# Chapter 7 Notes.

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Chapter 7 Notes

Electric Current The flow of charges through a wire or conductor is called an electric current. The flow of electrons Measure in amperes Flow from high to low.

Resistance Resistance 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 Law Defined as when the current in a circuit equals the voltage difference divided by the resistance.

Electrical Circuits Circuits 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: Series Parallel

Series Circuit Series 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 Circuit Parallel 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 system
Household 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 8 Section 1

Magnetism Magnetism: 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 Poles Magnetic 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 attract Earth has magnetic poles A compass needle is a small bar magnet that can freely rotate. A compass needle always points north.

Magnetic Materials Example of magnetic materials: iron, cobalt, nickel
The 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 Magnetism
Moving 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.

Electromagnet It 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 8 Sections 2/3

Producing an Electric Current
Electromagnetic 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.

Generator Generator—a device that produces electric current by rotating a coil of wire in a magnetic field The 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 Currents
Direct 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 of cycles per second, or 60 Hz. A 60-Hz alternating current changes direction 120 times each second.

Transformer Transformer—a device that increases or decreases the voltage of an alternating current Made 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!!!! 