5.2: Circuits, 5.2: Circuits, Ohm’s Law, Ohm’s Law,

Electrical PowerElectrical Power2/6/13

Part I: What Are Circuits?• electric circuit = an electrical device connected

so that it provides one or more complete paths for the movement of charges (bulb + battery + wires = circuit).• when a wire connects the terminals of the

battery to the light bulb, charges building up on one terminal of the battery have a path to follow (through the light bulb) to reach the opposite charges on the other terminal.

• this current causes the filament in the light bulb to give off light and heat.

• the inside of the battery (or any voltage source) is part of the closed path of current. Open circuits result when a complete path does not exist.

• the inside of the battery (or any voltage source) is part of the closed path of current. Open circuits result when a complete path does not exist.

• switches can be added to the circuit to create a way of turning the current on and off without having to disconnect wires from the power source.

• schematic diagram = a graphic representation of an electric circuit or apparatus, with standard symbols for the electrical devises.

• schematic diagram = a graphic representation of an electric circuit or apparatus, with standard symbols for the electrical devices.

Part II. Electric Resistance• resistance = the ratio of the voltage across a

conductor to the current it carries• most electric devices require the same voltage: 120V.

But light bulbs range from dim 40W bulbs to bright 100W. There is less current in a 40W bulb due to resistance.

• resistance is caused by internal friction, which slows the movement of charges. Also responsible for electric motors becoming warm after they have been on for a while.

• resistance equation =

• resistors = special conductor used to control current flow. Ex: light bulb filament

• conductors (metals) have low resistance because e- easily flow through them

resistance (in ) = voltage (in V) R = V current (in A) I

• conductors (metals) have low resistance because e- easily flow through them

• superconductor = have zero resistance when cooled past their critical temperature• CT ranges from -272C to -123C, depending on the

material. Used in magnets.• insulators have high resistance. They are used to coat

conducting material for safety.• ground wires carry excess current into the ground,

where it can spread out safely.• semiconductor = have electrical properties of both

conductors and insulators. Silicon and germanium are two common examples found in computer chips. Small impurities in the crystals allow electricity to flow, but only under certain conditions.

Part III. Series and Parallel Circuits• series circuit = a circuit or portion of a circuit

that provides a single conducting path. Ex: string of holiday lights that go out when one burns out or is removed.

• parallel circuit = a circuit or portion of a circuit that provides two or more conducting paths. Ex: string of holiday lights that do not go out when one burns out or is removed.

Part IV. Electric Power and Electrical Energy• electrical energy = the energy associated with

electrical charges, whether moving or at rest. Comes from sources like batteries and power plants.• when a charge moves in a circuit, it loses energy.

Some of the energy is transformed into useful work, such as turning a motor. The rest is lost as heat.

• the rate at which electrical work is done is called electric power. Equation:

SI unit of power= watt (W)

• power companies use kilowatt-hours (kWh) to track consumption of energy. SI unit of electrical energy= kilowatthour (kWh)

Power = current voltage P = IV

Energy = power time E = Pt

• power companies use kilowatt-hours (kWh) to track consumption of energy. SI unit of electrical energy= kilowatthour (kWh)Energy = power time E

= Pt

Part V. Fuses and Circuit Breakers• when too many appliances, lights, TVs, and other devises

are connected to a typical 120 V outlet, the overall resistance of the circuit is lowered. This means the wires are forces to carry more that what is considered a safe level of current.

• high currents in overloaded circuits can cause fires that start within the walls.

• worn insulation on wires can also be a fire hazard. If two side-by-side wires touch due to faulty insulation, an alternative pathway for the current is created. This is called a short circuit. The greatly reduced resistance allows too much current to pass though, possibly igniting a fire. Ground wires prevent this from happening.

• fuse = an electrical device containing a metal strip that melts when current in the circuit surpasses safe levels.• fuses are connected in series along a wire in the

circuit.

• fuse = an electrical device containing a metal strip that melts when current in the circuit surpasses safe levels.• fuses are connected in series along a wire in the

circuit.• example: a 20 A fuse will “blow” when the current in

the wire exceeds 20 A.• circuit breaker = a device that protects a circuit from

current overloads. • Made of a bimetallic strip (2 types of metals,

welded together) that bends (like a switch would) in different ways depending on the amount of current passing through it.

• The circuit breaker will “open” when there is too much current.