2.3.5Electricity
AC and DC — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions
5 min read 11 parts Lesson 14 of 18
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Two types of current
Direct current (d.c.)
- Direct current (d.c.) The charge always flows in one direction round the circuit.
- Cells and batteries supply d.c., and the potential difference stays the same sign.
Alternating current (a.c.)
- Alternating current (a.c.) The current repeatedly reverses direction, because the supply p.d. keeps changing between positive and negative.
- Mains electricity and generators such as alternators produce a.c.
Electrons in a.c.
- Electrons in a.c. The charges do not travel round the circuit;
- they oscillate back and forth about a fixed position while energy is transferred.
Part 2 of 4Mains values in the UK
| Quantity | Value |
|---|---|
| Frequency | 50 Hz (50 complete cycles every second) |
| Potential difference | About 230 V |
| Type of current | Alternating (a.c.) |
| Time for one cycle | 1 ÷ 50 = 0.02 s |
Part 3 of 4Reading a trace
1
A d.c. supply shows as a flat horizontal line at a steady voltage above or below zero.
2
An a.c. supply shows as a repeating wave that crosses zero, rising to a positive peak and falling to a negative peak.
3
Find the period T by measuring the time for one full wave, using the time-base setting of the oscilloscope.
4
Calculate frequency from f = 1 ÷ T, where f is in hertz (Hz) and T is in seconds.
5
Measure the peak voltage as the height of the wave above zero multiplied by the voltage scale on the y-axis.
Part 4 of 4Comparing the two
a.c. versus d.c.
Direction
- Direction D.c. is one-way; a.c. reverses 100 times a second on the mains (twice per cycle).
Typical sources
- Typical sources D.c.: cells, batteries, solar cells. A.c.: power stations, generators.
Voltage trace
- Voltage trace D.c. is a constant line; a.c. is a regularly repeating wave.
Why mains is a.c.
- Why mains is a.c. Alternating p.d. can be raised and lowered efficiently by transformers, making long-distance transmission cheaper.
Key terms
| Term | Meaning |
|---|---|
| Direct current | Current that flows in one direction only. |
| Alternating current | Current that repeatedly changes direction. |
| Frequency | The number of complete cycles per second, measured in hertz (Hz). |
| Period | The time taken for one complete cycle of an alternating supply. |
| Oscilloscope | An instrument that displays how a potential difference varies with time. |
Avoid these mistakes
1
Writing that a battery gives a.c. because the current 'moves' — batteries give d.c.
2
Claiming electrons travel all the way from the power station in a.c.; they only vibrate to and fro.
3
Using the period as the frequency; frequency is the inverse of the period.
4
Giving the UK mains voltage as 320 V or 110 V; the figure is about 230 V.
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