Electricity

A. Electric Charge 1. Static electricity is the accumulation of excess electric charges on an object. a. More e¯ = negative charge b. More protons = + charge 2. Charge is conserved (e¯ move from one object to another).

Law of Charges a. Opposite charges attract. b. Like charges repel.

Electric fields a.Electric fields exert force on objects within the field. b. Weaken with distance.

Transferring electric charge a.Conductors: electrons move easily through them b.Insulators: hold electrons tightly

c.Charging by Contact – charging materials by touching or rubbing (best with insulators). c.Charging by induction – when a charged object induces a charge on another.

How can a balloon be made to stick to a wall? When the charged balloon is brought near the neutral wall, it induces a positive charge, and the balloon will stick to the wall temporarily. When you rub a balloon on your hair, electrons are transferred to the balloon.

- A very large discharge of static electricity between the earth and clouds. -Lightning was found to be static electricity by Ben Franklin. -Grounding: a. Conductive path to Earth. b. Lightning rods & plumbing. Lightning

Although lightning and thunder occur together, light travels about a million times faster than sound, so you see it first. Thunder is caused by the rapid expansion of the air around the lightning bolt, which is heated to an extremely high temperature. Why do you see lightning before you hear thunder?

The electroscope can be used to detect electric charge

B. Electric Current HIGH LOW 1. The reason electric charge flows from one place to another is voltage. 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).

2. The flow of electric charge is called current. a.Current is measured in amperes, or amps (A). b. Voltage causes current.

Batteries are electron pumps provide a voltage difference to a circuit Types: wet-cells & dry-cells

5.Resistance- opposition to the flow of electrons. It changes electrical energy into thermal energy and/or light. Measured in ohms ( Ω) Conductors have less resistance than insulators.

Resistance in wire is greater for: 1) Longer wires 2) Thinner wires 3) Higher temperatures

Voltage Difference Current = Resistance I V R Ohm’s Law V = Potential Difference or Voltage Difference in volts (V) R = Resistance in Ohms ( Ω) I = current in amperes (A)

Ohm’s Law Example 1: Silver has a resistance of 1.98 x ohms. What voltage would produce a current of 100 amps? V = I x R = (1 x 10 2 A)(1.98 x Ω ) = 1.98 x V

Ohm’s Law Example 2: What resistance does a piece of aluminum wire produce which has a current of 150 amps and 120 V? R = V = 120 V = 0.80 Ω I 150 A

Ohm’s Law Example 3: Find the current when a 12 volt battery is connected through a resistance of 25 ohms. I = V = 12 V = 0.48 A R 25 Ω

Electrical Circuits A circuit is a conducting path. Series circuit a.One path b.Any break & all devices go out c.Current is the same throughout the circuit d.Bulbs will become dimmer as more and more bulbs are added in series.

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

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

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.

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

Electrical Power is the rate at which electricity is converted into another form of energy. Unit is the watt or kilowatt Electric Power = current x voltage P = I x V (watts) = (amps)(volts) Electric Power

Electrical Energy = Power x time E = P x t (kWh) = kW x h Unit is the kilowatt-hour (1000 watts for 1 hr) This is what we buy from the electric company. Electrical Energy

Calculate the electrical power used if 20 A of current flows through a 120-V outlet. Electric Power = current x voltage P = I x V P = (20 A)(120 V) P = 2400 Watts or 2.4 kW Electrical Power Example Problem:

Electrical Energy = Power x time E = P x t = 2.4 kW x 50 h = 120 kWh Calculate the electric bill for running this appliance if the rate is $0.05 per kWh. 120 kWh x $ 0.05 = $6.00 kWh Calculate electrical energy used if the appliance (on the previous slide) runs for 50 hours in a month.

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

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