Charge carries electrical effects.
Electrons carry negative charge. In metal conductors, some electrons can move through the material. Electric current describes a net movement of charge, not how much energy a device has stored.
A source such as a battery establishes a voltage difference. When a suitable complete path is connected, charge can flow and energy can be transferred to a load.
The amount of charge is measured in coulombs (C). A coulomb is a unit of charge, not a unit of energy. Current tells us how much charge passes a point each second.
Voltage, current and resistance.
Voltage is energy per unit charge between two points, measured in volts (V). Current is the rate of charge flow past a point, measured in amperes (A). Resistance, in ohms (Ω), describes opposition to current.
These quantities are connected but not interchangeable. A voltage can exist across an open switch even though no current flows through that gap.
- 1 ampere = 1 coulomb per second
- A current of 2 A means 2 C of charge passes a point each second. It does not mean that the circuit contains only 2 C.
- 1 volt = 1 joule per coulomb
- A 12 V difference corresponds to 12 J of energy transferred per coulomb passing through that voltage difference.
A first worked example.
For an ohmic resistor, V = I × R. Rearranging gives I = V ÷ R, so 12 ÷ 6 = 2 A.
Keeping the voltage at 12 V but increasing the resistance to 12 Ω reduces current to 1 A. This is a simple resistor model, not a complete description of an AC motor.
- In one second, 2 C passes through the resistor.
- The 12 V across it means 12 J per coulomb is transferred.
- 2 C × 12 J/C = 24 J in one second, or 24 W.
The resistor converts that electrical energy into heat. The charge is not used up: in this steady series circuit, charge flows into and out of the resistor at the same rate.
This links voltage and current to electrical power and energy. It also explains why charge flow and energy transfer are different quantities.
Resistance is only part of the AC picture.
The resistor example is a useful starting point. When voltage and current alternate, energy storage in a component also affects its response:
- Resistance converts electrical energy into heat.
- Inductance stores energy in a magnetic field and opposes changes in current.
- Capacitance stores energy in an electric field as charge separates, and opposes changes in voltage.
With a sinusoidal supply, current can reach its peak before or after voltage. This timing difference is called a phase difference. Inductive and capacitive effects produce reactance, which changes with frequency.
Impedance combines resistance and reactance into the overall AC response: both the size of the current and its phase relative to voltage. It is measured in ohms, but it is not usually found by simply adding the resistance and reactances as ordinary positive numbers.
An operating motor also exchanges energy with its mechanical load. Its current cannot be predicted from winding resistance alone. The AC and DC lesson introduces alternating supplies; watts, VA and power factor explains the energy consequence.
Optional: how the quantities fit together
For ideal components at one sinusoidal frequency, inductive reactance is XL = 2πfL and capacitive reactance is XC = 1/(2πfC). Frequency f is in hertz, inductance L in henries and capacitance C in farads. Both reactances are in ohms.
For a resistor, inductor and capacitor connected in series, the impedance magnitude is:
|Z| = √[R² + (XL − XC)²]
Then IRMS = VRMS / |Z|. RMS means root mean square, an effective value explained in the AC lesson. These equations describe that particular linear circuit in sinusoidal steady operation; they are not a complete model of a running lift motor.
Connect the terms to a lift.
A report might describe a supply voltage, motor current or a resistive connection. Each refers to a different property. Knowing the difference helps you ask what was observed without jumping to a diagnosis.
The next lesson follows the path that current takes through a circuit.
A closer look
Conventional current is described as flowing from positive to negative through the external circuit. Electrons in metal drift in the opposite direction. Circuit calculations normally use conventional current.
Check your understanding
Does a voltage difference always mean current is flowing?
Reveal the explanation
No. There can be voltage across a gap in an open circuit while no current flows through that gap.
Further reading.
Optional reading to explore this lesson in more detail.
- OpenStax: current and charge
Start with section 9.1, especially the current definition and circuit diagrams. The later calculus example is optional; the diagrams explain conventional current and electron movement.
- OpenStax: Ohm’s law and simple circuits
Section 20.2 develops voltage, current and resistance with worked calculations. A useful next step after the lesson’s 12 V example.
- OpenStax: voltage as energy per charge
Read the potential-difference definition and the volt = joule/coulomb relationship. These explain how charge and energy are related without being the same quantity.
- OpenStax: resistance, inductance and capacitance in AC circuits
Compare the resistor, capacitor and inductor sections. Their voltage/current plots show what leading and lagging mean; the calculus is optional.
- OpenStax: combining resistance and reactance
Advanced extension: equation 15.11 gives the impedance magnitude for a series RLC circuit. The surrounding phasor diagrams explain why the parts are not added as ordinary positive numbers.