Electrical Theory & DC Circuits
Ohm's Law, power calculations, series and parallel circuits, resistance, and fundamental electrical units.
- Apply Ohm's Law to calculate voltage, current, and resistance in DC circuits
- Calculate power in DC circuits using the power formula P = V x I
- Determine total resistance in series and parallel circuits
- Identify the units of capacitance, inductance, and impedance
- Solve practical circuit problems involving series-parallel combinations
Lección 1
Ohm's Law & Power Calculations
The Three Fundamental Quantities
Every electrical circuit involves three fundamental quantities: voltage (V), current (I), and resistance (R). Understanding the relationship between these three values is the foundation of all electrical work.
- Voltage (measured in volts, V) - the electrical pressure that pushes current through a circuit. Think of it as the force behind the electron flow.
- Current (measured in amperes, A) - the rate of electron flow through a conductor. One ampere equals one coulomb of charge passing a point per second.
- Resistance (measured in ohms, omega) - the opposition to current flow. Every conductor has some resistance, which converts electrical energy into heat.
Ohm's Law
Ohm's Law states: V = I x R (voltage equals current times resistance). This single formula, rearranged three ways, lets you solve for any unknown quantity when you know the other two:
A critical exam concept: if voltage remains constant and resistance increases, current will decrease. This follows directly from I = V / R. Doubling the resistance cuts the current in half.
Power Calculations
The formula for calculating power in a DC circuit is P = V x I (power in watts equals voltage times current). This is the most frequently tested power formula on the Red Seal exam.
Derived power formulas include:
- P = I2 x R - useful when you know current and resistance
- P = V2 / R - useful when you know voltage and resistance
Exam Example
A 240V electric heater draws 20 amps. What is its power consumption?
P = V x I = 240V x 20A = 4,800 watts
Units of Power
| Quantity | Unit | Symbol |
|---|---|---|
| Voltage | Volt | V |
| Current | Ampere | A |
| Resistance | Ohm | omega |
| Power | Watt | W |
| Energy | Watt-hour | Wh |
Power is the rate of energy consumption. A 4,800 watt heater running for one hour consumes 4,800 watt-hours (4.8 kWh) of energy.
Power Factor in DC vs. AC
In DC circuits, all power delivered to a load is consumed as useful work. In AC circuits, the power factor describes how effectively current is converted to useful power. A purely resistive load - such as a heater or incandescent lamp - has a power factor of 1.0, meaning all current produces useful work. Inductive loads like motors have a power factor less than 1.0 because some current oscillates between the source and load without doing work. Understanding this distinction prepares you for AC theory in the next module.
P = V x I is the formula for calculating power in a DC circuit. When voltage is constant and resistance increases, current decreases per Ohm's Law (I = V / R). Master these two relationships and you can solve most DC circuit problems on the exam.