What sets a wire's resistance, and circuit symbols
At a given temperature, the resistance of a wire is greater when the wire is longer, thinner or made of a material with a higher resistivity. Each material has its own resistivity. For the same material and thickness, 2 m of wire has twice the resistance of 1 m. This explains a bulb. Its filament is a long, thin, coiled tungsten wire, so it has a very large resistance, while the connecting wires are short and thick copper with a very small resistance. The same current passes through both, but the filament gets by far the most heat and glows, while the connecting wires stay cool. The American inventor Thomas Edison (born 1847, died 1931) is known for a long-lasting filament bulb, the phonograph and a motion-picture camera and projector; 1093 US patents were granted to him. In diagrams we use standard symbols, so that anyone can read them: a cell, a battery, an open switch, a closed switch, a bulb, a fixed resistor, a variable resistor (rheostat), an ammeter and a voltmeter. Careful: “thicker wire has less resistance” is the rule, not “thicker wire carries less current”.
| To get more resistance, make the wire… | because |
|---|---|
| longer | the charge has a longer path |
| thinner | less room for the charge to move |
| of a higher-resistivity material | the material resists more |
Symbol practice, on squared paper. Copy the symbols for a cell, a switch (open and closed), a bulb, a resistor, a rheostat, an ammeter and a voltmeter. Then draw three circuits from the descriptions: (1) a cell, a switch and a bulb; (2) the same with a rheostat; (3) the same with a voltmeter across the bulb. Swap papers with a friend and check each other. No real wires are needed.