Circuit symbols, charge and current
Circuit symbols
| Component | What the symbol looks like |
|---|---|
| Cell | A long thin line beside a short thick line (the long line is the positive side) |
| Two or more cell symbols joined in a row | |
| A gap in the wire with a hinged line: lifted when open, lying across the gap when closed | |
| A circle with a cross inside | |
| A small rectangle with the wire running through it lengthways | |
| An empty rectangle | |
| A resistor with a diagonal arrow through it | |
| A resistor inside a circle, with two arrows pointing in towards it | |
| A resistor with a diagonal line through it that has a short flat end | |
| A triangle pointing at a short line across the wire | |
| A diode with two arrows pointing out, away from it | |
| A circle with A inside | |
| A circle with V inside |
Electric current
is the . In metal wires, the charge is carried by electrons.
For a current to flow, you need a closed circuit with a source of , such as a cell. In a single closed loop, the current is the same at every point.
You measure current with an connected in series, in the loop with the component. You measure potential difference with a connected in parallel, across the component.
Common mistake
Ammeters go in the loop (series); voltmeters go across a component (parallel). Mixing them up is a very common mistake in circuit diagrams.
Key terms
Charge and current: Q = It
You must recall this equation:
, or Q = It
| Quantity | Symbol | Unit |
|---|---|---|
| Charge flow | Q | coulombs (C) |
| Current | I | amperes, or amps (A) |
| Time | t | seconds (s) |
Example: a current of 0.5 A flows through a lamp for 2 minutes. How much charge flows?
- Convert: 2 minutes = 120 s
- Q = It = 0.5 × 120
- Q = 60 C