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Electricity & Magnetism Physical Science

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Static Electricity Atoms are made of charged particles: – Electrons: orbit the nucleus of the atom and have a negative charge – Protons: bound in the nucleus of the atom and have a positive charge

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Static Electricity The accumulation of excess electric charge on an object is called static electricity

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Static Electricity Charges exert forces on each other: – Like charges repel – Opposite charges attract

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Static Electricity Charges do not have to touch to exert these forces Surrounding every electric charge, there is an electric field. Any charge in an electric field feels an electrical force.

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Charging Objects Charging by contact: transferring charge by touching or rubbing Just like transferring heat, transferring charges by touching is called conduction

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Induction Charging by Induction: the rearrangement of a neutral system caused by a nearby charge

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Grounding Grounding: charging or neutralizing an object by connecting negative charge from the earth

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Warm Up What are the three methods of charge transfer? Grounding Charge by contact Induction

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Circuits A Circuit is a closed path through which electric charge flows. Excess electric charges repel and flow toward a positively charged area

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Electric Flow Conductor: a material in which electrons are able to move easily Insulator: a material in which electrons are not able to move easily

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Electric Current Electric current (I) is the net movement of electric charges in a single direction Measured in Amperes (A)

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Electric Current and Switches As long as the system is closed, current flows. A switch can open and close the circuit to allow or prevent electric current

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Electric Current Voltage Difference (or Potential Difference, or Voltage) is the ability to move an electric charge through a resistance Voltage Difference is measured in Volts (V)

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Electric Current A battery is a device that stores chemical energy and makes it available as electrical energy A battery can provide voltage

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Resistance Resistance: the tendency of a material to oppose the flow of electrons The SI unit for resistance is Ohms (Ω) Resistance changes electrical energy into thermal energy and light

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Resistance Insulators have more resistance than conductors Thinner wires have more resistance Longer wires have more resistance Hotter wires have more resistance

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Warm-Up 1. If a tow truck has a mass of 16,000 kg and is accelerating at 5m/s 2 what is its pulling force? F=ma (16000)(5m/s 2) = 80,000 N 2. If the tow truck’s pulley can do 20,000 joules of work in 20 seconds how much power does it have? P= W/T (20000)(20) = 1000 Watts 3. If the same tow truck towed a car 5 km and did 100,000 joules of work how much force is applied? W= FD 100,000= (F) (5) =20000 N

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What Makes a Light Bulb Glow? The filament of a light bulb is a very thin wire. When current flows through the wire, electrons collide with metal atoms of the wire. Electrical energy is converted to thermal energy and the bulb becomes hot enough to glow.

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Circuit Symbols Current: I – measured in Amperes (A) Voltage Difference: V – measured in Volts (V) Resistance: R Measured in Ohms – ( Ω )

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Ohm’s Law Ohm’s Law is the relationship between voltage difference, current, and resistance I = V/R IR V

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Ohm’s Law Example: An iron with a resistance of 9.6-Ω is plugged into an electrical outlet with a voltage difference of 120-V. What current flows in the iron? I = V/R I = ? V = 120 R = 9.6 I = 120/9.6 = 12.5-A

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Practice Problems 1.How much resistance does a light bulb have if a 2.8 V battery draws.35 A of current? I= V/R.35= 2.8 V / R =.98 Ω 1.What is the voltage needed to provide a 3.2 kΩ resistor with 88.2 mA of current? I=V/R 88.2= V/3.2 kΩ V=

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Series Circuits Series circuits have one loop through which current can flow Example:

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Parallel Circuits Parallel Circuits have two or more branches through which current can flow

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Parallel Circuits In a parallel circuit: – Voltage difference is the same across all branches – More current flows through branches with lower resistance If one branch of a parallel circuit is cut, energy can still flow down another branch. Example:

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Parallel Circuits In your house: Parallel circuits with circuit breakers Circuit breakers have a main breaker in series with many parallel circuits. The parallel circuits each have circuit breakers. When the wires overheat, the circuit breaker switches off the current.

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Warm Up What is the voltage in a battery that provides a 50Ω resistor 80mA? How much resistance does a circuit have if 30 A flows through it with a volt difference of 100 V? I= V/R.080A= V/50Ω V= 4 I= V/R 30A= 100V/ I Ω= 3000

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Electric Power Electric Power: The rate at which electric energy is converted into another form of energy P = IV Power is measured in Watts Current in Amperes Voltage in Volts Appliances with heating elements usually use more electric power then things that don’t have heat

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Example Problem A 1.5 V battery provides 2A to a light bulb. How much power does the light bulb draw? P= VI P= (1.5) (2) P= 3 watts

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Electric Power A 9 Volt battery powers a digital clock. The current through the clock is 6.67 Amps. How much power does the clock use? A light bulb uses 60 Watts of Power. It is plugged into a 120 Volt electrical outlet. What current flows through the bulb? P= VI (9) (6.67) P= Watts P= VI 60 W= (120 V) (I) I = 0.5 A

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Warm Up 1.If a generator has 10 watts of power and can complete 50 J of work. How long would that take in minutes? 2.What is labeled on opposite ends of a magnet?

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Magnets Properties of Magnets: – Magnets have magnetic fields, which exert forces on other magnetic materials – Magnets have a north pole and a south pole

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Magnetic Fields Magnetic field lines travel from the magnet’s north pole to the south pole Magnetic fields are strongest at the magnet’s poles

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Magnetic Domains Electric particles have magnetic properties When different particles align their poles in similar directions, magnets are formed Domains are groups of atoms with particles with aligned magnetic fields

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Magnetic Domains Different objects have different natural states of their domains – This determines why a “magnet” is a “magnet” Difference between a nail and a permanent magnet

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Permanent v. Induced Domains When a magnetic field is brought close to another non-magnet, the domains line up to the permanent angle

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Electromagnets An electric current traveling through a wire produces a magnetic field around the wire An electric current traveling through a coil of wire produces a magnetic field inside the coil. This creates an electromagnet

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