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AC and DC

AC is alternating current, which changes direction periodically, while DC is direct current, which flows in one direction only.

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Student-friendly explanation

In DC, the flow of charge remains in the same direction. A battery gives DC in school-level circuits. In AC, the current changes direction again and again, so its value and direction vary with time. Household mains supply is AC. This difference matters because some devices and transmission systems are designed for one type more than the other.

How to write this in exams

  1. 1

    Start with the exact idea

    AC is alternating current, which changes direction periodically, while DC is direct current, which flows in one direction only.

  2. 2

    Then show how to use it

    1. Check whether current changes direction. 2. If it changes periodically, it is AC. 3. If it flows in one direction only, it is DC. 4. Give one example source. 5. Mention a use or advantage if asked.

  3. 3

    Add one concrete example

    A torch cell provides DC, while the electricity supplied to homes is AC.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is to say AC and DC are the same because both carry electric charge. They differ in direction and time variation.

Definition

AC is alternating current, which changes direction periodically, while DC is direct current, which flows in one direction only.

Example

A torch cell provides DC, while the electricity supplied to homes is AC.

Rule to remember

AC changes direction periodically; DC flows in one direction. AC is often described by time-varying current, while DC remains steady in direction.

Memory hook

AC alternates, DC directs one way.

Examples and method

Worked example

A battery-powered toy works on DC because the current flows in one direction. A home fan works on AC because the supply keeps changing direction periodically while still delivering usable power.

Method to apply

1. Check whether current changes direction. 2. If it changes periodically, it is AC. 3. If it flows in one direction only, it is DC. 4. Give one example source. 5. Mention a use or advantage if asked.

Diagram support

Draw a current-time graph for AC as a wave crossing zero and a straight positive line for DC.

How CBSE asks it

Differentiate AC and DC, identify a source as AC or DC, or compare their circuit use.

Avoid common mistakes

Common confusion

Students often think AC and DC differ only in strength. The key difference is direction and variation with time.

Common wrong answer

A common wrong answer is to say AC and DC are the same because both carry electric charge. They differ in direction and time variation.

Exam tip

For short answers, clearly state direction change for AC and single direction for DC, then give one example each.

Quick check

Why is household electricity usually supplied as AC instead of DC?

Household electricity is usually supplied as AC because it can be transformed to high or low voltages more easily for transmission and distribution. It also fits the design of the power grid better than DC in the school-level context.

Answer writing and exam use

1-mark answer

AC is alternating current, which changes direction periodically, while DC is direct current, which flows in one direction only.

2-mark answer

AC is alternating current, which changes direction periodically, while DC is direct current, which flows in one direction only. AC changes direction periodically; DC flows in one direction. AC is often described by time-varying current, while DC remains steady in direction. A torch cell provides DC, while the electricity supplied to homes is AC.

3-mark answer

In DC, the flow of charge remains in the same direction. A battery gives DC in school-level circuits. In AC, the current changes direction again and again, so its value and direction vary with time. Household mains supply is AC. This difference matters because some devices and transmission systems are designed for one type more than the other. AC changes direction periodically; DC flows in one direction. AC is often described by time-varying current, while DC remains steady in direction. A battery-powered toy works on DC because the current flows in one direction. A home fan works on AC because the supply keeps changing direction periodically while still delivering usable power. Differentiate AC and DC, identify a source as AC or DC, or compare their circuit use. A common wrong answer is to say AC and DC are the same because both carry electric charge. They differ in direction and time variation.
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