CH 7 Electricity

CH 7

Electricity

Section 1: Electric Charge

• Static electricity is the accumulation of excess electric charges on an object.

• According to the law of conservation of charge, charge can be transferred from object to object, but it cannot be created or destroyed.

• Remember—opposite charges attract and like charges repel.

• An electric field surrounds every electric charge. The electric field exerts the force that causes other electric charges to move.

• Any charge that is placed in an electric field will be pushed or pulled by the field.

Electric Field

Conductors and Insulators

• An excess of electrons can move more easily through conductors.

• What are the best conductors?– metals

• An insulator is a material that doesn’t allow electrons to move through it easily.

Transferring Electric Charge

• The process of transferring charge by touching or rubbing is called charging by contact.

• The rearrangement of electrons on a neutral object caused by a nearby charged object is called charging by induction.

Lightning and Thunder

• Lightning is a large static discharge—transfer of charge through the air between two objects because of a buildup of static electricity.

• Thunder is powerful sound waves generated by lightning.

• It is estimated that Earth is struck by lightning more than 100 times every second. It can cause power outages, injury, loss of life, and fires.

Section 2: Electric Current

• Charges flow from high-voltage areas to low-voltage areas.

• Voltage is like an electrical pressure that pushes charge.

• A voltage difference causes charges to flow and is measured in volts (V).

• The flow of charges through a wire or any conductor is called electric current.

• SI unit for current (I)—amperes (A)

Closed and Open Circuits

• A circuit is a closed, conducting path. Electric charge flows only when the wire makes a closed loop.

• When any part of a circuit is disconnected, no current flows through the circuit (called an open circuit).

Sources of Voltage Difference

• In order to keep the current moving continuously through a circuit, a device must be used to maintain a voltage difference. One common source of this is a battery. Another source is wall sockets.

Dry-cell and Wet-cell Batteries

• Each cell of a battery has an electrolyte (a chemical that conducts charge) and two electrodes (terminals). One electrode provides electrons to the electrolyte. The other electrode takes electrons from the electrolyte.

• In dry cell batteries, the electrolyte is the paste. The electrodes are the carbon rod and zinc container.

• In wet cell batteries, the electrolyte is a liquid. The electrodes are the lead dioxide and lead plates.

Resistance

• Electric current loses energy as it moves through the filament because the filament resists the flow of electrons.

• Resistance is the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light.

• SI unit for resistance = ohms (Ω)

Ohm’s Law

• Ohm’s Law is related to the flow of charges, or current, in a circuit.

• Ohm’s Law: current (A) = voltage difference (V)resistance (Ω)

• If the resistance in a circuit increases, the current decreases.

• If the voltage difference increases, the current increases.

Section 3: Electrical Energy

• Circuits typically include a voltage source, a conductor such as a wire, and one or more devices that use the electrical energy to do work.

Series Circuit

• In a series circuit, the current has only one loop to flow through.

• Series circuits are used in flashlights and some holiday lights.

Parallel Circuits

• Parallel circuits contain two or more branches for current to move through.

• The current splits up to flow through the different branches.

Fuses and Circuit Breakers

• Fuses– contain a small piece of metal that melts if the

current becomes too high– When it melts, it causes a break in the circuit,

stopping the flow of current through the overloaded circuit.

• Circuit Breaker– contains a piece of metal that bends when

it gets hot– The bending causes a switch to flip and

open the circuit.

Electrical Power

• The rate at which electrical energy is converted to another form of energy is electrical power.

• power = current x voltage differenceP (watts) = I (amperes) x V (volts)

• SI unit for power: watts (W)

Calculating Energy

• energy = power x time E (kWh) = P (kW) x t (h)

• unit of electrical energy: kilowatt-hour (kWh)

• One kilowatt is equal to 1000 watts.

• You can calculate your energy bill, by multiplying the energy used by the cost per kilowatt-hour.

Example Problems

• A dryer draws 40 A of current at a voltage difference of 240V. What is the power rating for the dryer?– 9600 W

• A 100 W lightbulb has a power rating of 100 W. How much energy in kWh is used when you leave it on for 5 h?– 0.5 kWh

• Determine the cost of using a 100 W lightbulb for 20 h at a cost of $0.09/kWh.– $0.18