Download Ch 20 Notes

Ch. 20 Notes – Electricity
Electric Charge and Static Electricity
Electric Charge – a property that causes subatomic particles such as protons and electrons to attract or
repel each other.
• There are two types of charge – positive and negative
• Protons have a positive charge
• Electrons have a negative charge
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LIKE CHARGES REPEL AND OPPOSITE CHARGES ATTRACT.
An excess or shortage of electrons produces a net electric charge. An atom is neutral because it has an
equal number of protons and electrons. If the atom were to gain an electron, it would become a negatively
charged ion, and if the atom were to lose an electron, it would become a positively charged ion.
The SI unit of charge is the coulomb (C) - One coulomb is equal to about 6.24 x 1018 electrons.
Electrically charged objects either attract or repel each other (pull or push) – this is Electric Force
The strength of electric forces depend the amount of charge and on the distance between them.
• Electric force is directly proportional to the net charge on each object
• Electric force is inversely proportional to the square of the distance between them.
- If you double the charge of one object, the electric force between them doubles
- If you double the charge of both objects, the electric force between them quadruples
- If you double the distance between the objects, the electric force is one-fourth as strong
Electric forces are much stronger than gravitational forces, but on a large scale, since most matter is neutral,
electric forces are usually close to zero.
Electric Fields – the effect an electric charge has on
other charges in the space around it
Static Electricity – the study of the behavior of electric charges,
including how charge is transferred between objects
Law of Conservation of Charge – the total charge in an isolated system is constant
• if one object gains charge, another object lost an equal amount of charge
Charge can be transferred between objects by friction, contact (conduction), and induction
Charging by Friction – If two objects rub together, one object may transfer electrons to the other. The
object that gains electrons becomes negatively charges and the object that
loses electrons becomes positively charged
Charging by Contact (Conduction) – If a charged object touches a second object, electrons can either
leave or enter the second object, giving it a net charge
Charging by Induction – If a charged object is brought near a neutral object, charge within the neutral
object can move and redistribute. If a ground is provided to the neutral
object, electrons can either leave or enter, giving the object a net charge
grounding - a path provided between an object and “the ground” allowing charge to transfer
Static Discharge occurs when a pathway through which charges can move forms suddenly
- when you rub your feet on the carpet you become charged through friction. When you touch the doorknob, a path is
provided for electrons to transfer. You feel a static discharge. Lightning is a more dramatic example of static discharge
Electric Current and Ohm’s Law
Electric Current – the continuous flow of electric charge. The SI unit of current is the ampere (A)
• One ampere (A) equals one coulomb of charge per second
There are two types of current – Direct Current (DC) and Alternating Current (AC)
Direct Current (DC) – current where charge flows only in one direction (ex. Batteries)
Alternating Current (AC) – the flow of electric charge that regularly reverses its direction (ex. outlets)
Charge flows more easily in some material than others:
electrical conductor – a material through which charge can flow easily (ex. most metals)
electrical insulator – a material through which charge cannot flow easily (ex. wood, plastic, rubber, air)
• Metals are typically good conductors because even though the ions in the metal are not free to move, each
has electrons that are not tightly bound to it. These free electrons can conduct charge. Insulators do not
have many free electrons so they do not conduct charge well. You have probably noticed that most current
carrying wires are surrounded by a material that is a good insulator – why do you think?
Resistance - opposition to the flow of charges in a material. The SI unit of resistance is the ohm (Ω)
• As electrons move through a conducting wire, they collide with other particles (electrons and ions). These collisions
convert some of the kinetic energy into thermal energy – which reduces the energy available to move electrons through the
wire. This reduces the current.
Besides how well a type of material conducts, there are other factors that affect the resistance of a material:
thickness – when a wire is thicker there is less resistance because there is more room for electrons to flow
length – when a wire is longer there is more resistance because the charges have to travel further
temperature – as temperature increases, resistance increases because the faster moving electrons collide more often
superconductor – a material that has almost zero resistance when it is cooled to very low temperatures
Voltage (or Potential Difference) – the difference in electric potential energy between two places in an
electric field. Because it is measured in units of joules per coulomb, also called volts (V), potential
difference is called voltage
• In order for charge to flow in a conducting wire, a source of electrical energy must be provided to maintain a
difference in electric potential energy between two places. This is provided by a voltage source.
Voltage Sources - devices that do work to increase the potential energy of electric charges
Ex. batteries, generators, solar cells
• A battery converts chemical energy into electrical energy. A chemical reaction inside the battery creates
an electric field that maintains a difference in electric potential energy between the two terminals of the
battery (one terminal being positive and one being negative). In a 9-volt battery the Voltage Drop, or
potential difference that is maintained between the two terminals is about 9 volts. When wires are
connected between the two terminals, it causes charges to move.
