GCSE · Physics · Edexcel · Spec 1PH0
Closed circuit with PD source has current
A loop of wire with a lamp and a closed switch. Every part connected, nothing missing from the loop. Yet no current at all. Why?
Physics · Will there be a current?
Judge the circuit
Pick a circuit, then pick where it belongs. Commit first, then read the reason.
Still to sort
There is a current (0)
A source of potential difference and a closed circuit.
Where the line is: Both conditions have to be met at the same time.
No current: no source of potential difference (0)
The loop is complete, but nothing in it supplies a potential difference.
Where the line is: The path is closed, so the problem is not a gap. It is the missing cell, battery or power supply.
No current: the circuit is not closed (0)
There is a source, but the path round is broken.
Where the line is: The source is there, so the problem is a gap in the path.
Six circuits, described in words. Is there a current, and if not, which condition is missing?
Predict, then check
One loop: a cell, a lamp and a switch. The switch sits on the far side of the cell from the lamp.
The lamp is lit. You open the switch on the far side of the cell. The lamp's own wires are still connected. What happens to the lamp?
Physics · The two-question test
Will there be a current?
Start at the top. Pick an answer at each fork and see where the circuit ends up.
Source of potential difference? → Closed circuit?
3 possible circuits.
Ask the questions in order. Only one path ends in a current.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
A loop of wire can look finished and still do nothing at all.
What you need to know
- A current needs two things at once: a source of potential difference and a closed circuit.
- A cell, a battery and a power supply are all sources of potential difference.
Have a goWhich of these is a source of potential difference: a lamp, a battery, a switch or a length of wire?
The battery.
A battery is something that supplies a potential difference. The lamp, the switch and the wire are only parts of the loop it is connected to.
- 'Closed' means one complete loop, with no gaps and no open switch.
- A loose wire or a missing wire is a gap too, not just an open switch.
Have a goKai says: 'The switch is closed, so my circuit is closed.' But one wire has fallen out of its terminal. Is Kai right?
No. The circuit is not closed.
A closed switch is only one part of the loop. A loose wire is a gap too, and a circuit is closed only when the whole loop is complete.
- A complete loop with no source has no current, however neat it looks.
Have a goA lamp and a closed switch sit in a loop of wire, but there is no current. What is the quickest fix: add a cell, use thicker wire, or add a second lamp?
Add a cell.
The loop is already closed. What is missing is a source of potential difference, and neither thicker wire nor a second lamp supplies one.
- A source with a gap in the loop has no current either.
- A gap anywhere in a single loop means no current anywhere in that loop.
- To describe the condition, say both parts: a source of potential difference, and a closed circuit.
The big picture
A current exists in a circuit when the circuit is closed and includes a source of potential difference. If there is no source, there is no current. If the circuit is not closed, there is no current. You need both at once.
Key points
Worked example
Problem
A student joins a bulb and a buzzer in a loop with wires. There is no cell or power supply in it, and one wire is not touching the buzzer. Is there a current? If not, say which conditions are missing.
⚠ Watch out
Thinking a complete loop of wire is enough, or that a cell on its own is enough. A current needs a source of potential difference and a closed circuit together.
Memory hook
Source AND loop. Miss one and the current is gone.
Check yourself
A power supply and a lamp are joined by two wires, but there is an open switch in the loop. Which condition is missing, and what would you do to get a current?
Flashcards
(6)What two things must be true for there to be a current in a circuit?
Name three sources of potential difference you could put in a circuit.
What does 'closed circuit' mean?
What does opening a switch do to a circuit?
Why is a complete loop alone not enough for a current?
A circuit has a battery but a gap somewhere in the loop. Where is there a current?
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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