Direct Current & Alternating Current
Key idea: Learn the difference between direct current (d.c.) and alternating current (a.c.), how to interpret waveforms, and how to use frequency and period in O Level questions.
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The core idea
On this page
Learning objectives
- Recognise and interpret circuit symbols for cells, batteries, switches, lamps, LEDs, resistors, fuses, ammeters and voltmeters
- Draw circuit diagrams with cells, batteries, switches, lamps, LEDs, fixed and variable resistors, fuses, ammeters and voltmeters
- Recognise and interpret circuit symbols for d.c. and a.c. supplies, potentiometers, bells, light-dependent resistors and thermistors
- Draw circuit diagrams with d.c. and a.c. supplies, potentiometers, bells, light-dependent resistors and thermistors
- Apply the same-current rule in series circuits
- Apply the potential-difference sum in series circuits
- Apply current conservation at parallel junctions
- Apply equal potential difference across parallel branches
- Calculate effective resistance in series
- Calculate effective resistance in parallel
- Solve whole-circuit problems using consistent quantities
- Describe variable-potential-divider action
- Describe NTC thermistor action as an input transducer
- Describe light-dependent resistor (LDR) action as an input transducer
- Solve simple NTC and LDR potential-divider problems
1. Definition
A. Direct current (d.c.)
Direct current (d.c.) is a current that flows in one direction only.
B. Alternating current (a.c.)
Alternating current (a.c.) is a current that reverses direction periodically (back and forth).
Topic 15 directly requires you to draw the d.c. supply and a.c. supply symbols. Waveform, period and frequency work on this page is useful supporting knowledge, not a separate Topic 15 learning outcome.
2. Key Ideas
| Feature | d.c. | a.c. |
|---|---|---|
| Direction | One direction only | Reverses direction |
| Sign on a current–time graph | Stays on one side of zero | Alternates between positive and negative |
| Common source | Cell or battery | A.C. generator or mains supply |
d.c. does not mean “constant current”. The size can change — it is still d.c. as long as it does not reverse direction.
3. Detailed Explanations
A. A reliable waveform test
Use the zero line as the boundary between opposite current directions:
- If the trace remains entirely above zero (or entirely below zero), the current is d.c. It may be steady or varying.
- If the trace takes both positive and negative values repeatedly, the current is a.c.
Touching zero is not enough on its own. The trace must pass to the other side of zero for the direction to reverse.
B. Frequency and period
Frequency, f (Hz), is the number of cycles per second.
- f = 50 Hz means 50 cycles per second
- Period, T (s), is the time for one cycle: T = 1/f
On a graph, measure T between two consecutive points in the same phase, such as peak to next peak. Then use f = 1/T.
For d.c. or a.c., discuss direction. For frequency or period, discuss repetition in time. These are different questions.
4. Common Mistakes
- Saying “d.c. means the current is constant” (d.c. means one direction).
- Calling a waveform a.c. merely because its size changes.
- Measuring half a cycle and using it as the period.
- Mixing up “electron flow vs conventional current” with “a.c. reverses direction” (different ideas).
- Saying “50 Hz means 50 seconds per cycle” (it is 50 cycles per second).
5. Exam Tips
- Compare by direction, then give a common source (battery versus mains).
- Mark two matching points before reading a period from a graph.
- Write the unit with the answer: hertz (Hz) for frequency and seconds (s) for period.
6. Worked Examples
Modelled example 1
Identify d.c. or a.c.
Problem
Study the worked solution
Classify the battery supply
Method
Identify the torch battery as d.c.Reason
Its conventional current has one direction through the external circuit.Working
Battery → d.c.Classify the mains supply
Method
Identify the wall-socket supply as a.c.Reason
The current reverses direction periodically.Working
Mains → a.c.
Guided practice 2
Frequency to cycles
Problem
Try this before viewing the solution
Hints
Hint 1: read hertz as a rate
View solution step by step
Translate frequency
Method
Use 50 cycles per second.Reason
Hertz is the unit of cycles per second.Working
f = 50 s⁻¹.Scale by time
Method
Multiply frequency by 2.0 s.Reason
The cycle rate is constant over the stated interval.Working
N = ft = 50 × 2.0 = 100 cycles
Common misconception 3
“Is it still d.c.?”
Learner claim
Try this before viewing the solution
View solution step by step
Use the correct criterion
Method
Check the sign or direction, not whether magnitude is constant.Reason
d.c. means one direction only.Working
The current remains positive.Correct the classification
Method
Classify it as varying d.c.Reason
It never reverses direction or crosses to the opposite side of zero.Working
Varying magnitude, one direction → d.c.
Examiner practice 4
Period from frequency
Examination question
Try this before viewing the solution
View solution step by step
Select the relationship
1 markMethod
Use T = 1/f.Reason
Period is time per cycle, reciprocal to cycles per second.Working
T = 1/f.Substitute
1 markMethod
Insert f = 60 Hz.Reason
Hertz is reciprocal seconds.Working
T = 1/60 s.Evaluate
1 markMethod
Give 0.0167 s or 0.017 s appropriately rounded.Reason
This is one cycle’s duration.Working
T ≈ 0.017 s.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark relationship, substitution and result.
Challenge 5
Waveform identification
Waveform transfer
Try this before viewing the solution
Hints
Hint 1: treat zero as the direction boundary
View solution step by step
Read direction from sign
Method
Observe that the trace never crosses below zero.Reason
Its current direction therefore never reverses.Working
Sign remains positive.Classify
Method
Call it d.c.Reason
Changing magnitude is compatible with one-direction current.Working
Varying d.c.
Challenge 6
Reading a waveform
Graph-reading transfer
Try this before viewing the solution
Hints
Hint 1: identify one complete cycle
View solution step by step
Read the period
Method
Take the matching-peak separation as one complete cycle.Reason
The two peaks are at the same phase.Working
T = 0.025 s.Find frequency
Method
Calculate f = 1/T.Reason
Frequency and period are reciprocals.Working
f = 1/0.025 = 40 Hz
7. Mind Stretchers
Mind stretcher 1: PeriodExtension
The mains is 50 Hz. Find the period T.
Show Answer
T = 1/f = 1/50 = 0.020 s
Mind stretcher 2: Direction changesExtension
How many times does a sinusoidal alternating current change direction during one complete cycle?
Show Answer
Twice: once from positive to negative and once from negative to positive.
8. Practice and next step
- Identify whether each waveform is d.c. or a.c. by checking whether it crosses the zero-current line, not whether its magnitude changes.
- Review the required d.c. and a.c. supply symbols.
- Continue with the core Topic 15 rules in Series & Parallel Circuits.
Continue with the next resource in this course.
Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027