Download 16.3 and 16.4

16.3 Electric Circuits
What did Ohm discover? In the 1800s, George Ohm performed many
experiments on electrical resistance. Ohm found that the current,
voltage, and resistance in a circuit are always related in the same way.
Ohm discovered that changing the voltage in a circuit changes the current
but does not change the resistance. Ohm concluded that conductors and
most other devices have a constant resistance regardless of the applied
voltage.
Ohm’s Law – Ohm created a law that describes how voltage, current, and
resistance are related. Ohm’s law says that resistance in a circuit is equal
to voltage divided by current.
Or rearranged:
Resistance (Ω)= Voltage (V)
Current (A)
Voltage = Current x Resistance
What is a circuit made of? Objects that use electricity contain circuits.
All electric circuits have these basic features: devices that run on
electrical energy, sources of electrical energy, and conducting wires.
 Batteries and power plants are examples of energy sources. They
supply the voltage that causes current to flow. When the energy
source is a battery, current flows from the positive end to the
negative end.
 Energy is always conserved in a circuit. Electrical energy doesn’t get
used up. It gets transferred into other forms of energy, such as heat,
light, mechanical, and sound energy.
 Electric circuits are connected by conducting wires. The conducting
wires complete the path of the current. They allow charges to flow
from the energy source to the device and back to the energy source.
 A switch is often included to control the current. Opening a switch
breaks the circuit, which shuts off the device.
Series Circuits – If all the parts of an electric circuit are connected one
after another along a path, the circuit is called a series circuit. It only has
ONE path for the current to take – so if one light goes out, the other
lights go out as well. The lightbulbs in the circuit will become dimmer as
more bulbs are added.
Parallel Circuits – different parts of the circuit are on separate branches.
There are several paths for current to take. Each bulb is connected by a
separate path from the battery and back to the battery.
If one light burns out in a parallel circuit, charges can still move through
the other branches, so the others remain lit.
Overall resistance decreases as new branches are added, so the
brightness of the light bulbs does not change. Remember, for a given
voltage, if resistance decreases, current increases.
16.4 Electric Power and Safety
https://www.youtube.com/watch?v=4SIT0aU_FRw
The rate at which energy is transformed from one form to another is
known as power. The unit of power is the watt (W).
Power Ratings: A bright light bulb might be 100W. The power rating of a
dimmer bulb might be 60W. The brighter bulb transforms (or uses)
electrical energy at a faster rate than the dimmer bulb.
Calculating Power: The power of a light bulb or appliance depends on
two factors: voltage and current. Power is calculated by multiplying
voltage by current.
Power (W) = Voltage (V) x Current (A)
Rearranged: Current = Power
Voltage
Paying for Electrical Energy – the electric bill that comes to your home
charges for the month’s energy use, not power. Power tells you how
much energy an appliance uses in a certain amount of time. The total
amount of energy used is equal to the power of the appliance multiplied
by the amount of time the appliance is used.
Energy = Power x Time
Kilowatt-hours = Kilowatts x Hours
So if a refrigerator averages a power of 0.075 kW (75W). Multiply by 720
(hours in one month). Energy equals 54 kWh
How can Electric Shocks be prevented? A short circuit is a connection
that allows current to take the path of least resistance. Touching a frayed
wire causes a short circuit, since current can flow through the person,
rather than through the wire. Since the new path has less resistance, the
current can be very high.
Many bodily functions, such as heartbeat, breathing, and muscle
movement are controlled by electrical signals. Because of this, electric
shocks can be fatal.
Shocks can be prevented with devices that redirect current or break
circuits.
Ground wires connect the circuits in a building directly to Earth, giving
charges an alternate path in the event of a short circuit. The third prong
you may have seen on electrical plugs connects the metal parts of
appliances to the building’s ground wire. A circuit connected to Earth in
this way is grounded.
The circuits in your home also contain devices that prevent circuits from
overheating, since overheated circuits can result in fires. Fuses are
devices that melt if they get too hot, which breaks the circuit. Circuit
breakers are switches that will bend away from circuits as they heat up.
Unlike fuses, which break when they are triggered, circuits breakers can
be reset.