Unit 18: Electricity
Key Terms
Electric Charge &
Static Electricity
Electric charge
Electric force
electric field
Static electricity
Law of conservation of
charge
Electric Current &
Ohm’s Law
Electric current
Direct current
Alternating current
Electrical conductor
Electrical insulator
Resistance
Superconductor
Potential difference
Voltage
Battery
Ohm’s law
Electric Circuits
Electric circuit
Series circuit
Parallel circuit
Electric power
Electric Charge and Static Electricity
Electric Charge
Electric energy is the energy associated with electric charge.
electric charge – a property that causes subatomic particles such as protons and electrons to attract or repel
other.
- an excess of shortage of electrons produces a net charge.
each
Electric Forces
EXAMPLE: rub a balloon on your clean dry hair. It will a attract
bits of paper. Now, rub a second balloon and bring the two
balloons close together. You can feel the balloons repel.
- like charges repel, and opposite charges attract.
- the effect an electric charge has on other charges is
the space around it is the charge’s Electric Field.
Static Electricity and Charging
Static electricity – the study of the behavior of electric charges, including how charge is transferred between objects.
- there are several ways that a net charge can build up on an object or move from one object to another.
- charge can be transferred by friction, by contact, and by induction.
Law of Conservation of Charge – the total charge in an isolated system is constant.
Charging by Friction: electrons move from your hair to the balloon because atoms in
rubber have a greater attraction for electrons than the atoms in hair. The balloon picks
up a net charge. Because your hair loses electrons, it becomes positively charged.
Charging by contact: a Van de Graff generator has charged a metal sphere. When the girl
touches the sphere, she acquires a charge large enough to make her hair stand on end. The
sphere is still charges, but its net charge is reduced.
Charging by Induction: reach for a doorknob after walking across a carpet. You keep
picking up extra electrons from the carpet, so your hand is negatively charged. The net
negative charge in your hand repels electrons in the metal doorknob.
induction – a transfer of charge without contact between materials.
Static Discharge
Static discharge occurs when a pathway through which charges can move forms suddenly.
- charges will not travel through air from your hand to the doorknob. But air becomes charged suddenly when
the gap between your fingers and the doorknob is small. This air provides a path for electrons to flow from your
hand to the doorknob.
Electric Current and Ohm’s Law
Electric Current – the continuous flow of electric charge
- the SI unit of electric current is the ampere ( A ), or amp, which equals 1 coulomb per second.
- there are 2 types of current: direct and alternating.
direct current – (DC)charge flows only in one direction.
- flashlights and most battery operated devices use DC.
alternating current – a flow of electric charge that regularly reverses its direction
- your home
Conductors and Insulators
electrical conductor – a material through which charge can flow easily.
electrical insulator – a material through which charge cannot flow easily.
Enduring Understandings: Some materials allow electricity to flow easily while others do not.

What types of materials allow electricity to flow and what types do not?
A metal is made up of ions in a lattice. The ions are not free to move. But each ion has one or more electrons that are
not tightly bound to it. These free electrons can conduct charge. Most materials do not easily conduct charge because
they don’t have free electrons.
Metals such as copper and silver are good electrical conductors.
Wood, plastics, rubber, and air are good electrical insulators.
Resistance – opposition to the flow of charges in a material.
- the SI unit of resistance is the ohm.
- resistance is lower if you make a wire thicker because more electrons can flow through a thicker wire.
- a material’s thickness, length, and temperature affect its resistance.
GREATER Resistance:
longer wires – the charges travel farther
temperature increases – electrons collide more often.
LESS Resistance:
shorter wire – charge travels a shorter distance
temperature decreases – electrons collide less often
superconductor – a material that has almost zero resistance when it is cooled to low temperatures.
- the best superconductors found thus far must be cooled to about 138K

Why are some materials classified as semiconductors?
semiconductor - material has an electrical conductivity value between a conductor, such as copper, and an
insulator, such as glass.
- allow electricity to flow better than insulators but not as well as conductors.
- Semiconductors are the foundation of modern electronics.
increase conductivity
decrease conductivity
increase temperature
decrease temperature
- this is opposite to that of a metal

What are some uses of semiconductors?
Semiconductor devices can display a range of useful properties:
- passing current more easily in one direction than the other
- showing variable resistance
- sensitivity to light or heat
EXAMPLES: silicon, germanium
USES:
diodes, transistors, solar cells
Enduring Understandings: The amount of electricity that flows through a conductor depends on several factors

What are the factors that affect the flow of electricity through a conductor?
GREATER Resistance:
longer wires – the charges travel farther
temperature increases – electrons collide more often.
thickness reduced – less electrons can flow

LESS Resistance:
shorter wire – charge travels a shorter distance
temperature decreases – electrons collide less often
thickness increases – more electrons can flow
What is electrical power?
Voltage
In order for a charge to flow in a conductive wire, the wire must be connected in a complete loop that includes a source
of electrical energy.
Potential Difference – (voltage) the difference in electrical potential energy between two places in an electric field
- measured in joules per coulomb, or volt.
Voltage Source
battery – a device that converts chemical energy to electrical energy
- have terminals that can connect to wires in a circuit.
- one terminal is positive and one is negative
- a voltage drop, or potential difference, is maintained across the terminal.
EXAMPLE: the voltage drop is a 9-volt battery is about 9 volts.
Ohm’s Law – the voltage ( V ) in a circuit equals the product of the current ( I ) and the resistance ( R ).
- the unit of resistance, the ohm, is named after the German scientist Georg Ohm (1789-1854)
- he determined how resistance and current affect voltage.
- he discovered that voltage is not the same everywhere in a circuit.
- he hypothesized that resistance reduces voltage.
V=I xR
or
I = V/R
V = voltage ( volts )
I = current ( A )
R = resistance ( ohm )
- increasing the voltage increases the current. Keeping the same voltage and increasing the resistance
decreases the current.
Electric Circuit
electric circuit – a complete path through which charge can flow.
- circuit diagrams use symbols to represent parts of a circuit, including a source of electrical energy and devices that are
run by the electrical energy.
Series circuit – charge has only one path through which it can flow.
- if one light bulb goes out in a series circuit, it becomes an
open circuit.
- if one element stops functioning in a series circuit, none of
the elements can operate.
Parallel circuit – an electrical circuit with two or more paths through
which charges can flow.
- if one bulb burns out, the rest can still operate
-if one element stops functioning in a parallel circuit, the
rest of the elements still can operate.
Power and Energy Calculation
Electric power – the rate at which energy is converted to another form of energy
- unit of electric power is joules per second, or watt ( W ).
- power is often measured in thousands of watts, or kilowatts ( kW )
- electric power can be calculated by multiplying voltage by current.
Electric Power:
P (watts) = I (amps) x V (volts)
Electric Energy:
E (kWh) = P (watts) x t (seconds)
or
- power companies usually charge by kilowatt-hours (kWh)
- 1 kWh = 3,600,000 joules.
E=Pxt