Electric Circuits Notes

Ohm's Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R), represented by the formula V = I × R. This fundamental relationship helps in analyzing and designing electric circuits, as it allows engineers to calculate the necessary parameters for desired circuit functionality.

  • Conductors have the same resistance regardless of applied voltage.

Components Related by Ohm's Law

  • Ohm's law relates voltage, current, and resistance in a circuit.

  • When voltage changes, current changes as a result.

  • Resistance does not change when voltage is altered.

Ohm’s Law Equation

  • R=VCR = \frac{V}{C}

  • Current is measured in Amps.

  • Resistance = Voltage / Current

  • R=VCR = \frac{V}{C}

  • Voltage = Current x Resistance

  • V=CRV = CR

Basic Features of a Circuit

  • Devices that run on electricity.

  • Sources of energy.

  • Conducting wires.

Simple Electric Circuit

  • Battery (+)

Energy Sources

  • Batteries, power plants.

Energy Conservation

  • Energy is always conserved.

  • Electrical energy is converted, not used up (e.g., heat, light, mechanical, sound).

Conducting Wires

  • Complete the path for the current.

Switch

  • Controls the current.

Series Circuit

  • Parts are connected one after another along a single path.

Pros:

  • Easy to build and design.

Cons:

  • If one bulb burns out, they all do due to a break in the circuit.

  • Lights become dimmer as more are added.

Circuit Diagram

  • Components: Energy source, wire, switch, resistor.

Effect of Voltage Change

  • If the voltage of the battery were doubled, the current through each of the bulbs would double as well.

  • Bulbs would become brighter.

Parallel Circuit

  • Different parts of the circuit are on different branches.

Pros:

  • If one bulb goes out, the others remain lit.

  • Switches can be added to branches to control specific bulbs.

  • Adding branches doesn't change bulb brightness.

Resistance in Parallel Circuit

  • Overall resistance decreases as more branches are added.

  • This is because the current has more paths to travel.

Electric Power and Safety

Electric Power and Energy

  • The rate at which energy is transformed is known as power.

  • Examples: 100W vs. 60W light bulb (Lumens - light emitted, Watts - power consumed).

Calculating Electric Power and Energy

  • Dependent on two factors: Voltage x Current

  • P=V×IP = V \times I

  • Current = Power / Voltage

  • I=PVI = \frac{P}{V}

  • Unit for power: Watts.

  • Standard wall outlets in the US are 120V.

Current Calculation Examples:

  • 4,000W washing machine

  • 85W laptop

  • 300W TV

  • 850W Toaster

Example Calculations

  • Washing Machine: I=4000120=33.33I = \frac{4000}{120} = 33.33 Amps

  • Laptop: I=85120=0.71I = \frac{85}{120} = 0.71 Amps

  • TV: I=300120=2.5I = \frac{300}{120} = 2.5 Amps

  • Toaster: I=850120=7.08I = \frac{850}{120} = 7.08 Amps

Paying for Electrical Energy

  • Total energy use equals the power of the appliance multiplied by the amount of time the appliance is used.

  • E=P×tE = P \times t

Safety Measures

  • Shocks can be prevented with devices that redirect current.

  • The third prong on power cords connects to a ground wire.

  • Fuses melt when they get too hot, breaking the circuit and preventing fire.

Circuit Breakers

  • Switches that bend away from the circuits as they heat up.

  • They can be reset since they are a switch instead of breaking the circuit entirely.

Calculating Electrical Energy Challenge

  • Energy is Power x Time (E=P×tE = P \times t).

  • Power is usually measured in kilowatts (kW).

  • To convert watts to kilowatts, divide by 1,000.

  • Kilowatt-hours (kWh) = (kW x hours).

Example: Refrigerator Energy Usage (30-day month)

  • A refrigerator averages a power of 0.075kW (75W).

  • Energy used in one month: E=0.075×(30×24)=54kWhE = 0.075 \times (30 \times 24) = 54 kWh

Electrical Energy Challenge Round 2

  • Energy is Power x Time (E=P×tE = P \times t).

  • Power is usually measured in kilowatts (kW).

  • To convert watts to kilowatts, divide by 1,000.

  • Kilowatt-hours (kWh) = (kW x hours).

Example: Stereo Energy Usage (28-day month)

  • A stereo averages a power of 340W (0.340kW).

  • Energy used in one (28-day) month: E=0.340×(28×24)=228.48kWhE = 0.340 \times (28 \times 24) = 228.48 kWh