ELECTROMAGNETISM

• Static Electricity

• Atomic Model of Electricity

• Electrical Forces and Fields

• Forces on Moving Charges

• Generators

• Transformers

STATIC ELECTRICITY• Static electricity can be seen at work

when hair is combed on a cold, dry day.

• As the comb is pulled through the hair, strands of hair stand out stiffly.

• Some kind of force seems to pull the hair upward toward the comb.

• To understand the nature of this force it is necessary to know something about the structure of atoms and the concept of electric

charge.

STATIC ELECTRICITY and ELECTRICAL CHARGES

• When a charge is stationary or static, it produces forces on objects in regions where it is present, and when it is in motion, it produces magnetic effects.

• Electric and magnetic effects are caused by the relative position and movement of positively and negatively charged particles of matter. So far as electrical effects are concerned, these particles are either neutral, positive, or negative.

• Electricity is concerned with the positively charged particles, such as protons, that repel one another and the negatively charged particles, such as electrons, that also repel one another. Negative and positive particles, however, attract each other.

• This behavior may be summarized as follows: Like charges repel, and unlike charges attract.

ELECTRICAL FORCES

• Two rods that carry the same kind of charge repel each other. To observe this, obtain two rods that are made of the same kind of material (glass stirring rods, for example). Rub both rods in the same way (with a piece of silk, for example).

• If they are made of the same material and have been rubbed in the same way, the rods should carry the same kind of charge. Hang one rod from a thread so that it is free to rotate. Bring the other rod near. The first rod should rotate away from the second, demonstrating that like charges repel.

• If the rods had different kinds of charges, the first rod would rotate toward the second, demonstrating that unlike charges attract.

ATOMIC MODEL and ELECTRICITY

• How does the simple atomic model relate to the static electricity experiments?

• Rubbing action creates charged objects because it tears electrons loose from some kinds of atoms and transfers them to others.

• In the case of plastic rubbed with wool, electrons are taken from the wool and pile up on the plastic, giving the plastic a net negative charge and leaving the wool charged positively.

• When glass is rubbed with silk, the glass loses electrons and the silk gains, producing glass that is charged positively and silk

that is charged negatively.

ELECTRICAL FORCES

Electrical Forces are proportional to the product of the charges, and inversely proportional (reduced), by the square of the distance between charges.

CHARGES

• An electroscope is used to detect the presence of electric charges, to determine whether these charges are positive or negative, and to measure and indicate their intensity. This schematic drawing shows the basic parts of the device:

• (a, a-) are thin leaves of metal foil, suspended from (b), a metal support;

• (c) is a glass container, while (d) is a knob that collects electric charges.

• Electric charges (either positive or negative) are conducted to the leaves at the bottom via the metal support. Because like charges repel one another, the leaves fly apart. The amount of the charge is calculated by measuring the distance the leaves are forced apart.

ELECTRICAL CHARGES MAKE FIELDS

In “A,” like charges produce field lines that repel and veer away from each other. In “B,” unlike charges are attracted, and field lines move towards each other.

MOVING CHARGES IN AN ELECTRIC FIELD

A simple electric field occurs in the space between two oppositely charged flat plates. The field lines are equally spaced between the plates, showing that the electric field strength is the same everywhere. Such a field is called a uniform electric field. An electron placed in such a field at any spot in the field will accelerate at a constant rate toward the positive plate because the electrical force on it is constant. (It should be noted that the electron, which is negatively charged, moves in a direction opposite to that of the field lines.) If an electron enters a uniform field parallel to the plates, it will veer toward the positive plate. The stronger the field is, the more the deflection. Fields such as these are used to control the scan of the electron beam in televisions and computer screens.

MAGNETIC FIELDS

A solenoid makes a magnetic field by an electric current flowing through wire. In a bar magnet, ferromagnetism produces an identical field. On a larger scale, the earth, through geomagnetism produces a similar field.

MAGNETISM and ELECTROMAGNETISM• All magnetism arises from moving electric charge.

• If a current flows in a helical coil, called a solenoid, the magnetic field will be directed through the solenoid and out one end. The field curves around and reenters the other end of the solenoid.

• This is similar to the shape of the magnetic field around a bar magnet with a south and north pole and led the French physicist Andre-Marie Ampere to speculate in the early 1820s that the magnetic field of a bar magnet is produced by circulating currents in the magnet.

• Today it is believed that those circulating currents are caused by the motions of electrons, particularly by their spin within individual atoms. The tiny magnetic fields of the individual atoms align themselves into domains in

which the magnetic effects add together.

MAGNETIC FIELDS and CURRENTS• The movement of a compass needle, near a conductor through

which a current is flowing, indicates the presence of a magnetic field around the conductor.

• When currents flow through two parallel conductors, the magnetic fields of the conductors attract each other when the current flow is in the same direction in both conductors, and repel each other when the flows are in opposite directions.

• The magnetic field caused by the current in a single loop or wire is such that if the loop is suspended near the earth, it will behave like a magnet or compass needle and swing until the wire of the loop is perpendicular to a line running from the north and south magnetic poles of the earth.

ELECTROMAGNETISM• The red bar represents a current

carrying wire (electrons are flowing through it).

• The blue lines represent the magnetic field produced by electrical currents (moving electrons).

• The circles represent compasses, which show the direction of the field.

