Download Chapter 19 - Electricity

Electricity
The basis for the study of electricity begins with the electron. It is a small, negatively charged
particle located outside the nucleus in all atoms. The nucleus of the atom is positively charged
due to the protons within. The electrons and protons have an attraction for each other due to
their opposite charges.
In a neutral atom, there are as many electrons as protons, the charges balance. If an electron is
removed, the atom has one more proton than electron and the atom is positively charged.
Electrons can be removed from atoms rather easily.
Rub a balloon on your hair and stick the balloon on the wall. Why does it
stick? The rubbing has removed electrons from your hair and transferred
them to the balloon. Your hair has lost electrons so it is electron deficient. It
has a positive charge. The balloon has an excess of electrons and it has a
negative charge. The number of electrons changes due to the transfer of
electrons.
Electric Current
When a metal bar is heated at one end, the heat flows from high temperature to low
temperature since there is a difference in temperature at the two ends. In an electrical circuit,
there is an electrical potential difference at opposite ends of the circuit. Without this difference,
electricity will not flow. Heat will flow until the temperatures are equal. Current will flow until
the electrical potentials are equal.
If I become charged by rubbing my feet on the carpet and then touch someone we get a
shock. The electric potential is different between the two of us. The shock acts to equalize the
potentials between us. Water will flow from a reservoir to a container until the water levels
are equal. Electrical current will flow until electrical charges are equal at the two ends of the
circuit.
Electrons are the carriers of electrical charge which move within the circuit. The rate of electron
flow is the current. More electrons per second, the greater the current. The unit of current is
the ampere.
Sources of Current
Charging my body by rubbing my feet on the carpet is not a good source of electricity. I
discharge myself in one large shock so I cannot release this charge slowly at a steady rate.
Batteries are much better at this. Batteries contain two metallic compounds, one which is an
electron source, one which is electron target. Electrons will flow from one substance to another
due to a chemical reaction occurring within the battery. A piece of carbon is placed between
the two chemicals to carry the electrons which separates the two compounds. As electrons flow
from one end of the carbon rod to the other, we can use these electrons to light a bulb by
making the electrons flow through a wire as they make their way from one chemical
compound to the other.
Electrons will only flow if we connect the two end of the carbon rod together with a wire,
0therwise, electrons do not flow within the battery. Batteries supply the work done to pump
electrons from one area of high electric potential to low electric potential. Work and potential
energy have units of joules. The greater the potential difference in the battery, the more
energy the electrons have. Voltage is the measure of energy per charge. The greater the
potential difference, the greater the voltage.
Let’s use an analogy of water flowing from one container to another. A 2-liter soda bottle is
filled with water. Some holes are punched near the top, some near the bottom of the bottle.
As water flows out of the holes, the water from the top hole does not flow out very far. The
water pressure is not very great. The difference in height between the water and the hole is
the potential difference. We can say the voltage is low. Water leaving the hole near the
bottom of the bottle is under greater pressure, it flows out farther than the top hole. We can
say the voltage is high.
Ah, you say the amount of water per second flowing out of the top hole is less. The current is
less. Let’s make the hole on the top larger to let more water out. The pressure is the same, but
more water per second flows out. Now the amounts of water flowing out of the two holes is
the same, but the hole on the bottom is under greater pressure. Equal amounts of water,
equal current. Different pressures, water flows out more forcefully. More force, more voltage.
When speaking of an electrical circuit, we say current flows through the circuit. Voltage does
not flow through a circuit. Voltage is a measure of the difference in electrical potential. Water
flows out of the bottle. The pressure doesn't flow out of the bottle. Pressure is a measure of the
difference in height of the water and the hole. Voltage provides the pressure to push electrons!
No voltage, no pressure. No pressure, no electrons. What is measured in a circuit is the number
of electrons flowing: The current. This is what you pay for.
Electrical Resistance
Let's go shopping!! As I push my way through the crowd, I squeeze in through the door of the
store. I wonder, why doesn't the manager open another door! More people could get in faster!
The door is the resistance, I am the electron, or the current. The more people in line behind me,
who are pushing to get in is the voltage.
Every appliance which uses electricity allows electrons to pass at a certain rate. Some easily, or
with low resistance, and some more difficult, or high resistance. The resistance of a wire
depends on its length and its thickness. A short wire, low resistance (short hallway). A long wire,
high resistance (long hallway). A thick wire, low resistance (large door). A thin wire, high
resistance (small door). Electrical resistance is measured in Ohms.
Ohm's Law
The amount of electricity flowing through a conductor is dependent upon the voltage
(pressure), the current (number of electrons), and the resistance.
Voltage = current x resistance
Current = voltage / resistance
Resistance = voltage / current
If the resistance in a circuit is constant, the current and voltage are directly proportional. Twice
the current, twice the voltage. If the resistance is doubled for a given circuit (constant voltage),
then the current is halved.
For a household light bulb, the filament (wire inside the light bulb) is around 100 Ohms. With
120 volts, the amperage is about 1.2 amps (120 volts/100 ohms). High resistance in the filament
causes the filament to become hot. You could almost say it glows!!
What Causes Electrical Shock?
The damaging effect of an electrical shock is produced by current. Human skin when dry has a
resistance of 500,000 Ohms, and about 1000 Ohms when wet (water is a good conductor).
