Electricity and magnetism CHAPTER 7 Electricity and magnetism

Electric Charge and Forces Section 1 Electric Charge and Forces

Electric Charges Atoms are made of electrons moving around a nucleus (protons and neutrons). Protons have a positive charge. Neutrons have a neutral or no charge. Electrons have a negative charge. Atoms are electrically neutral in charge having equal protons and electrons. Losing an electron makes the atom positively charged. Gaining an electron makes the atom negatively charged.

Forces Between Charges Like charges repel and unlike charges attract Distance and amount of charge determines the force between objects Greater distance = less force Greater amount = greater force

Electric Field and Forces Objects do not have to touch to exert electric forces on each other Electric fields surround every proton and electron Electric fields are invisible and at every point in space surrounding a charge.

Electrically Charging Objects Charging by contact – objects must come into direct contact with each other Electrons are gained by one object and lost by the other Charging by induction – rearranging electric charges due to the presence of an electric field One part of the object is positively charged while another part is negatively charged

Conductors and Insulators Conductors hold electrons so loosely that they can move through the material easily Silver, gold, copper Insulators hold electrons so tightly that they cannot move through the material easily Plastic, glass, rubber, wood

Static Charge The imbalance of electric charges on an object is called static charge. Occurs during charging by contact The movement of static charge from one place to another is called electric discharge. Lightning – may occur between cloud and ground, within cloud, or between clouds

Electrostatic potential 30,000 volts removing sweater 10,000 volts using a comb 3,000 volts walking on carpet 1,000 volts putting on coat LUCKY FOR US, these high volts have very low amperage.

Lightning Safety 30-30 rule If there is 30 seconds between thunder and lightning, seek shelter and stay away from metals. Wait 30 minutes after last lightning strike before leaving shelter. Storm clouds need not be present for lightning to strike. Grounding is providing a path to drain excess electric charges into Earth Lightning rods

FUN FACTS 50-100 lightning bolts hit the ground every second on Earth. Lightning discharge 100 million volts of electricity, heating the air to 60,000 degrees F. Bolts can be up to 100 miles long Steel turns into a magnet if struck be lightning.

Section 2 Electric Current

Electric Current The flow of electric charges Electrons in a wire moving along the wire Equal numbers of protons and electrons in wire making it electrically neutral Equal numbers of electrons entering wire and exiting the other end Measured in amperes (A) = the amount of electrons flowing in and out of wire every second [about 6 billion billion]

Simple Electric Circuit Electricity is either static or current Static - electrons accumulate and "stand" on the surface Current - electrons must "flow" through something Direct current (DC) - flow in one direction (battery) Alternating current (AC) - reverses its flow many times a second (household power) Electric current will flow continually on a closed path known as an electric circuit Broken wires, opened switch, broken filament will stop current flowing

Flowing Electric Charges A force must be exerted on electric charges to make them flow An electric field in the circuit will move electrons in a single direction A battery will produce an electric field when a chemical reaction in the battery occurs. Charges terminals of battery – and + Electrons flow from – to +

Electrical Resistance Electrons have trillions of collisions every second inside batteries changing their direction and speed The electric field keeps the electrons flowing in the direction of the current The measure of how difficult it is for an electron to flow in an object is known as electric resistance. Measured using ohms (Ω)

Electrical Energy Transfer Electrons flowing through a circuit have kinetic energy Every collision causes some energy to be converted into heat and light As the strength of the electric field increases so does the kinetic energy of the flowing electrons

Voltage The measure of electrical energy transferred by an electron as it moves from one point to another in a circuit Measured using a voltmeter Battery voltage indicates that there is more electrical energy transferred in higher voltage

Ohm’s Law Shows relationship between voltage, current, and resistance As voltage increases, current in the circuit increases. Increasing resistance slows down the current flow voltage(V) = current(A) X resistance(Ω) V = IR

Series and Parallel Circuits Devices connected so there is only one closed path for current to follow is a series circuit. Any part of path broken, current will not flow. Devices connected so there is more than one closed path for current to follow is called a parallel circuit. If one path is broken, current will continue to flow through another path.

Electric Cost Companies charge by the kilowatt-hour (kWh) That is the amount of energy equal to 1kW of power continuously for 1 hour About ten 100-W light bulbs for 1 hour

Section 3 Magnetism

Magnetic Poles Every magnet has a north and a south pole. Magnetic poles are like electric charges. Like poles repel; unlike poles attract. Forces are stronger the closer they are to each other. Magnetic fields are like electrical fields. Fields are stronger at the poles of magnets.

Magnetic Materials Metals that contain iron, nickel, cobalt and a few other rare-earth elements Objects that do not contain the above elements are not effected by magnetic fields.

Magnetic Domain A group of atoms that have their magnetic poles pointing in the same direction has magnetic domain. With magnetic domains aligned in a single direction, the magnetic field is stronger. Materials become magnetized when magnets exert forces on the materials magnetic domains.

Electromagnetism Discovered in early 1800s, a wire carrying an electric current is surrounded by a magnetic field Magnetic fields can be made stronger by wrapping the wire around an iron core. Increasing current, increases the magnetic field. Electromagnets are temporary and can be controlled.

Generating Electric Current Moving a magnet through a wire loop that is part of a circuit makes an electric current flow – electromagnetic induction Generators convert mechanical energy to electrical energy. Power plants use steam or moving water to convert mechanical energy to electrical energy.