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Electricity and Magnetism
Transcript of Electricity and Magnetism
Magnetism I. Electric Charges – all things have electric charges. – the number of protons and electrons are equal.
Protons – positive
Electrons – negative – material such as carpet or hair loses electrons easily and transfers electrons.
Static electricity is the accumulation of excess electric charges on an object.
Van de Graaff generator – a device for producing high-voltage static electricity
Law of Conservation – applies to charges in the fact that charges cannot be created or destroyed, but only transferred. C. Conservation of Charge Opposites attract – opposite charges attract and like charges repel.
Electric Fields – an electric field surrounds every charge. A. Neutral charges B. Building a Charge II. Conductors and Insulators A. Conductors allow electrons to move easily.
Metals are excellent conductors.
Semiconductor – material such as silicon and germanium that are neither a good conductor nor insulator. Only can be used as a conductor if impurities are added. B. Insulator – material that does NOT allow electrons to move through easily. III. Storms and Electricity A. Lightning is a static discharge. Static discharge is a transfer of electric charges through the air between two objects because of a buildup of charges. B. Thunder – caused by the heat energy of lightning. What Gives Lightning it's Zap? The accumulation of electric charges has to be great enough to overcome the insulating properties of air. When this happens, a stream of negative charges pours down towards a high point where positive charges have clustered due to the pull of the thunderhead. What Gives Lightning it's Zap? The connection is made and the protons rush up to meet the electrons. It is at that point that we see lightning and hear thunder. A bolt of lightning heats the air along its path causing it to expand rapidly. Thunder is the sound caused by rapidly expanding air. What Gives Lightning it's Zap? C. Lightning happens when the negative charges (electrons) in the bottom of the cloud are attracted to the positive charges (protons) in the ground. D. Grounding – connecting an object to Earth with a conductor. The earth is a large neutral object such that if a conductor is connected to the Earth, it will ground out or become neutral.
Lightning rods are used in homes to ground out. IV. Electrical Pressure A. A voltage difference is the push that causes charges to move and is measured in volts (V).
B. Charges flow from high voltage areas to low voltage areas. V. Circuits A. Closed Circuits – a closed, conducting path.
The flow of charges through a wire is the electric current.
The electric current is measured in amps.
Current is almost always the flow of electrons. B. Electric Circuits To use electrical energy, a complete circuit must be made.
Circuits typically include a voltage source, a conductor such as wire, and one or more devices that use the electrical energy to do work. C. Series Circuits In a series circuit, the current only has one loop to flow through.
Series circuits are used in flashlights and holiday lights. D. Parallel Circuits Parallel circuits contain two or more branches for current to move through.
Most houses are wired with parallel circuits. E. Household Circuits Most household wall sockets are 120 V while the dryer and stove requires a 220 or 240 V.
All household circuits contain either a fuse or a circuit breaker. VI. Circuit Overload Protection A. Fuses An electrical fuse contains a small piece of metal that melts if the current becomes too high.
Too many appliances in use at the same time is the most likely cause for the overheating of the circuit. VI. Circuit Overload Protection B. Circuit breaker – a circuit breaker is a guard against overheating. A circuit breaker contains a piece of metal that bends when it gets too hot.
The bending causes a switch to flip and open the circuit. VII. Batteries – a battery is a device used to maintain a voltage difference. A. Dry-Cell Batteries A dry cell battery is the typical individual batteries used. Ex (AA,D)
There is a positive and negative terminal on each battery, causing a voltage difference. The voltage difference causes a circuit to flow. B. Wet Cell Batteries A wet cell battery contains two connected plates made of different metals in a conducting solution.
An example of a wet celled battery is a battery used in a car.
As a car is driven, the alternator recharges the battery by sending current through the battery in the opposite direction to reverse the chemical reaction. VIII. Resistance – the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light. Resistance is measured in Ohms (Ω).
Thinner wires have a greater resistance to electron flow, as opposed to thicker less resistant wires. Longer wires have more resistance than shorter wires. IX. Ohm’s Law Current (I) is measured in Amperes (amps).