ELECTRICAL CHARGES in a MAGNETIC FIELD

• If an electrical charge moves parallel to the lines of a magnetic field, it will travel in straight lines.

• If the charge crosses any of the magnetic field lines, it will curve and deflect.

PREDICTING HOW A CHARGE WILL MOVE IN A MAGNETIC FIELD

• Put the thumb of your right hand in the direction a positive charge is moving.

• Point the fingers of your right hand in the direction of magnetic field lines.

• Curl your fingers. The charge will go in the direction your fingers now point.

• For a negative charge in the same field, repeat the process using your left hand.

SO WHY DOES IT MATTER?• In television picture tubes, magnetic fields are used to steer the electrons from the cathode.

As the magnetic field strength is varied, the electrons are deflected so that they scan across the screen.

• In a loudspeaker, the current from the amplifier is fed to a coil of wire attached to the speaker cone. The coil is arranged so that it is in line with a permanent magnet. As current in the coil is varied, the moving charges are deflected by the field of the permanent magnet. As the coil moves, the cone of the speaker vibrates, causing sound waves to be produced.

• The magnetic field of the Earth deflects and traps charged particles that travel from the sun and other stars toward the Earth. These trapped charged particles have formed two doughnut-shaped regions known as the Van Allen radiation belts. Some particles not trapped by the Earth's magnetic field are steered by that field into the atmosphere near the poles. It is believed that the aurora borealis is produced as these deflected charges crash into molecules of gas in the Earth's atmosphere.

A DC ELECTRIC MOTOR

• The electric motor demonstrates a common application of the interaction between moving charge and a magnetic field.

• In a motor, electrical energy is converted into energy of motion. A simple motor can be represented as a loop of wire attached to a source of direct current (DC). The loop is pivoted to rotate in a magnetic field. As electric charge moves along the loop, deflecting forces begin to cause the loop to rotate. To keep the loop rotating, the direction of current in the loop must be reversed every 180 degrees. A device called a split-ring commutator is used for this purpose.

A Direct Current Generator

• A generator is a motor working in reverse: a motor changes electrical energy into mechanical energy, but a generator produces electrical energy from mechanical energy.

• Superficially the diagram of a generator appears identical to that of a motor. Each consists of a loop that can rotate in a magnetic field. In a motor, electric current is fed into the loop, resulting in rotation of the loop. In the generator, the loop is rotated, resulting in the production of electric current in the loop.

• For 180 degrees of the rotation, electron deflection produces an electric current in the loop that moves in one direction; for the next 180 degrees, the electron deflection is reversed.

• As the current leaves the loop to an external circuit, the current will be observed to move in one direction and then the other. This is called alternating current.

• For a generator to generate direct current it is necessary to use a split-ring commutator at the point where the generator feeds current to the external circuit. The current in the

loop is still alternating, but it is direct in the external circuit.

GENERATING ELECTRICITY

A. If the magnet does not move, no current is generated. B. If the magnet moves, there is a current generated.C. Why did the current change direction?

• Michael Faraday, the English scientist, and Joseph Henry of the United States independently showed in 1831 that moving a magnet through coils of wire would generate a current in the wire.

• If the magnet was plunged into the coil, current flowed one way. When the magnet was removed, the current direction was reversed.

• This phenomenon is called electromagnetic induction, and it is the principle underlying the operation of the generator. As long as the magnet and the coil move relative to each other, a potential difference is produced across the coil and current flows in the coil.

• A potential difference is also produced if the magnetic field through the coil grows stronger or weaker. The greater the rate at which the magnetic flux through the coil changes, the greater the potential difference produced. The key is that the magnetic field through the coil must be changing.

TRANSFORMERS• If the secondary coil, for example, is wound

with 100 times as many turns as the primary, the voltage across the secondary will be 100 times larger than that across the primary.

• A transformer used in this way is said to be a step-up transformer. If there are fewer turns on the secondary, it is a step-down transformer.

• It might seem that a step-up transformer gives more energy than it uses: after all, the larger secondary voltage means a larger energy change per charge moved in the field. But that voltage increase comes at the expense of a reduced current.

TRANSFORMERS• A simple transformer consists of a coil of wire fed by a voltage source such

as a generator. The coil is wound around one side of an iron frame. This is the primary coil. The other side of the iron frame is wound with another coil, the secondary, that feeds electricity to a separate circuit.

• As alternating current from the generator flows in the primary coil, the magnetic field in that coil strengthens, weakens, and then reverses direction as the alternating current changes. The iron core intensifies these magnetic-field changes.

• As the magnetic field in the secondary coil changes with time, electric charges in the secondary are deflected and current is produced. The alternating voltage produced in the secondary depends on the relative number of turns in the secondary compared to the primary coil.

TRANSFORMERS in TRANSMISSION

• The action of a transformer makes possible the economical transmission of electric power over long distances.

• If 200,000 watts of power is supplied to a power line, it may be equally well supplied by a potential of 200,000 V and a current of 1 amp or by a potential of 2000 V and a current of 100 amp, because power is equal to the product of voltage and current.

• The power lost in the line through heating is equal to the square of the current times the resistance. Thus, if the resistance of the line is 10 ohms, the loss on the 200,000 V line will be 10 watts, whereas the loss on the 2000 V line will be 100,000 watts, or half the available power.