A. Law of electric charges. Electricity and Electromagnetism SOL review Scan for a brief video The law of electric charges states that like charges repel and opposite charges attract. Because protons and electrons are oppositely charged, they are attracted to each other. An electric force is the force between charged objects. Its strength depends on: the size of the charges and the distance between them. The greater the size and the closer the distance, the greater the electric force. An electric field is a region around a charged particle that can exert a force on another charged particle. B. Three ways an object can be charged. 1. Friction rubbing two objects together can cause electrons to be wiped from one object and transferred to the other. For example, when you rub a balloon against your hair, electrons from your hair are transferred to the balloon and the balloon becomes negatively charged while your hair is positively charged. 2. Conduction when electrons are transferred from one object to another by direct contact. For example, if you touch an uncharged piece of metal with a positively charged glass rod, electrons from the metal will move to the glass rod. 3. Induction when charges in an uncharged object are rearranged without direct contact with a charged object. For example, when a positively charged object is near a neutral object, the electrons in the neutral object are attracted to the positively charged object and move toward it.
- Charges can be detected by a device called an electroscope. An electroscope is a glass flask that contains a metal rod inserted through a rubber stopper. There are two metal leaves at the bottom of the rod. The leaves hang straight down when the electroscope is not charged but spread apart when it is charged. C. Conductors, insulators, and semiconductors. Conductors material in which charges can move easily. Examples of good conductors are metals such as copper, silver, aluminum, and mercury, and water. Insulators material in which charges cannot easily move. They do not conduct charges very well because electrons are tightly bound to the atoms of the insulator and cannot flow freely. Examples of insulators are plastic, rubber, and wood. - Semiconductors materials that are in-between a conductor and an insulator such as silicon. o Diode a semiconductor device that is two-pronged and that acts like a one way valve to control the flow of electricity in electrical circuits. Examples are solar cells and light emitting diodes (LED s). Solar cells produce DC when light strikes them. LED s emit visible light or infrared radiation when current passes through them. o Transistor a semiconductor device that is three-pronged and made from silicon that are used to amplify electrical signals (in stereos or radios) or to act like a light switch turning the flow of electricity on and off.
D. Static electricity Static electricity is the buildup of electric charges on an object. For example, your clothes are charged by friction as they rub against each other inside a dryer. Positive charges build up on some clothes, and negative charges build up on other clothes. Because clothing is an insulator, the charges stay on each piece of clothing, creating static electricity. Electric discharge is the loss of static electricity as charges move off an object. Electric discharge can occur slowly such as separating clothes stuck together, or quickly such as when you walk on a carpet and touch a metal doorknob. Lightning is an electric discharge. During a thunderstorm, negative charges build up at the bottom of the cloud and positive charges build up at the top. The negative charge at the bottom of the cloud induces a positive charge on the ground. The large charge difference causes a rapid electric discharge we call lightning. Lightning rods are a pointed rod connected to the ground by a wire. They are always mounted so that they stick out and are the tallest point on a building. When lightning strikes a lightning rod, the electric charges are carried safely to the Earth through the rod s wire preventing damage to buildings. E. Electrical energy and electric current and the difference between a cell and a battery. An electric current is a flow of charges. Current flows from the positive end of the energy source to the negative. A cell is a device that produces an electric current by converting chemical energy into electrical energy. Cells can be divided into two groups: 1. Wet cells contain liquid electrolytes. An example of a wet cell is a car battery. 2. Dry cells contain electrolytes that are solid or pastelike. Examples of a dry cell would be cells used in portable radios and flashlights. Potential difference is the energy per unit charge as a charge moves between two points in a path of a current. It is also known as voltage and is expressed in volts (V). The greater the electric current, the greater the potential difference and vice versa. F. Two types of electric current. 1. Direct current (DC) the charges always flow in the same direction. Examples of direct current are batteries and cells. 2. Alternating current (AC) the charges continually switch from flowing in one direction to flowing in the reverse direction. An example of alternating current would be the outlets in your home. G. Resistance. Resistance the opposition to the flow of electric charge. It is expressed in ohms (Ω, the Greek letter omega ). In equations, the symbol for resistance is the letter R. Resistance can be thought of as electrical
friction. The higher the resistance, the lower the current and vice versa. Good conductors have low resistance. An objects resistance depends upon: the object s material, thickness, length, and temperature. Material Good conductors have low resistance. Thickness and length thick and short wires have less resistance than thin and long wires. Temperature the resistance of metals increases as temperatures increase because atoms move faster at higher temperatures and get in the way of the flowing electric charges. A superconductor is certain materials that have been brought to an extremely low temperature making the resistance nearly 0 ohms. H. Ohm s Law. Ohm s Law is an equation that shows how the units of current, voltage, and resistance are related. The equation is: Current (I) = Voltage (V)/Resistance (R) and is represented by the following units: I. Electric power. Amperes (A) = volts (V) Ohms (Ω) Electric power is the rate at which electrical energy is used to do work. The unit for power is the watt (W), and the symbol for power is the letter P. Electric power is calculated using the following equation: Power (P) = voltage (V) x current (I) DON T FORGET: The symbol used in the equation is different than the symbol that represents each area. For example, watts is used for power, volts is used for voltage, and amperes is used for current. J. Know what an electric circuit is and the parts of a circuit. An electric circuit is a complete, closed path through which electric charges flow. All circuits consist of: energy source, a load, and wires to connect the other parts together. Energy source can be a battery, cell, or electric generator. It is where the electric energy comes from. Load a device that uses electrical energy to do work. Examples are light bulbs, appliances, televisions, and motors. All loads offer some resistance. Wires connect the other parts of a circuit together. A switch is used to open and close a circuit. For charges to flow through a circuit, the switch must be closed or turned on. If a switch is open, or off, the loops of the circuit is broken and no charges can flow through the circuit. Examples of switches are light switches, power buttons on radios, and keys on calculators and computers. K. Two types of circuits. Series circuit Parallel circuit 1. Series circuit a circuit in which all parts are connected in a single loop. All loads share the same current. Because the current in a series circuit is the same, all light bulbs would glow with the same brightness. If more light bulbs were added, the resistance would increase, the current decrease, and the light bulbs would become dimmer. Examples of a series circuit are the automatic door at the grocery store, bank alarms, some types of street lights, and certain computer circuits.
