This group covers the basic vocabulary of electricity: what current and voltage are, the units used to measure them, which materials carry current, and how alternating current differs from direct current. These ideas underpin every other part of the exam.
Voltage, current and power
Voltage is the electrical pressure, or potential difference, that pushes electrons through a circuit. A difference in voltage is what causes electron flow; with no difference, nothing moves. Voltage is measured in volts.
Current is the flow of electrons through a circuit. It is measured in amperes (amps). Do not confuse it with “amperes per second” or “volts per second”, which describe how fast current or voltage is changing, not the flow of electrons itself.
Power is the rate at which electrical energy is used. It is measured in watts. Watt-hours are a different quantity: they measure total energy used over time, like the amount of fuel burned rather than how fast it burns.
Resistance is the opposition to current flow, measured in ohms. Resistance opposes every kind of current: direct current, alternating current and RF current alike, turning electrical energy into heat.
Conductors and insulators
Metals are good conductors because they have many free electrons that are loosely bound to their atoms and can drift when voltage is applied. Density or stiffness has nothing to do with it, and protons are fixed in the nucleus and do not move through a wire.
An insulator has very few free electrons, so it blocks current. Glass is a good insulator, as are plastic, rubber, ceramic and dry air. Sea water conducts because of dissolved salts, stainless steel is a metal, and graphite (carbon) conducts reasonably well.
Direct and alternating current
Direct current (DC) flows in one direction only, as from a battery.
Alternating current (AC) reverses direction repeatedly, swinging between positive and negative directions. It does not just fall to zero and come back the same way; it actually reverses. Household power and radio signals are alternating.
One complete trip through the positive direction and the negative direction, ending where it started, is a cycle. Frequency is the number of complete cycles per second, and its unit is the hertz (Hz), meaning one cycle per second. Wavelength is a distance, not a rate, so it is not a frequency term.
Key facts to memorize
- Current: amperes (A); voltage: volts (V); power: watts (W); resistance: ohms
- Frequency: hertz (Hz), cycles per second
- A voltage difference causes electron flow
- Current is the flow of electrons
- Power is the rate at which electrical energy is used
- Metals conduct because of free electrons; glass is a good insulator
- AC alternates between positive and negative directions
- Resistance opposes DC, AC and RF current
Memory hooks
- Think of a water pipe: voltage is pressure, current is flow rate, resistance is a narrow pipe.
- Watts are speed of energy use; watt-hours are the distance traveled.