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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.
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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.
