GCSE · Physics · AQA · Spec 8463
Direct and alternating potential difference
A battery pushes one way, all the time. Your wall socket keeps turning its push round, back and forth, many times every second. Same unit, very different pattern.
Follow the mains p.d. as it changes direction
Drag the point from 0 to 40 ms. Above zero the p.d. pushes one way; below zero it pushes the other way (1 and −1 are its biggest values each way). Up, down and back to the start is one full cycle: find where it ends.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Direct and alternating potential difference
What you need to know
- The one difference between direct and alternating p.d. is direction.
- Mains electricity is an ac supply.
- UK mains: a frequency of 50 Hz and a p.d. of about 230 V.
- What 50 Hz means: 50 complete cycles every second.
The big picture
Potential difference (p.d.) comes in two patterns. A direct p.d. always acts in the same direction; cells and batteries supply one. An alternating p.d. keeps reversing direction. Mains electricity is an ac (alternating) supply: in the UK it has a frequency of 50 Hz, which means 50 complete cycles every second, and a p.d. of about 230 V.
Key points
Worked example
Problem
A student looks at a p.d.–time trace from a supply. The trace crosses zero again and again, and exactly 4 complete cycles fit into 0.08 s. Is the p.d. direct or alternating, and what is its frequency?
⚠ Watch out
Saying an alternating p.d. 'switches on and off'. It never switches off: it passes through zero for an instant and carries on in the opposite direction. The difference from a direct p.d. is direction, not on and off.
Memory hook
Direct: Doesn't Change direction. Alternating: Always turning round. UK mains is '50 and about 230': 50 Hz and about 230 V.
Check yourself
Cover the page. In one sentence, what is the only difference between a direct and an alternating p.d.? Then give the two numbers for UK mains, each with its unit.
Flashcards
(8)What is a direct potential difference?
What is an alternating potential difference?
Is mains electricity an ac or a dc supply?
What is the frequency of the UK domestic electricity supply?
What is the p.d. of the UK domestic electricity supply?
What does a frequency of 50 Hz mean?
Name a supply that gives a direct p.d.
On a p.d.–time graph, how can you tell an alternating p.d. from a direct one?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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