Overview
This lecture covers the calculation and direction of electric current, explains the concept of voltage as potential energy, and introduces electrical power.
Calculating Current
- Current (I) is defined as the change in charge (Q) over the change in time (t): ( I = \frac{Q_2 - Q_1}{t_2 - t_1} ).
- Example: If charge changes from 2 C to -4 C in 300 s, current is (-20) mA.
- Negative current means flow is opposite to defined positive direction.
Current Direction
- Positive current is defined as flowing to the right; negative current means flow to the left.
- If current in one direction is +6 A, the same magnitude in the opposite direction is -6 A.
- Current direction is important for circuit analysis.
Understanding Voltage
- Voltage is always measured between two terminals and can be positive or negative.
- Ground is typically defined as 0 volts.
- Voltage can be thought of as electrical potential energy that can drive current, similar to pressure from water in a raised tank.
- Increasing the "height" (potential) increases the ability to push current.
- Voltage measurement requires specifying which terminal is positive and which is negative.
Voltage Measurement Examples
- In a circuit, swapping the positive and negative leads flips the sign of the measured voltage.
- Current usually flows into the positive terminal of a device.
Practical Example: Static Shock
- Static shocks occur due to a high voltage difference, which can ionize the air and allow electricity to flow.
- High voltage can force current to flow even through normally non-conductive materials.
Power in Circuits
- Electrical power (P) is defined as ( P = V \times I ), where V is voltage and I is current.
- Power is measured in watts (W), indicating energy delivered or absorbed per unit time.
- If current flows into the positive terminal, the device absorbs (uses) power.
Key Terms & Definitions
- Current (I) — Rate of charge flow, defined as change in charge over time.
- Voltage (V) — Electrical potential difference between two points, related to potential energy.
- Power (P) — Rate of energy transfer, calculated as voltage times current.
- Ground — Reference point defined as 0 volts.
Action Items / Next Steps
- Review the formulas for current and power.
- Practice determining the direction and sign of current in sample circuits.
- Prepare for further examples and practice problems on power in the next lesson.