Safety Paul says "Don't play with the toaster in the bath tub")
Touching a 120 V outlet gives you a shock, but not enough to kill you because your resistance
is high. Using Ohm's Law: Current = Volts / Resistance, the current received is quite low. Getting
wet decreases your resistance, therefore the current which flows is increased. This will kill you!
The current must flow from one point of your body to the other (high potential to low
potential). Birds sit on high voltage lines all the time. Electricians have been killed by high
tension lines. Why is this so?
Birds can sit on the wires because there is no area of high and low potential in their bodies,
they are not touching anything else for electricity to flow through it. They feel nothing. If we
touch high tension lines, we are usually touching something else. If one hand is on a ladder and
the other touches the wire, electricity flows from the hand on the wire to the hand holding the
ladder, and zap, your dead.
Direct Current vs Alternating Current
Batteries provide a constant flow of electrons. Electrons flow in one direction, from the
negative terminal towards the positive terminal. (remember that electrons are charged
negatively). This is dc or direct current. Household current is ac, or alternating current.
Electrons don't flow like water in a pipe, they move back and forth in cycles. First in one
direction, then in the other around a fixed point. The number of back and forth motions an
electron makes is determined by the power company. For the U.S., it is 60 cycles per second, or
60 Hz. In Europe, it is 50 Hz.
So power companies sell energy to vibrate electrons back and forth, they don't sell electrons.
Batteries are sources of high energy electrons. We buy these.
Power
The rate of energy usage is power. Remember running up the stairs. Work is done in a shorter
time, or it takes more power to run up the stairs than walk. The amount of work done is the
same.
Power = Work (Energy) / time
Power = joules / second
The unit of power is a Watt. A 100 W light bulb uses more power than a 25 W bulb.
It uses the same voltage (120 V).
Power = amperage x voltage
Power = (charge/time) x (energy/charge)
Power = energy/time
Problem:
How many amps are flowing through a 100 W light bulb?
Power = amperage x volts, so
Amperage = power / volts
Amperage = 100 W / 120 V = 0.83 Amps
What is the resistance of the light bulb?
Voltage = Current x Resistance, or
Resistance = Voltage / Current
R = 120 V / 0.83 Amps = 144.6 Ohms
The power company sells electrical energy in a unit of kilowatt-hours. If I run a 1000 W hair
dryer for 1 hour, this is 1 kilowatt-hour. Joules (energy) = power (joules/time) x time
Problem:
I spent the night preparing these notes. I ran a 100 W light bulb (not a candle) for 5 hours.
How many kilowatt hours of energy did I use?
Energy (kilowatt hour) = power x time
kWh = 0.1 kW (100 W) x 5 hours = 0.5 kWh
If the electric company sells a kWh for $0.16, how much did it cost me to use the light bulb?
Cost = Cost/kWh x kWh used,
Cost = 0.5 kWh x $0.16 / kWh
Cost = $0.08, or 8 cents.
Series and Parallel Circuits
A series circuit is shown to the right.
1.
A
B
There is a single pathway for the electrons to flow.
The electrons flowing (the current) through each
bulb is the same.
2. The total resistance is dependent upon the number of the bulbs.
3. The current is equal to the voltage supplied divided by the resistance (Ohm's Law)
4. The voltage supplied is divided by each device. Thus the energy supplied is divided equally
by each bulb.
5. The decrease in voltage is proportional to the resistance. The greater the resistance, the less
energy available to run the light bulbs.
If one lamp filament burns out, then the circuit is broken (opens the circuit) and all of the
lights go out. Adding more lamps increases the resistance, which decreases the current, and the
lights are dimmer.
Let's go shopping again.
If only one door is open, then only a certain amount of people can enter the store. If people
must go through successive doorways to get into the store, this slows the people down even
more. Less people move through the door. As the number of bulbs is increased in the circuit,
each electron must pass through each bulb, which increases the resistance, thus decreasing the
current.
Parallel Circuits
A parallel circuit is shown to the right.
1. Each lamp is connected to the same terminals of
the battery. The voltage across each lamp is the same.
There are individual circuits for each lamp.
2. The voltage in each circuit is the same. The current is inversely proportional to the resistance.
I (current) = V (voltage) / (R) resistance
3. The total current supplied is equal to the current passing in each bulb.
4. As the number of bulbs is increased, the overall resistance of the circuit is decreased. Overall
resistance is lowered for each path added. The overall resistance is less than the resistance for
any one path.
Let's go shopping again.
As more doors are opened to the store, the resistance is less, therefore more people can enter
the store. The flow of people is greater. As more circuits are added, there are more circuits for
electrons to flow through, thus the resistance decreases. More electrons can flow, thus the
current increases.
Fuses
Fuses are installed in our houses to protect the wires in the walls and the outside lines from
carrying too much current. Too much current, causes the wires to get hot. If they get too hot,
then they start fires. Fuses are short pieces of wire inside a glass case. As the current increases,
the fuse becomes hot. If it gets too hot, it melts or burns and the circuit is opened. No electricity
flows through the circuit shutting off the current to the house.
Fuses are rated in terms of amps. Each fuse can carry a certain amount of current. If the
current flowing through the fuse exceeds the rating of the fuse, then the fuse blows!