Electricity and Magnetism

The Sciences

chapter 5

Electricity

• If you walk across a rug and pick up a charge from the carpet, are you negatively or positively charged?

Electricity

describes charged particles • • • • Rub a balloon with wool, picks up electrons Rub glass with silk, loses electrons Objects attract or repel when “electrically charged” “static electricity”

Figure 5-1

The two kinds of electrical charges. Opposite charges attract, while like charges repel.

Electricity

describes charged particles • • These charged particles can be at rest (“static electricity”) or they may be moving (“current electricity”)

Ben Franklin & Electric Charge

• • • 1746, 1 st to use “negative” and “positive” in e’statics 1752 famous kite experiment Invented lightning rod

Lightning

• • • Result of charges that become separated in thunder clouds Negative charge on cloud induces positive charge on ground Discharge causes lights and heats air

The movement of electrons

• • What were Franklin’s “negative fluids”?

Need to know atomic structure to understand electricity

Draw an atom. Label its parts. What do the different parts do?

• • • Protons Neutrons Electrons

• Every atom has a positively charged nucleus surrounded by negatively charged electrons.

• The electrons of all atoms are identical. Each has the same quantity of negative charge and the same mass.

• Protons and neutrons compose the nucleus. Protons are 1800X more massive than electrons, but carry an amount of positive charge equal to the negative charge of electrons

• Neutrons have slightly more mass than protons and have no net charge.

Static Electricity

Coulomb’s Law

• •

The electric force between two charged particles varies directly as the product of their charges and inversely as the square of the separation distances.

force (newtons) = k x 1 st charge x 2 nd charge / distance 2

Coulomb’s Law

• • •

…observed that if 2 electrically charged objects are moved farther away from each other, the force between them gets smaller, just like gravity.

If the distance between the 2 objects is doubled, the force decreases by a factor of 4.

(inverse square relationship)

Electrical Field

• A kind of “aura” or “force field” around every electric charge • Extends radially away from the proton and in opposite direction about the electron.

…distinguish between conductors and insulators

• • • Conductors Material that electrons are able to pass through Metals, ionic solutions • • • Insulators Material that electrons do not through easily Glass, wood, rubber, plastics

Electrical potential and electric current • • • • • • Movement of electric charge creates electric current Charges move as current only when energy is supplied to them Circuit Switch Voltage Current

If we use the water analogy… • Voltage = the pressure • Current = the rate of flow

Voltage

• • • • • The push that makes electrons move.

1.5 V “D” cell or 6 V lantern battery Higher voltage = greater push on electrons Water analogy Voltage causes current.

• • • • •

Electric Current

Voltage creates current Current is the amount of charge passing a point in a circuit in a second Metric unit = Ampere (A) Measures by an ammeter Different devices often carry different amounts of current

…distinguish between

DC and AC

• • • Direct current Current moves in one direction From dry cells or batteries • • • • Alternating current Pumped to us by Cobb EMC Oscillates back and forth at 60 Hz wall sockets

Ohm’s Law

• • How is current related to voltage?

Direct relation between the two led to discovery of “resistance” • Voltage / Current = Resistance (V / I = R)

Resistance

• measures how hard it is for current to move through a conductor (unit = Ohm).

• Easier for electrons to move through thick wires than thin wires • Light bulb filament, thin, high resistance, heats up and glows

…

distinguish between Series Circuits & Parallel Circuits

• • • Only one path for current flow Same amount of current thru entire circuit Cheap string of decorative lights • • • Alternate paths for current flow Current divides up among the paths Wiring system for lights and elec outlets in homes & buildings

Electrical Safety

• • • • Fuses Circuit breakers Ground-fault interrupter “Atoms Family” ElecSafety link

• • • •

Electric Power

Power = energy used / time Also calculated as product of current and voltage Watts = amp’s x volts Ex: 60W bulb draws .5 A on a 120V line 120W lamp draws 1A on a 120V line  If a 120V line to a socket is limited (by a fuse) to 15A, will it operate a 1200W dryer?

Solution…

• If a 120V line to a socket is limited (by a fuse) to 15A, will it operate a 1200W dryer?

• 1200W = A x 120V A = 10, yes

Review Questions from chap 5: Electricity and Magnetism

• • • Try discussion questions 1-11, p. 114.

Problems 2,3,5,6,7, p. 114.

Answers are posted in study guide.