CHAPTER 17

ELECTRICITY

ELECTRIC CHARGE

• Charges Exert Force• Atoms are composed of particles with charges. The

law of electric charges states that like charges repel and opposite charges attract.

THE FORCE BETWEEN PROTONS AND ELECTRONS

• Protons and electrons have opposite charges, they are attracted to each other

THE ELECTRIC FORCE AND THE ELECTRIC FIELD

• Electric force- the force of attraction or repulsion on a charged particle that is due to an electric field

• Electric field – the space around a charged object

in which another charged object experiences an electric force

CHARGE IT!

Friction• Friction- occurs when

electrons are “wiped” from one object to

another

Conduction• Conduction – electrons

move from one object to another by direct contact.

CHARGE IT!

• Induction – charges in an uncharged metal object are rearranged without direct contact with a charged object

CONSERVATION OF CHARGE

• When you charge something by any method, no charges are created or destroyed. The numbers of electrons and protons stay the same.

• Detecting Charge • You can use a device called an electroscope to

see if something is charged

MOVING CHARGES

• Conductors- a material in which charges can move easily• Examples: copper, aluminum

• Insulators- a material in which charges cannot move freely

• Examples: plastic, rubber, wood

STATIC ELECTRICITY

• Static electricity- electric charge at rest; produced from friction or induction

• Electric Discharge – the release of electricity stored in a source

17.2 ELECTRIC CURRENT & ENERGY

• Electric Current (I) – rate at which charges pass a given point• When you flip a switch, an electric field is set up

in the wire at the speed of light. The electric field causes the electrons in the wire to move.

• This electric field is created so quickly that all electrons start moving through the wire instantly. • Think of the electric field as a command to the

electrons to charge ahead.

TYPES ELECTRIC CURRENT

Direct Current (DC)• Direct Current (DC) –

charges always flow in one direction.• Ex: batteries used in

appliances

Alternating Current (Ac)• Alternating Current

(AC) – charges shift from flowing in one

direction to flowing in

the reverse direction• EX: Wiring in house

VOLTAGE AND ENERGY

• Voltage – the potential difference between points• SI unit : Volts (V)

• You can think of voltage as the amount of energy released as a charge moves between two points in the path of a current.

• As long as there is a voltage between 2 points, charges will flow in the wire. The size of the current depends on the voltage.

Varying Nature of Voltage

• Different devices need different levels of voltage.

1.5 V 9 V 12 V

Resistance

• Resistance (R) – opposition to the flow of electric charge• SI unit: ohm’s (Ω)

• Good conductors, such as copper, have low resistance• Poor conductors, such as iron, have high resistance

• Resistance depends on thickness, length, & temperature• Thick, short wires have less resistance than long, thin wires.

• The resistance of metals increases as temperature increases

Resistance

• Resistance (R) – opposition to the flow of electric charge• SI unit: ohm’s (Ω)

• Good conductors, such as copper, have low resistance• Poor conductors, such as iron, have high resistance

• Resistance depends on thickness, length, & temperature• Thick, short wires have less resistance than long, thin wires.

• The resistance of metals increases as temperature increases

Resistance

Generating Electrical Energy

• Remember energy is not created or destroyed – it only changes forms

• Parts of a Cell • A cell contains a mixture of chemicals called an

electrolyte. Every cell also has a pair of electrodes made from conducting materials

Types of Cells

• There are 2 types of cells:• Wet – have an electrolyte

• Example: batteries – sulfuric acid is the electrolyte

• Dry – have a dry electrolyte• Example: small radios or flashlights

Generating Electrical Energy

• Cells change chemical or radiant energy into electrical energy. Batteries are made of one or more cells.

• Thermocouple can take thermal energy and transfer it to electrical energy.

• Photocell convert light energy into electrical energy.

17.3 ELECTRICAL CALCULATIONS

• How fast is a nanosecond? A nanosecond (ns) is one-billionth of a second. Electrical signals travel at 30 cm/ns. Calculate how far electrical signals travel in 1 second.

•

CONNECTING CURRENT, VOLTAGE, AND RESISTANCE

• Georg Ohm Ohm (1789 –1854) studied the resistance of materials. He measured the current that resulted from different voltages applied to a piece of metal wire.

• Ohm’s Law – the ratio or voltage (V) to current (I) is the resistance (R) of a material• V = I x R Unit SI Unit

Voltage (V) V - volt

Current (I) A- Amps Resistance

(R)Ohm - Ω

Electric Power

OHM’S LAW CALCULATIONS

• Ex: 1 What is the voltage if the current is 2 A and the resistance is 12 Ω ?

• Ex: 2 Find the voltage if the current is 0.2 A and the resistance is 2 Ω

OHM’S LAW CALCULATIONS

• Ex 3: The resistance of an object is 4 Ω. If the current in the object is 9 A, what is the voltage used?

• Ex 4: An object has a resistance of 20 Ω. Calculate the voltage needed to produce a current of 0.5 A.

ELECTRIC POWER

• Electrical Power (P) – the rate at which electrical energy is converted into other forms of energy• Si Unit: Watts (W)

• Power = voltage x current• P = V x I

Unit SI UnitVoltage (V) V - volt

Current (I) A- Amps Power (P) W - Watts

ELECTRICAL POWER CALCULATIONS

• Ex 1: A toaster draws approximately 10 A of current. A home receives 120 V at each electrical outlet. What is the power of the toaster?

• Ex 2: A car uses a 12 V battery. One headlight draws 3.0 A. What is the power of the headlight?

ELECTRICAL POWER CALCULATIONS

• EX 3: A light bulb draws a 0.5 A current at a voltage of 120 V. What is the power rating of the light bulb?

ELECTRICAL ENERGY

• Measuring Household Energy Use • Different amounts of electrical energy are used

each day in a home. Electric companies usually calculate electrical energy by multiplying the power in kilowatts by the time in hours.

17.4 ELECTRIC CIRCUIT• Just like a roller coaster, an electric circuit always forms a

loop— it starts and ends at the same place

• Parts of an Electric Circuit• Energy source (Battery)• Wires• Load (light bulb, appliance)

SWITCHES

• Sometimes, a circuit also contains a switch

• Switch – opens or closes a circuit• Closed – electrons can flow• Open – electrons cannot flow

TYPES OF CIRCUITS

• Series Circuit – a circuit in which the parts are joined one after another such that the current in each part is the same.• One path for charges to follow – charges must flow

through each part of the circuit• The voltage across each load is different

Series circuits are useful in wiring burglar alarms. If any part of thecircuit fails, there will be no current in the system and the alarm will sound.

TYPES OF CIRCUITS

• Parallel Circuits - a circuit in which the parts are joined in branches so that the voltage (potential difference) across each part is the same• Loads do not have the same current• Charges have more than one path to follow

Parallel Circuits are used in almost all appliances and decorative string lights

HOUSEHOLD CIRCUIT SAFETY

• Circuits can fail if they are overloaded or a wire breaks or has water damage. • To prevent fire:

• Fuses• A thin strip of metal in circuit that the charges flow through. If the current is too high, the metal will melt and the circuit is broken

• Circuit Breakers• A switch that automatically opens if the current is too high. A

metal strip heats up, bends, and opens the switch stopping the flow of current