Electricity

What is Electricity? a form of energy resulting from the existence of charged particles (such as electrons or protons) either static as an accumulation of charge or dynamic as a current.

Electric Current Static Electricity: build up of charges that pass QUICKLY to another object Electric Current: continuous flow of charges through a conductor

Static Electricity Objects can acquire a static electric charge through: 1)Friction (when an object whose electrons are loosely held rubs against another object) 2)Conduction (when an object with an excess of electrons touches a neutral object) 3)Induction (a neutral object acquires a charge from a charged object close by without contact being made)

Friction

Induction

Conduction Charging by contact!

Transferring electric charge a. Conductors: e¯ move easily. b. Insulators: hold e¯ tightly. c. Contact charging is done when two materials are rubbed together (best with insulators). d. Charging by induction is done when one charged object induces a charge on another.

Lightning a. Large static discharge between the earth and clouds. b. Lightning was found to be static electricity by Ben Franklin. Grounding a. Conductive path to Earth. b. Lightning rods & plumbing.

The electroscope can be used to detect electric charge

B. Electric Current 1. The reason electric charge flows from one place to another is voltage. HIGH LOW a. Voltage is the difference in electrical potential between two places where e¯ are flowing. b. Voltage is the “push” that makes electric charges move. c. Measured in volts (V).

Electricity and Voltage - Electricity is the flow of electrons (-) Charges (-) flow from HIGH voltage areas to LOW voltage areas –Voltage is like electrical pressure that pushes and pulls charges –Voltage Difference: the push/pull that causes charges to move and is measured in volts (V)

Voltage Voltage is created by –a chemical cell (battery) when it changes chemical energy to electrical energy – by a generator when it changes mechanical energy to electrical energy – by a solar cell when it changes light energy to electrical energy.

Voltage and Current When a wire connects the terminals of a battery or generators, then the voltage will push and pull electrons through a conductor. –One terminal has extra electrons thus a negative charge. The other terminal has a deficit of electrons and thus a positive charge. –Electrons in the wire are pushed by the negative terminal and pulled by the positive terminal through the wire Circuit: a closed, conducting path For changes to flow, the wire must always be connected in a circuit Electric Current: the flow of charges through a wire or any conductor. Measured in Amperes (A=Amps) Current is almost always the flow of electrons What happens if we break the circuit?

Current - The flow of electric charge a. measured in amperes, or amps (A). b. Voltage causes current. The amount of electric charge is measured in coulombs. a. 1 coulomb is the charge carried by 6.24 x 10^18 e¯. b. 1 amp is 1 coulomb per sec.

Batteries are e¯ pumps. a. They provide a voltage difference to a circuit. b. Types: wet-cells & dry-cells

Check for Understanding What is voltage? How is voltage generated? (3 ways) What is current?

Resistance Resistance: the tendency for a material to oppose the flow of electrons Changes electrical energy into thermal energy and light Ex: lightbulb filament Resistance is measured in Ohms (Ω)

Resistance a. Opposition to the flow of e¯. b. It changes electrical energy into thermal energy and/or light. c. Measured in ohms. d. Conductors have less resistance than insulators.

What Affects Resistance?

Wire resistance greater for: 1) Longer wires 2) Thinner wires 3) Higher temperatures

IV. Control the Flow A voltage difference causes the charges to flow Flow of charges = current (Amps or A) Electrical resistance restricts the movement of charges Resistance = current Pressure = current (Voltage Difference)

Ohm’s Law Current = voltage difference Resistance I = V/R or V=IR I= current (units = A, amps) V= voltage (units = V, volts) R = resistance (units = ohms Ω)

6. Ohm’s law Voltage Difference Current = Resistance I V R

Electrical Circuits - A circuit is a conducting path. 1. Series circuit a. One path b. Any break & all devices go out c. Current is the same throughout the circuit

3. Parallel circuit a. Multiple paths b. A break in one branch & the other branches stay on c. Voltage is the same in each branch, but current and resistance may be different

Electrical Energy I. Series Circuit: the current has only one loop to flow through –things are wired one right after the other –If one thing (bulb) goes out every thing goes out –If the circuit is broken the entire flow of current stops

Series Circuit Continued…. Current is the same at each point in the circuit When another resistor (light bulb) is added in series, the total resistance increases. When resistance increases, current will decrease. Decreased current means dimmer light.

Series Circuit Concepts (assuming 3 resistors) R TOTAL = R1 R1 + R2 R2 + R3R3 V drop for each R is VX VX = I RXRX so, V TOTAL = I R 1 + I R 2 + I R3R3 or, V TOTAL = I (R 1 + R2 R2 + R3)R3) since I = V ÷ R then… I CIRCUIT = V ÷ (R 1 + R2 R2 + R3)R3)

II. Parallel Circuit: contains two or more branches for current to move through –current splits up to flow through the different branches –because all branches connect the same two points of the circuit – the voltage difference is the same in each branch –more current flows through the branches that have the lower resistance

Parallel Circuit Concepts (assuming 3 paths) V is the same everywhere in the circuit I TOTAL = I1 I1 + I2 I2 + I3I3 since I TOTAL = V ÷ R and I1 I1 = V ÷ R1R1 then V ÷ R = V ÷ R 1 + V ÷ R 2 + V ÷ R3R R TOTAL R 1 R 2 R 3

Household Electrical Safety In a house, many appliances draw current from the same circuit –If more appliances are connected to a circuit, more current will flow through the wires –More current in wires = more heating in the wires –More heat causes insulation on wires to melt, which increases chances of fire To protect a house from this, all household circuits have a: 1.Fuse, or 2.Circuit breaker

Household Circuits: Fuse: a small piece of metal that melts if the current becomes too high Circuit Breaker: contains a small piece of metal that bends when it gets hot bending causes a switch to flip and opens the circuit

5. Electrical safety devices a. Fuses - one time use. b. Circuit breakers - can be reset and reused.

4. Household circuits a. Mostly parallel. b. 120 V in the U.S. c. More devices plugged in a circuit mean more current. d. More current means more heat in the wiring.

Let’s Compare Series and Parallel Circuits Series Circuits _______ path(s) for current Current ________________ Voltage ________________ Break in circuit _______________________ Adding resistance in series ______________________ Parallel Circuits _______ path(s) for current Current ________________ Voltage ________________ Break in circuit _______________________ Adding resistance in parallel _______________________

Let’s Compare Series and Parallel Circuits Series Circuits 1 path(s) for current Current is the same at every point Voltage drops at each resistor Break in circuit stops all current Adding resistance in series decreases total current (dimmer light bulbs) Parallel Circuits multiple path(s) for current Current can be different in each branch Voltage same across each resistance Break in circuit does not affect other bulbs Adding resistance in parallel increases total current

Comparison of series and parallel circuits The same voltage battery Notice the brightness of the bulbs

Electric Power and Energy 1. Electrical power is the rate at which electricity is converted into another form of energy. a. Power = current x voltage b. Unit is the watt or kilowatt. Electrical energy a. Energy = power x time b. Unit is the kilowatt-hour. (1000 watts for 1 hour) c. This is what we buy from the electric company.

Light Bulbs Incandescent a. Have a tiny filament that resists the flow of electrons. b. The filament gets hot and glows to produce light. c. Very hot and inefficient.

Fluorescent making it give off U-V rays. c. Phosphor absorbs U-V rays and glows to make light. d. Very cool, efficient, and last a long time. a. Filled with gas and coated with phosphor. b. Electricity excites the gas