or (hour)
Energy consumption page 308
For household use the units we use are kW•h
Energy consumed then is measured in a kW•h
Power in kW (kilowatts) and time in hours.
(the numbers would be too massive otherwise)
How much energy is consumed if a 40 kW oven was in use for two hours?
Energy = Power x Time
Energy = 40kW x 2 hours
Energy = 80 kW•h
Cost of energy used is then the cost per kW•h multipled by the number of kW•hs.
For example if the power company charged 10cents per kW•h.
To run the above oven would cost 0.10 X 80 kW•h = 8.00 dollars
Using the same charges how much would it be to run a 2kW oven for 15 mins?
Energy = power X time
= 2 kW x 0.25 hours
= 0.50 kW•h
Cost = 0.10 X 0.50 kW•h
= 0.05 cents
Complete 9-2B page 310
Electrical Energy is converted to many forms including light, heat and sound.
For example the electrical energy that goes into a burner is converted to heat
(transferred to the pot) light (glows red) and sound (sometimes little crackles as it
heats up
Careers related to Electricity
- Electrician
- Photocopier technician
Electrical Efficiency
Electrical energy may be converted to other forms such as light, heat, sound
Efficiency is the % of energy that is converted to useful form.
Efficiency = useful energy output x 100%
Total energy input
For example, an incandescent light bulb left on for 10.0 h/week uses 7.0 kW-h of energy to
supply 0.4 kW-h of light. A compact fluorescent bulb uses 2.0 kW-h in the same amount of time
to supply the same amount of light.
% efficiency = Energy output x 100%
% efficiency = Energy output x 100%
Energy input
Energy input
Incandescent: 0.4 kW-h x 100% = 6%
Compact
efficiency
fluorescent: 0.4 kW-h x 100% = 20%
7.0 kW-h
efficiency
2.0 kW-h
The Compact fluorescent is more efficient because it converts more electrical energy to light.
Complete practice problems page 318 and 319
Reducing Electrical consumption
- Improve home insulation
- Turn off lights when not in use
- Use energy efficient bulbs
- Air dry clothes when possible
Energy Generators: electrical generator (made up of a coil of wire and a magnet)
Electrical Generating Stations page 327
1. Hydroelectric - water (mechanical energy) runs the generator
2. Thermal – heat (steam) from burning of fossil fuels runs the generator
3. Nuclear – heat (steam from nuclear reactor runs the generator
Getting power
Transformers are electrical devices that changes voltage so that it can be used by
homes (120V and 240V). They can step up (increase) voltage or step down
(decrease) voltage. Ex. Power adaptors are transformers (have to have the proper
adaptor to rum your game system)
Problems
1. safety 2. cost of production 3. degree of environmental impact
Alternate Energy Sources
- Wind Generators
- Solar Energy
- Fuel Cells
Constraints of new Technologies
1. cost
2. availability of materials
3. properties of materials
See page 330-4