Exam 2 Flashcards

Formula: Ohm's Law

V = IR, where V is Voltage (Volts), I is Current (Amps), and R is Resistance (Ohms).

Formula: Electric Power (DC)

P = VI, where P is Power (Watts), V is Voltage, and I is Current.

Formula: Transmission Line Power Loss

P_loss = I²R. This shows that power loss is proportional to the square of the current.

Formula: Specific Heat Equation

Q = mcΔT, used to calculate the energy (Q) needed to change the temperature of a mass (m) by a specific amount (ΔT).

Formula: Heat Loss Rate

P = UΔT, where P is the power lost, U is the heat loss coefficient, and ΔT is the temperature difference between inside and outside.

Formula: Energy from Power and Time

E = P * t. Energy equals Power multiplied by Time.

Formula: Solar Energy Generation

E = P_peak * t * l_f, where l_f is the Load Factor (or Capacity Factor).

Formula: Total Energy in a Battery

E_total = m * e_d, where m is mass and e_d is Energy Density.

Definition: Watt (W)

The SI unit of power, equivalent to one Joule per second (1 W = 1 J/s).

Definition: Kilowatt-hour (kWh)

A unit of energy equal to 1,000 Watt-hours or 3.6 million Joules. It is commonly used for electricity billing.

Definition: Specific Heat Capacity (c)

The amount of heat energy required to raise the temperature of one unit of mass of a substance by one degree Celsius.

Definition: Energy Density

The amount of energy stored in a given system or region of space per unit mass or volume.

Definition: Load Factor

The ratio of actual energy output over a period of time to the maximum possible output if operating at full capacity.

Definition: Efficiency

The ratio of useful power output to total power input, usually expressed as a percentage.

Concept: High Voltage Transmission

Increasing voltage allows power to be transmitted with lower current, which significantly reduces resistive heat loss (I²R) in the wires.

Concept: Voltage Drop

The decrease in electric potential along the path of a current flowing in a circuit, calculated by V_drop = I * R.

Concept: Relationship between Voltage and Current (Constant Power)

For a constant amount of power (P=VI), Voltage and Current are inversely proportional. If Voltage increases by 10x, Current decreases by 10x.

Concept: Useful Power Calculation

To find useful power, multiply the input power by the efficiency rating (e.g., 1000 W * 0.90 = 900 W useful).

Calculation: Time to Heat

Time (t) is calculated by dividing the Total Energy required (Q) by the Power (P) of the heater: t = Q/P.

Impact of doubling voltage on transmission loss

Doubling the voltage halves the current. Since loss is I²R, halving the current reduces loss to (1/2)² = 1/4 of the original loss.

Unit Conversion: kW to W

1 kW = 1,000 Watts.

Unit Conversion: kJ to J

1 kJ = 1,000 Joules.

Trivia: Specific Heat of Water

Water has a very high specific heat (4.184 J/g°C), meaning it takes much more energy to heat up compared to metals like copper or aluminum.

Trivia: Joule vs. Watt

A Joule is a unit of energy (an amount), while a Watt is a unit of power (a rate of flow). Watts are Joules per second.

Mnemonic: Power Loss

"Twinkle Twinkle Little Star, Power equals I squared R." (Helps remember P_loss = I²R)