AC & DC

Types of Current

Direct Current (DC) is an electric current that flows in one direction only. The electrons move steadily from the negative terminal to the positive terminal of a power source.

Alternating Current (AC) is an electric current that continually changes direction. The electrons oscillate back and forth, reversing direction many times per second.

In DC, the flow of electrons is steady and unidirectional, while in AC, the flow changes direction periodically.

Common sources of DC include batteries and cells, which provide a constant voltage and current in one direction. AC is typically supplied by the mains electricity in homes and businesses, generated by power stations.

Characteristics of AC

AC voltage and current vary continuously with time. The voltage rises from zero to a maximum positive value, falls back through zero to a maximum negative value, and repeats this cycle.

The number of complete cycles per second is called the frequency, measured in hertz (Hz). In the UK, mains electricity has a frequency of 50 Hz, meaning the current changes direction 50 times every second.

The maximum value of the voltage or current in a cycle is called the amplitude.

The shape of the AC waveform is usually a sine wave, which smoothly oscillates between positive and negative values.

For example, the UK mains supply voltage varies between approximately +325 V and -325 V (peak voltage), with a root mean square (rms) voltage of about 230 V. The rms voltage is the effective voltage that delivers the same power as a DC voltage of the same value.

Characteristics of DC

DC voltage and current remain constant over time, providing a steady flow of electrons in one direction.

This means the voltage does not change polarity or magnitude, unlike AC.

Common DC sources include batteries, cells, and solar panels. These are used in devices like torches, mobile phones, and laptops.

For instance, a typical AA battery provides a constant voltage of about 1.5 V DC.

Applications of AC and DC

AC is mainly used for:

• Mains electricity supply to homes, schools, and businesses.
• Powering large appliances like washing machines, ovens, and lights.
• Transmission over long distances because AC voltage can be easily transformed to higher or lower voltages, reducing energy loss.

DC is mainly used for:

• Battery-powered devices such as phones, laptops, and flashlights.
• Electronics that require a steady voltage, like radios and computers.
• Electric vehicles and solar power systems.

Advantages of AC:

• Easier to change voltage levels using transformers.
• Efficient for long-distance power transmission.

Disadvantages of AC:

• Can be more dangerous due to high voltages and changing direction.

Advantages of DC:

• Provides a steady voltage, ideal for sensitive electronics.
• Simple and safe for low-voltage applications.

Disadvantages of DC:

• Not practical for long-distance transmission without conversion.
• Voltage cannot be easily changed.

Example: The UK mains supply is 230 V AC at 50 Hz. This means the voltage oscillates 50 times per second, reaching a peak voltage of about 325 V, but the effective voltage measured (rms) is 230 V.

Inline example: If a battery provides 9 V DC, the voltage remains constant at 9 V and does not change direction or magnitude over time.