Chapter 7Notes

Electric CurrentThe flow of charges through a wire

or conductor is called an electric current.The flow of electronsMeasure in amperesFlow from high to low.

ResistanceResistance is the tendency for a material to

oppose the flow of electrons, changing electrical energy into thermal energy and light.All materials have some electrical resistance.Measured in Ohms.Making wires thinner, longer, or hotter

increases the resistance.

Ohm’s LawDefined as when the current in a

circuit equals the voltage difference divided by the resistance.

Electrical CircuitsCircuits rely on generators at power plants to

produce a voltage difference across the outlet, causing the charge to move when the circuit is complete.

There are two types of circuits: SeriesParallel

Series CircuitSeries Circuit: the current only has one loop

to flow through.The parts are wired one after another, so the

amount of current is the same.Open Circuit: if any part of a series circuit is

disconnected, no current flows through the circuit.

Example: holiday string lights

Parallel CircuitParallel circuit is a circuit that contains two

or more branches for current to move through.Individual parts can be turned off without

affecting the entire circuit.Example: electrical system in your house.

Parts of an electrical systemHousehold circuits are parallel circuits. It enters your home at the circuit breaker

and then branches out.Electric fuse: melts if the current becomes

too high, stopping the flow of the current.Circuit Breaker: bends when it gets hot,

opening circuit and stopping the flow of the current.

Chapter 8Section 1

MagnetismMagnetism: the properties and interactions

of magnets.Interaction between two magnets, called

magnetic force, increases as magnets move closer together.

A magnetic field surrounds a magnet and is strongest closest to the magnet.

Magnetic PolesMagnetic poles are regions of a magnet

where the magnetic force exerted by the magnet is strongest.All magnets have north and south poles.Like poles repel; opposites attractEarth has magnetic poles

A compass needle is a small bar magnet that can freely rotate.

A compass needle always points north.

Magnetic MaterialsExample of magnetic materials: iron, cobalt,

nickelThe magnetic field created by each atom

exerts a force on nearby objects.Magnetic domains: groups of atoms with

aligned magnetic poles.Like poles of all domains point in the same

direction.Permanent magnets are made by placing a

magnetic material in a strong magnetic field, forcing a large number of magnetic domains to line up.

Electricity and MagnetismMoving charges, like those in an electric

current, produce magnetic fields.The magnetic field forms a circular pattern.The direction of the field depends on the

direction of the current.The strength of the magnetic field depends on

the amount of current flowing.

ElectromagnetIt is a temporary magnet made by placing a

piece of iron inside a current-carrying piece of wire.Magnetic field is only present when current is

flowing through the wire.Increase strength by adding more turns or

increasing current.Properties can be controlled by changing the

electric current.Converts electrical energy into mechanical.

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Chapter 8Sections 2/3

Producing an Electric CurrentElectromagnetic induction—the production of an electric current by moving a loop of wire through a magnetic field or moving a magnet through a wire loop.

GeneratorGenerator—a device that produces electric

current by rotating a coil of wire in a magnetic fieldThe wire coil is wrapped around an iron core and

placed between the poles of a permanent magnet. Coil is rotated by an outside source of mechanical

energy.As the coil turns within the magnetic field of the

permanent magnet, an electric current flows through the coil.

Direction of the current in the coil in a generator changes twice with each revolution.

Direct and Alternating CurrentsDirect current (DC) is current that flows in

only one direction through a wire.Alternating current (AC) reverses the

direction of the current flow in a regular way.In North America, generators produce

alternating current at a frequency ofcycles per second, or 60 Hz.A 60-Hz alternating current changes direction

120 times each second.

TransformerTransformer—a device that increases or

decreases the voltage of an alternating currentMade of two coils (primary and secondary)

wrapped around the same iron core.Alternating current in a primary coil creates a

changing magnetic field around the iron core, which induces an alternating current in the secondary coil.

A step-up transformer increases voltage. The secondary coil has more turns of wire than the primary coil does.

Transformer Continued…A step-down transformer decreases voltage.

The secondary coil has fewer turns of wire than the primary coil does.

Power carried in power lines as high as 750,000 V is reduced by step-down transformers to household current (AC) of 120V.

THE END!!!!