Voltage is the cause of Current and Resistance opposes current. This relationship was expressed
mathematically by a German scientist – Georg Ohm, and is expressed in what is known as Ohm’s Law
Ohm’s Law
Ohm’s Law – the voltage (V) in a circuit equals the product of the current (I) and the resistance ®
• Voltage has a direct relationship on current
• Resistance has an indirect relationship on current
Voltage = Current • Resistance
(volts)
(amps)
(ohms)
(V)
(A)
(Ω)
Ex. What current is produced when 9.0 volts is applied to
a circuit with 3.0 ohms of resistance?
Given
Formula
Solution
V=I•R
V
or
I
=
R
Ex. What is the resistance of a wire with a current of 4.5 amps
when plugged into a 120-volt outlet?
Given
Formula
Solution
Electric Circuits
Electric Circuit – a complete path through which charge can flow
Circuit Diagrams use symbols to represent parts of a circuit
Including a voltage source and devices run by the electrical energy
Series Circuit – a circuit where charge has only one path through
which it can flow (picture on left)
• If one element stops functioning in a series circuit, all stop since current
has to be the same everywhere in the circuit
• Adding bulbs to a series circuit, increases the overall resistance, which decreases
the overall current, and each bulb shine less brightly
• The available voltage is divided among the elements (directly proportionally to the resistance)
Parallel Circuit – a circuit with two or more separate paths through which charge can flow (picture on right)
• If one element stops functioning in a parallel circuit, the rest of the elements operate unchanged
• Adding bulbs to a parallel circuit, decreases the overall resistance, increasing the current the voltage source puts out.
However, current at other branches is not affected.
• Each parallel branch receives the available voltage – it is not divided among the branches.
Electric Power
We have defined power as the rate of doing work, which is measure in joules per second, or watts (W).
Electric Power similarly, is the rate at which electrical energy is converted into another form of energy.
• Electric power is often measured in thousands of watts, or kilowatts (kW)
• Electric Power can be calculated by multiplying voltage by current
Power = Current • Voltage
(watts)
(amps)
(volts)
(W)
(A)
(V)
Ex. How much power is used by a calculator with a 1.5 V
battery the provides .75 A of current?
Given
Formula
Solution
P=I•V
Ex. How much current is drawn by a 75-watt light bulb that
is plugged into a 120-volt outlet?
Given
Formula
Solution
Power is the rate at which energy is used. To find the amount of energy we need a different equation.
Energy = Power • time
(kilowatt-hour)
(kilowatts)
(hours)
(kW•hr)
(kW)
(hr)
E=P•t
One kilowatt-hour equals
3,600,000 joules
Ex. If you use a 350 Watt stereo for 3 hours, how much
Ex. If you use 9500 W of power on average per day, and the power
Energy did you use (give answer in kW-hr and joules)?
company charges you $0.08 per kW-hr, how much does it cost?
Given
Formula
Solution
Given
Formula
Solution
Electrical Safety
To be able to operate appliances independently, most homes are wired with parallel circuits. As a result, as
more devices are connected to a circuit, the overall current increases, which also increases the temperature
of the wire. To avoid exceeding the circuits safety limit, certain devices are usually used.
Fuse – a device connected in series with the entire parallel circuit. To avoid overloading the circuit with current, a
wire in the center of the fuse melts at a certain current level, which stops current in the entire circuit.
Circuit Breaker – similar to a fuse, but instead of melting a thin wire, a switch opens when the current is too high
Insulation – the protective coating surrounding wires prevents us from directly touching the wire. It also prevents
the wires from touching each other which provides a very low resistance path for current – a short circuit
Three-prong plug – the third prong on many appliances provides a ground in case a charge builds up on the
appliance or a short-circuit develops – providing a path for current to the ground rather than through your body
Ground-fault Circuit Interrupter (GFCI) – monitors current flowing to and from an outlet or appliance. If the
two currents are not equal (which they should be) it means current is escaping and the GFCI opens to prevent shocks
Electronic Devices
Electronics – the science of using electric current to process or transmit information
Electronic Signal – information sent as patterns in the controlled flow
of electrons through a circuit
analog signal – smoothly varying signal produced by continuously changing
the voltage or current in a circuit (diagram A)
digital signal – information encoded as a string of 1’s and 0’s (diagram B). Can be done by
pulsing a current on and off (off represents a 0 and on represents a 1)
Digital signals are more reliable than analog signals – unless severe damage, the signal is often still readable
Vacuum Tubes – used to control electron flow, change AC into DC, increase the strength of a signal, or
turn a current on or off. One example is a cathode-ray tube (CRT)
Capacitors – device that stores and releases charge
Semiconductors – a crystalline solid that conducts only under certain conditions. Two types, n-type and ptype, that when used together can control the flow of electrons in various ways
Diodes – a solid-state component (two joined semiconductors) that only allows current into one direction
– converts AC into DC
Transistors – a solid-state component with three layers of semiconductors that can be used as a switch or as an amplifier,
Integrated Circuits (microchips) – tiny pieces of silicon with many solid-state components like diodes,
transistors and capacitors. Used in communication devices to store and process information very quickly.