2. Parallel circuit a circuit in which different loads are located on separate branches. Charges travel through more than one path with a parallel circuit. Because the current in a parallel circuit is different, each light bulb glows at full brightness, no matter how many bulbs are connected in parallel. Examples of parallel circuits are each electrical outlet in your home, a television, and stereo. - Overloaded A circuit may become overloaded when too many loads, or electrical devices, are attached to it. Plugging too many devices into one outlet may cause the temperature of the wires to increase and cause a fire. - Short circuit occurs when charges bypass the loads in the circuit. For example, if the insulating plastic around a cord is broken, the two wires inside can touch. The charges can then bypass the load and travel from one wire to the other. - Fuses and circuit breakers safety features built into circuits in your home. L. Magnet and the properties of magnets. A magnet is any material that attracts iron or materials containing iron. All magnets have the following three properties: All magnets have two poles, north and south. The pole of the magnet that points to the north is called the magnet s north pole. The opposite end of the magnet points to the south and is called the magnet s south pole. Magnetic poles always occur in pairs. You will never find a magnet with only a north or south pole. All magnets exert forces. Magnetic poles are similar to electric charges in that like poles repel and opposite poles attract. The force of repulsion or attraction between the poles of magnets is called the magnetic force. All magnets are surrounded by a magnetic field. A magnetic field exists in the region around a magnet in which magnetic forces can act. Magnetic field lines show the shape of a magnetic field around a magnet. The closer together the field lines are, the stronger the magnetic field is. These lines are closest at the poles showing that the magnetic force is strongest at the poles. M. Making materials magnetic. Moving electrons produce magnetic fields that can give an atom a north and south pole. Most materials, including copper and aluminum, have magnetic fields that cancel each other out, so the materials aren t magnetic. Materials like iron, nickel, and cobalt, have atoms grouped together in tiny regions called domains. In the domain, atoms are arranged so that the north and south poles of all the atoms line up and create a strong magnetic field. Domains are like tiny magnets of different sizes within an object. If the domains in an object are randomly arranged, the magnet fields of the individual domains cancel each other out, and the object overall has no magnetic properties. If most of the domains in an object are aligned, the magnetic fields of the individual domains combine to make the whole object magnetic.
Domains of a magnet may not always stay aligned. Several ways that this can happen include: dropping a magnet, striking it too hard, or increasing the temperature of a magnet. A magnet can be made from an unmagnetized object made of iron, cobalt, or nickel by aligning the domains in the object. For example, rubbing a nail in one direction with one pole of a magnet will magnetize the nail. If you cut a magnet into pieces, each piece will still be a magnet with two poles, one north and one south. N. The different types of magnets. 1. Magnets made of iron, nickel, cobalt, or alloys of these metals have strong magnetic properties and are called ferromagnets. Magnetite is an example of a naturally occurring ferromagnet. 2. An electromagnet is a magnet, usually with an iron core, produced by an electric current. 3. Temporary magnets are made from materials that are easy to magnetize but tend to lose their magnetization easily. 4. Permanent magnets are difficult to magnetize but tend to retain their magnetic properties better. O. How electricity and magnetism are related. Electric current produces a magnetic field and the direction of the magnetic field depends on the direction of the current. The interaction between electricity and magnetism is call electromagnetism. There are two devices that strengthen the magnetic field created by a current-carrying wire and so have useful applications. A solenoid is a coil of wire that produces a magnetic field when carrying an electric current. The strength of the magnetic field produced by a solenoid increases as more loops are added and as the current in the wire is increased. An electromagnet is a magnet that consists of a solenoid wrapped around an ion core. The strength of an electromagnet can be made stronger by increasing the number of loops in the solenoid, by increasing the size of the iron core, and by increasing the electric current in the wire. P. Electric motors and generators. Electric motor a device that changes electrical energy into kinetic energy. Generator a device that uses electromagnetic induction to convert kinetic energy into electrical energy.