Electricity and Magnetism

Chapters 11 and 12

Central High School

Physical Science

Electric Charge

• Rule 1: Like charges repel one another

• Rule 2: Unlike charges attract one another

Electric Charge

• Every atom has a positively charged nucleus surrounded by negatively charged electrons

• The electrons of all atoms are identical

• Protons and neutrons make up the nucleus. Protons are 1800x heavier than an electron

Electric Charge

• A charged atom is called an ion. Ions are made when electrons are moved.

• Adding electrons make a negative (-) ion.

• Removing electrons make a positive (+) ion.

• Ions may have charges ranging from -4 to +6. (-4, -3, -2, -1, +1, +2, +3, +4, +5, +6)

• Hydrogen without an electron is a +1 ion.

Coulomb’s Law

• The force between the two charged particles varies directly as the product of their charges and inversely as the square of the separation distance. (Remember Inverse-Square Law?)

• F= k q1q2/d2 where k is the proportionality constant equal to9,000,000,000 Nm2/C2; q is the charge in

coulombs (C); and d is in m

Electric Charge

• The charge on 1 electron is -1.602x10-19 C

• The charge on 1 proton is +1.602x10-19 C

• What is the force on an electron on a Helium atom with a radius of 128 x 10-12 m? (Helium will have 2 protons in the nucleus for a charge of +3.204 x 10-19 C)

Charge Polarization

• A polarized object has no net electrical charge. The “centers of charge” moves to the actual charge that it nears.

Electric Current

• The flow of electrons is what makes a current of electricity.

• Rate of the electrons is measured in Amperes and is 1.00 Coulombs of charge in 1.00 seconds.

Voltage

• Voltage is a pressure that forces electrons in a circuit.

• V=PEele/charge or V=J/C or a Volt

• Voltage does not flow through a circuit but is applied across a circuit from something like a battery that stores electro-chemical potential energy.

Electrical Resistance

• The resistance is the friction of a circuit. This is the force that causes electrons to use energy in the form of heat, light, or mechanical motion.

• Any substance with a resistance of zero is called a superconductor.

• Scientists measure resistance in Ohm’s or the symbol or omega.

Ohm’s Law

• The relationship between voltage, resistance, and current is found in the equation R=V/I Where I is the current V=IR is a more familiar rearrangement of this equation. Units are Volts, Amps, and Ohm’s.

Direct Current

• Direct current (dc) is a flow of electrons between the negative terminal to the positive terminal of a circuit. The electrons flow in only one direction. The current is usually very slow.

Alternating Current

• Alternating current (ac) is the periodic switching of a terminal from positive to negative that also switches the flow of electrons. Household electrical wiring uses ac current for the efficiency and speed with which electrons may be made to power useful appliances. 60 cycles of these switches per second or 60Hertz is the most common US power arrangement.

Series Circuit

• A series circuit is one in which all resistance or voltage follows a single path.

Series Circuit

In a series circuit

The current has but one pathway.

The current is resisted by all devices in a connected link one behind the other.

The current, resistance, and voltage follow Ohm’s Law.

The resistance around the circuit is the sum of all the resistance.

Parallel Circuit

• A parallel circuit is a circuit in which the paths diverge (branch) from the source to the end.

Parallel Circuit

A parallel circuit will The device connects the same two points A and B of the circuits. The voltage is the same across the devices.The total current is divided by each of the branches. Each branch is proportional to the inverse of the resistance.As the overall number of branches increases the resistance is reduced.

Parallel Circuit

• To find the overall resistance of a parallel circuit the following equation applies.

• 1 1 1 1

• ____= _____ + ____ + ______

• Rtotal R1 R2 R3

Overloading Circuits

• An overloaded circuit carries a current too large to be safely delivered. Once a circuit reaches a threshold a breaker is deployed to shut off all power to the circuit.

• Ground Fault Disrupters GFD are found in the actual outlet and are located in rooms with readily available water supplies.

• Fuse boxes and circuit breakers are set up for whole areas of a house to provide safety and prevent fires.

Magnetism

• Magnetic forces are similar to electrical forces. They cause attractions and repulsions of objects without actually touching.

• Magnetic poles produce magnetic forces.

Magnetism

• Like poles repel and opposite poles attract

• If you break a magnet in half; two poles will form

Magnetism

• A magnet is formed by like atoms aligning with one another. These are called magnetic domains.

• In order for a magnet to lose its force, the magnetic domains must be disordered.

Electromagnets

• Electrical current through a coil of wire will acquire a magnetic field. The pattern that is produced is influenced by the direction of the current.

• Large electromagnets are used in junk yards to lift automobiles.

• Long superconducting electromagnets are used for transportation and high energy particle physics. (TiNb)

Faraday’s Law

• A magnetic field can be used to create a charge.

• The induced voltage in a coil is proportional to the number of loops multiplied by the rate at which the magnetic field changes within those loops.

• Measurement for magnetically induced charges is a galvanometer.