Chapter 5 Energy in the Home
5.1 Work, Energy and Power
Work (joules) = force (newtons) x distance (m)
J (a.k.a. V x m)
N
m
Where
10 Newton (N) = 1 kg or 1 Newton (N) = 100 g
Example -a person at a gymnasium lifts two 200 N weights vertically
1 meter in the air.
figure
1m
How much work has he done?
2(200N) x 1 m = 400 J
1 joule = 1 N x m
energy - the ability to do work
figure
P.E.
KE
The weight now has a Potential
Energy increase of 400 J.
(STORED ENERGY)
If you could use the stored
energy when you lowered it - you
could make it do 400 J of work.
Potential energy - the stored energy of position
Kinetic energy - the active energy of motion
ENERGY AND WORK are both measured in Joules.
Instant practice - p. 89
Power - the rate at which energy is produced, absorbed, or
transferred. Measured in WATTS.
Power (watts)
=
energy (joules)
time (seconds)
1 watt
=
1 joule
sec.
* You can increase the potential energy of an object only if you push
it UPHILL.
(Weight lifter)
How can you tell if you increased the potential energy of
something?
Ask: Did it move up hill?
Electrical Energy and Power
Since electricity is the flow (or motion) of electrons, these principles
can also be applied to it.
1) The more electrons that are moved, the more work is done
2) The more quickly the electrons are move the more power is
used.
Energy
=
voltage (V) x current (I) x time (t)
Energy = VIt
example = a carpenter uses a saw rated @ 115V, 13A for ten
seconds.
How much energy did he use?
Energy = VIt
Power (w) = VI
= 115V x 13A x 10 sec
= 14, 950 J
so, the power of that saw is
Power = 115V x 13A
= 1,495 W
Instant Practice p.92
Measuring Electrical Energy
Because we use so much energy in homes and industry we use
larger units to describe power.
a kilo _______ means 1,000 _________
So, we use kilowatt - hours.
That is - the amount of kilowatts x the number of hours used.
1 kW x h = 1,000 W x 3600 s = 3,600,000 J !
Meters
Review 5.1
Activity 5C (at home)
HWK
Energy Efficiency
Efficiency - the ability of a device to convert electrical energy to
another desired form of energy
Eg. a light bulb produces 5 J of light for every 100 J of electricity used
Efficiency = __energy output____
energy input
Q: Where did the rest of the energy go?
Review 5.2
Household Circuits
Similar to those studied except
__5 J__
100 J
= 0.05
1) Use AC current
(60 Hz or cps)
2) Use higher voltages with wires
(120V Red
Neutral
120V Black)
See other overhead/handout
So…household circuits can operate between 120V and 240V
Circuit Breakers
Circuit breakers - switches that automatically turn off if too much
power goes through the individual circuit (branch)
- caused by:
a) circuit overload - too many appliances in
use at the same time
b) short circuits - where bare wires touch and
cause an undesired low resistance
pathway for electricity
* both can cause wires to get too hot and cause a fire
Main circuit breaker - turns off electricity in all branch circuits at the
same time
How they work -
figure
Outlets - places in the circuit where appliances can temporarily be
introduced into
Polarized - when (+) and (-) terminals are connected differently in an
outlet
Polarized plugs - when plugs have one prong wider than the other in
order to properly fit polarized outlets.
* This ensures that the switch runs off the “hot” wire and that
there will be no voltage left inside the appliance once it is
turned off. - safety device
Grounds - where an extra wire is connected from a circuit to deep in
the ground to provide an “emergency escape” for current to prevent
against electrocution.
figure
GFCI - like an extra circuit breaker in the wall
- deflects all electrical differences between hot and neutral wire
and shuts off
- used in outdoor or “near water” situation
Overloading figure
- In parallel circuits (houses) the total current is equal to the sum of
the currents in each appliance. I total = I1 + I2 + I3
- Power (watts) = I (Amps) x V (volts)
In houses; the maximum power is
P= 15A x 120V
= 180W
- The power in the above circuit is
= 1200W + 100W + 60W
= 1360
- Is the circuit overloaded?
- What would happen in I used a 900W toaster?
Read safety check list
Review 5.3
Chapter Review