medium importancemedium difficulty6 min

AC and DC

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

In one line: AC alternates, DC directs one way.

Practice This Concept

Understand it

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.

Example

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

Formula and rule

AC changes direction periodically; DC flows in one direction.

AC is often described by time-varying current, while DC remains steady in direction.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1A battery-powered toy works on DC because the current flows in one direction.
2A home fan works on AC because the supply keeps changing direction periodically while still delivering usable power.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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.

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.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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

1
Enough for 1 mark
AC is alternating current, which changes direction periodically, while DC is direct current, which flows in one direction only.
2
Add for 2 marks
Example: A torch cell provides DC, while the electricity supplied to homes is AC.
3
Add for 3 marks
In DC, the flow of charge remains in the same direction.

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