Presentation on theme: "Electricity. All of us agree the importance of electricity in our daily lives. But what is electricity?"— Presentation transcript:
All of us agree the importance of electricity in our daily lives. But what is electricity?
Electric Charge and Electrical Forces: Electrons have a negative electrical charge. Protons have a positive electrical charge. These charges interact to create an electrical force. Like charges repel each other (e.g. electron and electron or proton and proton repel each other). Unlike charges attract each other (e.g. electron and proton attract each other).
Model of an atom Most of the mass in a small, dense center = nucleusNucleus = positively charged protons + neutral neutrons. Negatively charged electrons move around the nucleus. Atoms are neutral because there is a balance between the number of positively charged protons and negatively charged electrons.
Electrostatic Charge: Electrons move from atom to atom to create ions. positively charge ions result from the loss of electrons = cations. Negatively charge ions result from the gain of electrons = anions.
(A) A neutral atom has no net charge because the numbers of electrons and protons are balanced. (B) Removing an electron produces a net positive charge; the charged atom is called a positive ion (cation). (C) The addition of an electron produces a net negative charge and a negative ion (anion).
Combing transfers e-s from the hair to the comb by friction, resulting in a - charge on the comb and a + charge on the hair.
The charge on an ion = electrostatic charge. An object becomes electrostatically charged by Friction,which transfers electrons between two objects in contact, Contact with a charged body which results in the transfer of electrons, Induction which produces a charge redistribution of electrons in a material.
Electrical Conductors and Insulators: Electrical conductors - materials that can move electrons easily. Metals. Copper is the best electrical conductor. Electrical nonconductors (insulators) - materials that do not move electrons easily. Wood, rubber etc. Semiconductors - materials that sometimes behave as conductors and sometimes behave as insulators. - Silicon, arsenic, germanium.
Measuring Electrical Charges: The fundamental charge is the electrical charge on an electron = X C (Electrical charge is measured in coulombs).
Force Fields: The condition of space around an object is changed by the presence of an electrical charge. The electrical charge produces a force field, that is called an electrical field since it is produced by electrical charge.
Lines of force diagrams for (A) a negative charge and (B) a positive charge when the charges have the same magnitude as the test charge.
Electrical Potential: An electrical charge has an electrical field that surrounds it. In order to move a second charge through this field work must be done. Bringing a like charge particle into this field will require work since like charges repel each other and bringing an opposite charged particle into the field will require work to keep the charges separated. In both of these cases the electrical potential is changed.
Coulombs law: Electrical force - Proportional to the product of the electrical charge and inversely proportional to the square of the distance. Mathematically, where, F - force k - constant = 9.00 x 10 9 N m 2 /C 2 q 1 and q 2 - electrical charge of object 1 and object 2 d - distance between the two objects.
Electric Current Flow of charge in the same way that a water current flows.
The Electric Circuit: Charges flow through the circuit from a higher to lower potential. Charges flow through wires to make a continuous path. Switch = means of interrupting or completing the circuit. Source of the electrical potential = voltage source.
Current depends on: # of electrons that are moved through the unit volume of conducting material. Voltage = measure of the potential difference between 2 places in a circuit. Voltage measured in joules/coloumb.
Electrical Resistance: Electrical resistance - Resistance to movement of electrons being accelerated with an energy loss. Materials have the property of reducing a current = electrical resistance (R). Resistance - Ratio between the potential difference (V) between 2 points and the resulting current (I). Resistance is measured in the unit ohm ( ).
The relationship between voltage, current, and resistance = Ohms Law. V =I R The magnitude of the electrical resistance of a conductor depends on 4 variables: Length of the conductor. Cross-sectional area of the conductor. Material the conductor is made of. Temperature of the conductor.
Electrical Power and Electrical Work: All electrical circuits have three parts in common. A voltage source. An electrical device Conducting wires. Work done (W) by a voltage source = work done by the electrical field in an electrical device, Work = Power x Time. Electrical work - measure in joules. A joule/second = unit of power called the watt. Power = current x potential Or, P = I V
This meter measures the amount of electric work done in the circuits, usually over a time period of a month. The work is measured in kWhr.
