Electricity. All of us agree the importance of electricity in our daily lives. But what is electricity?. Electric Charge. Electric Charge and Electrical Forces: Electrons have a negative electrical charge. Protons have a positive electrical charge.
A very highly simplified model of an atom has most of the mass in a small, dense center called the nucleus. The nucleus has positively charged protons and neutral neutrons. Negatively charged electrons move around the nucleus at much greater distance. Ordinary atoms are neutral because there is a balance between the number of positively charged protons and negatively charged electrons.
(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).
Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before and (B) after combing. Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair.
Charging by induction: The comb has become charged by friction, acquiring an excess of electrons. The paper (A) normally has a random distribution of (+) and (-) charges. (B) When the charged comb is held close to the paper, there is a reorientation of charges because of the repulsion of the charges. This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb.
Examples are silicon, arsenic, germanium.
q = n e
where e is the fundamental charge.
Electrical force is proportional to the product of the electrical charge and inversely proportional to the square of the distance. This is known as Coulomb’s law.Mathematically,
A positive test charge is used by convention to identify the properties of an electric field. The vector arrow points in the direction of the force that the test charge would experience.
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.
The falling water can do work in turning the water wheel only as long as the pump maintains the potential difference between the upper and lower reservoirs.
A simple electric circuit has a voltage source (such as a generator or battery) that maintains the electrical potential, some device (such as a lamp or motor ) where work is done by the potential, and continuous pathways for the current to follow.
I = q/t.
A conventional current describes positive charges moving from the positive terminal (+) to the negative terminal (-). An electron current describes negative charges (-) moving from the negative terminal (-) to the positive terminal (+).
What is the nature of the electric current carried by these conducting lines?It is an electric field that moves at near the speed of light. The field causes a net motion of electrons that constitutes a flow of charge, a current.
(A) A metal conductor without a current has immovable positive ions surrounded by a swarm of randomly moving electrons. (B) An electric field causes the electrons to shift positions, creating a separation charge as the electrons move with a zigzag motion from collisions with stationary positive ions and other electrons.
R = V/I
V =I R
This is known as Ohms Law.
The four factors that influence the resistance of an electrical conductor are the length of the conductor, the cross-sectional area of the conductor, the material the conductor is made of, and the temperature of the conductor.
Work = Power x Time.
Power = current x potential
Or, P = I V
What do you suppose it would cost to run each of these appliances for one hour?(A) This light bulb is designed to operate on a potential difference of 120 volts and will do work at the rate of 100 W. (B) The finishing sander does work at the rate of 1.6 amp x 120 volts or 192 W. (C) The garden shredder does work at the rate of 8 amps x 120 volts, or 960 W.
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.
Like magnetic poles repel and unlike magnetic poles attract.
Every magnet has ends, or poles, about which the magnetic properties seem to be concentrated. As this photo shows, more iron filings are attracted to the poles, revealing their location.
These lines are a map of the magnetic field around a bar magnet. The needle of a magnetic compass will follow the lines, with the north end showing the direction of the field.
Our Earth is a big magnet.
The earth's magnetic field.Note that the magnetic north pole and the geographic North Pole are not in the same place. Note also that the magnetic north pole acts as if the south pole of a huge bar magnet were inside the earth. You know that it must be a magnetic south pole since the north end of a magnetic compass is attracted to it and 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.
Oersted discovered that a compass needle below a wire (A) pointed north when there was not a current, (B) moved at right angles when a current flowed one way, and (C) moved at right angles in the opposite direction when the current was reversed.
(A) In a piece of iron, the magnetic domains have random arrangement that cancels any overall magnetic effect (not magnetic). (B) When an external magnetic field is applied to the iron, the magnetic domains are realigned, and those parallel to the field grow in size at the expense of the other domains, and the iron becomes magnetized.
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.
When a current is run through a cylindrical coil of wire, a solenoid, it produces a magnetic field like the magnetic field of a bar magnet. The solenoid is known as electromagnet.
A galvanometermeasures the direction and relative strength of an electric current from the magnetic field it produces. A 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.
A current is induced in a
coil of wire moved
through a magnetic field.
The direction of the
current depends on the
direction of motion.
The magnitude of the induced voltage is proportional to: