Bellringer

What happens when you turn the lights on? What allows lights to be turned on and off? Write your answer or draw an explanatory picture for later reference in your science journal. What happens beyond the confines of the room when you flip the light switch?

Electrical calculations and Circuits

Connecting Current, Voltage, and Resistance

• Georg Ohm (1789 –1854) studied the resistances of materials. He measured the current that resulted from different voltages applied to a piece of metal wire.

• Ohm’s Law Ohm found that the ratio of voltage to current is a constant for each material. This ratio is the resistance of the material.

R VI, or V IR

Connecting Current, Voltage, and Resistance, continued

Electric Power

• What Is Electric Power? The rate at which electrical energy is changed into other forms of energy is electric power. Electric power is calculated using the following equation:

• Watt: The Unit of Power The unit for power is the watt (W). Another common unit of power is the kilowatt (kW). One kilowatt is equal to 1,000 W.

power voltage current, or P V I

Electric Power, continued

• Power Ratings When you read the wattage label on a light bulb, you are reading the bulb’s power rating. The power rating describes the rate at which an electric device uses electrical energy.

Measuring Electrical Energy

• The amount of electrical energy used in a home depends the power of the electrical devices in the house and the length of the time that those devices are on. The equation for electrical energy is as follows:

electrical energy power time, or E P t

Measuring Electrical Energy, continued

• Measuring Household Energy Use Different amounts of electrical energy are used each day in a home. Electric companies usually calculate electrical energy by multiplying the power in kilowatts by the time in hours.

• How to Save Energy Every appliance uses energy. Replacing items that have high power ratings with items that have lower ratings is a way to save energy.

Measuring Electrical Energy, continued

• It’s All About the Bulb All of the lighting in a home could make up as much as 25% of a home’s energy consumption. So, buying and using energy–efficient light bulbs can make a big difference in the amount of energy used by a household.

• Energy–Saving Programs Many governments and organizations around the world have started programs to help save energy and natural resources.

Parts of an Electric Circuit

• Forming a Loop An electric circuit is a complete, closed path through which electric charges flow.

• All circuits need three basic parts: an energy source, wires, and a load.

Parts of an Electric Circuit, continued

• A Switch to Control a Circuit Sometimes, a circuit also contains a switch. A switch is used to open and close a circuit.

Types of Circuits

• Series Circuits A series circuit is a circuit in which all parts are connected in a single loop.

• Uses for Series Circuits Series circuits are useful in wiring burglar alarms. If any part of the circuit fails, there will be no current in the system and the alarm will sound.

Types of Circuits, continued

• Parallel Circuits A parallel circuit is a circuit in which loads are connected side by side.

• Uses for Parallel Circuits Almost all appliances are built with parallel circuits so that they will keep working if part of the system fails.

Household Circuit Safety

• Circuit Failure Broken wires or water can cause a short circuit. In a short circuit, charges do not go through one or more loads in the circuit.

• Fuses A fuse has a thin strip of metal. Fuses keep charges from flowing if the current is too high.

Household Circuit Safety, continued

• Circuit Breakers A circuit breaker is a switch that automatically opens if the current is too high. Charges stop flowing.

• Electrical Safety Tips Do not overload circuits by plugging in too many electrical devices. Do not use electrical devices near water.