All of us are familiar with magnets. In a magnet we have magnetic poles – the north and the south pole. A North seeking pole - North Pole. A South seeking pole - South Pole. Like magnetic poles repel and unlike magnetic poles attract.
Every magnet has ends, or poles, about which the magnetic properties seem to be concentrated. More iron filings are attracted to the poles, revealing their location.
Magnetic Fields: A magnet that is moved in space near a second magnet experiences a magnetic field. A magnetic field can be represented by field lines. The strength of the magnetic field is greater where the lines are closer together and weaker where they are farther apart.
Lines are a map of the magnetic field around a bar magnet. A magnetic compass needle will follow the lines, with the north end showing the direction of the field.
The Source of Magnetic Fields: Permanent Magnets: Moving electrons produce magnetic fields. Most materials these magnetic fields cancel one another + neutralize the overall magnetic effect. Materials such as iron, cobalt, and nickel, the atoms behave as tiny magnets because of certain orientations of the electrons inside the atom. Atoms are grouped in a tiny region- magnetic domain.
Our Earth is a big magnet. Earths magnetic field - thought to originate with moving charges. Core – Probably composed of iron and nickel, which flows as the Earth rotates, creating electrical currents that result in the Earths magnetic field.
The earth's magnetic field. Magnetic north pole + the geographic North Pole - not in the same place. Magnetic north pole acts as the south pole of a huge bar magnet were inside the earth. It must be a magnetic south pole since the north end of a magnetic compass is attracted to it + opposite poles attract.
A bar magnet cut into halves always makes new, complete magnets with both a north and a south pole. The poles always come in pairs. You can not separate a pair into single poles.
Electric Currents and Magnetism
Use (A) a right-hand rule of thumb to determine the direction of a magnetic field around a conventional current and (B) a left-hand rule of thumb to determine the direction of a magnetic field around an electron current.
Current is run through a cylindrical coil of wire, a solenoid, it produces a magnetic field.. Solenoid =Electromagnet.
Applications of Electromagnets: Electric Meters: Strength of the magnetic field produced by an electromagnet is proportional to the electric current in the electromagnet. Galvanometer measures electrical current by measuring the magnetic field. Measure current, potential difference, and resistance.
Galvanometer measures the direction + relative strength of an electric current from the magnetic field it produces. Coil of wire wrapped around an iron core becomes an electromagnet that rotates in the field of a permanent magnet. The rotation moves pointer on a scale.
Electric Motors: Electrical motor - Electromagnetic device that converts electrical energy into mechanical energy. Motor has 2 working parts - a stationary magnet = field magnet + a cylindrical, movable electromagnet = armature. Armature - Axle + rotates in the magnetic field of the field magnet. Axle - Used to do work.
Induced Current: Loop of wire -Moved in a magnetic field a voltage is induced in the wire. Voltage = induced voltage + the resulting current = induced current. Induction = Electromagnetic induction. Current is induced in a coil of wire moved through a magnetic field. Direction depends on the direction of motion.
The magnitude of the induced voltage is proportional to: # of wire loops cutting across the magnetic field lines. Strength of the magnetic field. Rate at which magnetic field lines are cut by the wire. Applications: DC and AC Generators, Transformers (step-up and step-down).
AC vs. DC Current Direct current (DC)- The positive and negative terminals of a battery are always, respectively, positive and negative. Current always flows in the same direction between the terminals. Batteries, fuel cells and solar cells Batteriesfuel cellssolar cells Alternating current (AC)- The direction of the current reverses, or alternates, 60 times per second (in the U.S.) or 50 times per second (in Europe, for example).Europe Power that is available at a wall socket in the United States is 120-volt, 60-cycle AC power.United States Power plant
Power companies convert alternating current to very high voltages for transmission then drop it back down to lower voltages for distribution inside the house for safety. 1 million volts to1,000 volts and finally down to 120 volts It's a lot harder to kill someone with 120 volts than with 1 million volts