Electricity and Magnetism Circuits Electromagnets Chapter 17 and 18

How can you produce electricity?... right now!

The Law of Electric Charge Like charges repel – push away Opposites attract

Electric Field A region around a charged object in which an electric force is exerted on another charged object. How do we create charge?

Friction electrons are “wiped” from one object to another

Conduction Conduction – electrons move through direct contact (a shock!)

Induction when charges are rearranged without direct contact

Circuits Series Parallel You will have more than one loop. You should be able to unscrew one bulb and the other stays lit. Never connect more than one wire to each side of the battery! Each part of the circuit is in a series or in one loop.

What Is Static Electricity? A stationary electrical charge that is built up on the surface of a material

Electricity Static

Static Discharge Human body can not feel less than 2,000 volts of static discharge Static charge built up by scuffing shoes on a carpet can exceed 20,000 volts.

What is an electric circuit? A complete loop (begins and ends at the same place) through which electric charges flow.

The Essential Parts of a Circuit An energy source – battery etc. Wires – to carry the electric charge Load – light bulb, radio etc.

Types of Circuits The parts of a circuit can be connected in different ways. The two main types are: Series Parallel

Series Circuits All parts are connected in a single loop All loads in a series circuit share the same current

Parallel Circuit A circuit in which loads are connected side by side Current in a parallel circuit has more than one path Each load receives the full amount of energy the energy source can provide (voltage) and they will use as much current as they need Ex. 45 vs. 60 Watt light bulb

Switches Some circuits in include a switch to open and close the circuit (turn it on and off) Open – off (the loop is broken) Closed – on (the loop is closed or complete)

Series Circuit Will all bulbs burn with the same brightness? Yes, all receiving the same electric current. What will happen if I add a bulb? They will all dim b/c they will receive less electric current. What happens if one bulb burns out? They will all go out, the loop is no longer complete.

Parallel Circuits Will all bulbs burn with the same brightness? No, it depends on their wattage. What will happen if I add a bulb? They will continue to burn with the same brightness. What happens if one bulb burns out? The remaining bulbs will stay lit.

Questions to Consider: Which circuit would be more useful in lighting a home or building? Parallel, you want to be able to have the lights on in one room and off in another. Which circuit is used in Christmas tree lights? It depends, they used to be made in series, if one burned out all went out. Nowadays many are made in parallel so the remaining bulbs will continue to burn. Why are series circuits useful? Burglar alarms What drawbacks are there in using parallel circuits?

Voltage Think 9 volt battery The unit of measure for potential difference is the Volt (V) Think 9 volt battery The device that provides the potential difference is considered the voltage source;

How does the flow begin? Electrons in a circuit have potential energy The potential energy of each charge is called electrical potential In a battery, the electrical potential of one terminal is higher than the other terminal The difference in this potential energy is called potential difference This potential difference causes the loose electrons to be pulled away from their atoms and flow through the material

Current The current is the rate at which a charge passes a given point The unit for current is the ampere or Amp (symbol - A) An amp is the amount of current that flows past a point in one second

Resistance The opposition to the flow of charges in a circuit. Measured in ohms - Ώ

Ohms law, Defines the relationship between voltage, current and resistance.

EXPLORING MAGNETISM

What Do Magnets Do? Attract or repel other magnets (exert a force) Attract other magnetic metals Have at least 2 distinct ends (poles) each

Magnetic Field This is the area in which a magnetic force can be exerted. Magnetic force is stronger the closer you are to the magnet.

Like poles repel, opposite poles attract Magnetic Force Like poles repel, opposite poles attract

Magnetic Materials What makes some things magnetic, while other things can’t be magnetized? Spinning electrons cause small magnetic fields around each atom. Magnetic materials have atoms whose magnetic fields can be lined up in the same direction. Areas where atoms’ magnetic fields line up are called magnetic domains. magnetic domain Randomly arranged domains = No magnet! Magnetic domains lined up = Magnet!

Permanent Magnets A permanent magnet is a material that keeps its magnetic properties even when it is NOT close to other magnets.

Electricity to Magnetism In 1820, H.C. Oersted discovered that an electric current flowing through a wire had a magnetic field around it. Electricity can cause magnetism! Electromagnets are powerful magnets that can be turned on and off. You can make an electromagnet stronger by (1) putting more turns of wire in the coil or (2) making a larger soft iron core, or (3) increasing the current through the wire.

What is a solenoid? Electricity and magnetism are closely related. This was first seen when a solenoid was developed. A solenoid is simply a coil of wire that when current is flowing through it will generate a magnetic field.

Electromagnet: A stronger version of the solenoid! It creates a temporary magnet when a current-carrying wire coil surrounds a magnetic metal core. Useful because they can be turned off Adding and removing coils can increase or decrease strength

Electromagnet Induction Moving a loop of wire through a magnetic field produces an electric current. This is electromagnetic induction. A generator is used to convert mechanical energy into electrical energy by electromagnetic induction.

Electromagnetic Devices Generators: convert mechanical energy into electrical using an electromagnet Electric Motors: use electrical energy through electromagnet to create mechanical energy (i.e. to turn an axle)

What are electric motors? An electric motor is a device which changes electrical energy into mechanical energy.

What is a generator?

Uses for electromagnets Current meters also use permanent magnets and electromagnets. When current flows through a wire, it makes an electromagnet. The force between the electromagnet and the permanent magnet makes a needle move on the meter. A simple DC electric motor contains a permanent magnet, an electromagnet, and a commutator. When current flows through the electromagnet, it turns within the magnetic field of the permanent magnet, changing electricity to mechanical energy.

Magnetism to Electricity Joseph Henry and Michael Faraday discovered that magnetism could also produce electric current. This is called electromagnetic induction. If a magnet is moved back and forth through a coil of wire, current can be made to flow through the wire. This is the idea behind electric generators and transformers. Current moves right in wire. Current moves left in wire.

Uses for Electromagnetic Induction Generators produce AC current for home and industrial use. Water, wind, or steam are used to move large electromagnets through the coils of wire to produce current. Transformers are used to step up voltage of electricity that must travel long distances through wires. Other transformers then step down the voltage before it enters our homes.

Direct Current If the voltage is maintained between two points in a circuit, charge will flow in one direction - from high to low potential. This is called direct current (DC) Battery-powered circuits are dc circuits.

Alternating Current If the high & low voltage terminals switch locations periodically, the current will flow “back and forth” in the circuit. This is called alternating current (AC). Circuits powered by electrical outlets are AC circuits.