Voltage (V) difference is measured in Volts.
Resistance (R) is measured in Ohms. X. Electrical Power – the rate at which electrical energy is converted to another form of energy. Power is measured in Watts.
Current is measured in amps.
Voltage is measured in volts. XI. Electrical Energy – the amount of electrical energy you use depends on two things:
How long it is used Calculating Electrical Energy
Energy is measured in kilowatt-hours (kWh).
Power is measured in kilowatts (kW).
Time is measured in hours (h). I. Magnetism – refers to the properties and interactions of magnets A. Magnetic force is the interaction between two magnets.
A magnet is surrounded by a magnetic field that exerts the magnetic force.
The magnetic field is strongest closer to the magnet. B. Magnetic Field of a Bar Magnet Un-magnetized iron brought near a magnet becomes magnetized.
Iron filings line up around the bar magnet. This shows the shape of the magnetic field. C. Magnetic Poles – regions where magnetic force is the strongest
All magnets have a north and south pole.
Magnetic field lines always connect the north pole and the south pole of a magnet. D. Earth’s Magnetic Poles Earth is like a bar magnet, with its south magnetic pole near its geographic south pole.
Scientists are NOT sure what produces Earth’s magnetic field. E. Compass Needles A compass contains a small bar magnet for a needle.
If you place a bar magnet near a compass, north and south poles will attract. F. Magnetic Materials Only a few metals such as iron, cobalt, and nickel are attracted to magnets or can be made into a permanent magnet.
These particular metals are magnetic because the magnetic properties of the electrons don’t cancel out. II. Magnets A. Making a Metal a Magnet In order for a metal to become a magnet, the atoms in the metal must align their magnetic domains in the same direction.
If you place a permanent magnet near a nail, the magnet aligns the magnetic domains and the nail becomes a temporary magnet.
A permanent magnet can be made by placing the metal in a strong magnetic field.
Permanent magnets can lose their magnetic behavior if heated or dropped. B. What happens when a magnet is broken? In un-magnetized iron, the domains are randomly oriented.
In slightly magnetized iron, there is incomplete alignment of domains. In strongly magnetized iron, virtually all of the domains are aligned.
Even if the magnet were divided into bits as small as a single domain, it would still have north and south poles. C. North Poles Cannot Be Separated from South Each piece of broken magnet still has a north and south pole.
No matter how many times a bar magnet is cut in half, there is always a north and south pole, even in the smallest piece. III. Electromagnets A. Moving charges and Magnetic Fields When electric current flows through a wire, a magnetic field forms around the wire.
The direction of the magnetic field depends on the direction of the current in the wire. B. Electromagnet – temporary magnet made by placing a piece of iron inside a current
The more coils there are, the more current the electromagnet has.
The gauges in a car use galvanometers, which are devices that use an electromagnet to measure electric current. If the current in the coil is switched, the direction of the coils magnetic field also switches. The north and south poles of the magnet trade places.
The coil is repelled by and attracted once again to the poles of the permanent magnet. C. Electric Motors – a device that changes electrical energy into mechanical energy.
A battery causes an electric current to flow through the coil of the electromagnet.
Unlike poles of the two magnets attract to each other and the like poles repel. This causes the coil to rotate until the opposite poles are next to each other. E. Current Direction
A direct current (DC) flows through only one direction through a wire
An alternating current (AC) reverses the flow of the current flow D. Generators
A generator produces electric current by rotating a coil of wire in a magnetic field.
Mechanical energy is turned in to electrical energy *** This is how electricity gets to houses in power lines. F. Transformers
A transformer is a device that increases or decreases voltage of an alternating current. Made of two coils of wire called primary and secondary coils.
If the secondary coil has more wires, then the transformer, increases, or steps up the voltage.
If the primary coil has more wires, then the transformer steps down, or reduces voltage. G. Power Transmission and Transformers
In the U.S., power lines can carry up to 750,000 volts.
This amount of power is dangerous and cannot be used with household appliances.
Step down transformers convert voltage to 120 V—a usable amount of power for household appliances.