Electricity & Magnetism Ch 9 6 th grade

How do charges interact? Atoms contain charged particles called electrons (-) and protons (+). If two electrons come close together, they repel each other. Protons act the same way. If a proton and an electron come together, they attract one another because the two have opposite electric charges.

The interaction between electric charges is called electricity. The force between charged objects is called electric force. An electrical field is a region around a charged object where the object’s electric force is exerted on other charged objects.

Electric Fields Electrical fields and forces get weaker the farther away from the charge. An electrical field is invisible. Field lines point away from positive charges and toward negative charges. Single charges have straight field lines, but if multiple charges are present, these forces combine to make more complicated field lines.

How does charge build up? Most objects have no charge because each atom’s positive charge is balanced by a negative charge. An uncharged object can become charged by gaining or losing electrons –If it looses electrons, it will have an overall negative charge. –If it gains electrons, it will have an overall positive charge.

Static electricity This build up is called static electricity. Charges build up on an object, but they do not flow continuously. Charges are neither created nor destroyed. This is known as the law of conservation of charge. If one object loses electrons, another object must pick them up.

Methods Friction: uncharged objects are rubbed together and moves electrons from one to the other Conduction: charged object touches another object and transfers electrons Induction: reaction to the electric field of a charged object without touching the object* Polarization: electrons react to electric fields resulting in individual atoms having charged ends that are attracted to charged objects* *parts of objects end up charged

Static discharge Positively charged objects gradually gain electrons and negatively charged gradually loose electrons and the objects eventually become neutral. Objects hold on to charge better in dry air. Lightening bolts are also discharge from the charge that builds up within the clouds.Lightening bolts are also discharge from the charge that builds up within the clouds.

Electric current Static electric charges do not flow continuously, but when electric charges are made to flow through a material, they produce an electric current. The amount of charges that passes through a wire in a given period of time is the rate of electric current, called an ampere, or amp (A).

Circuit In order to maintain an electric current, charges must be able to flow continuously in a loop, called an electric circuit.

What is a magnetic field produced by a current like? The magnetic field produced by a current can be turned on or off and can change directions. To turn on or off the magnetic field, turn on or off the electric current. To change the direction, reverse the direction of the electric current.

There are two ways to change the strength of a magnetic field. Increase the amount of current in the wire. Make a loop or coil in the wire. The magnetic field lines bunch close together inside the loop. Every additional loop strengthens the magnetic field.

What are the characteristics of solenoids and electromagnets? A coil of wire with a current is called a solenoid. Two ends of a solenoid act like the poles of a magnet. A solenoid with a ferromagnetic core (such as an iron nail) is called an electromagnet. Both the current in the wire and the magnetized core produce the magnetic field. The overall magnetic field of an electromagnet is much stronger that that of a solenoid.

Regulating Strength Strength can be increased by: –Increase the current in the solenoid, –Increase the number of loops, –Wind the coils of the solenoid closer together, –Use a material that is more magnetic than iron, such as alnico. solenoids and electromagnets Uses Trains

How can an electric current be produced in a conductor? A magnet can make, or induce, current in a conductor, such as a wire, as long as there is motion. Electromagnetic induction produces an induced current.

Induction of electric current Motion in a magnetic field either by moving the conductor or the magnet will induce a current without any voltage source. The rotation of a wire loop in a magnetic field is the basis of a galvonometer, a device that measures small currents.galvonometer The direction of an induced current depends upon the direction that the coil or magnet moves.

Alternating & Direct Current A current with charges that flow in one direction is called direct current, or DC. A battery produces direct current when placed in a circuit. If a wire in a magnetic field changes direction repeatedly, the induced current also keeps changing direction called alternating current, or AC.

Alternating current has a major advantage over direct current because the AC voltage can be easily raised or lowered. High voltage can be used to send electrical energy over great distances and then lowered to safe level for everyday use in homes, schools, and offices.

How does a generator work? A generator uses motion in a magnetic field to produce current. An AC generator has an armature that rotates in a magnetic field that induces a current. Slip rings turn which allows current to flow into brushes. When brushes are connected to a circuit, the generator can be used as an energy source.

Electric Companies The electric companies use giant generators to produce most of the electrical energy used. Water, steam, or hot gas spin huge turbines which turn armatures of generators that produces electric current.

What does a transformer do? A transformer is a device that increases or decreases voltage. It consists of two separate coils of insulated wire wrapped around an iron core. The primary coil is connected to an AC voltage source while the secondary coil is connected to a separate circuit without a voltage source.

The changing current in the primary coil produces a changing magnetic field which induces a current in the secondary coil. The change in voltage depends upon the number of loops in each coil and is expressed as a ratio: Voltage primary = Coils primary = 120v = 20 Voltage secondary Coils secondary 6v In step-up transformers, the primary coil has fewer loops than the secondary coil causing an increase in voltage. In step-down transformers, the primary coil has more loops causing a decrease in voltage.

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