Text lesson

Unit 2 — Electricity and Magnetism

Science — Third Preparatory

1.6 Electric resistance

is the opposition that a conductor offers to the flow of electric charges through it.
Its unit is the ohm (Ω); it is measured with an ohmmeter.

A fixed resistor has a constant resistance.
A variable resistor (rheostat) has a resistance that can be changed by moving a slider, which changes the length of wire through which the current flows. A longer length of wire gives larger resistance, so the total circuit resistance increases and the current decreases. Application: the volume control knob of a radio is a variable resistor: turning it changes the resistance, hence the current and the sound volume.
Factors affecting the resistance of a conductor

(at a given temperature).
In a light bulb, the filament is a very thin, long tungsten wire wound into a coil, while the lead-in wires are short, thick copper.
When the same current passes through both, only the filament gets white-hot.
The reason the filament has far larger resistance.
Resistance increases when:

the length of the wire through which the current flows increases;
the cross-sectional area (thickness) decreases;
the material has a larger resistivity (each material has its characteristic value; tungsten has a higher resistivity than copper).

Also, the higher the resistance of a wire carrying a given current, the more heat is produced in it.
That is the principle of heaters, irons and incandescent lamps.

Scientist profile. Thomas Edison (1847–1931), an American inventor with 1093 US patents, including the phonograph, the motion-picture camera and projector, and a long-lasting incandescent lamp.
He contributed to the development of electric power systems.

Unit 2 — Electricity and Magnetism · 7 / 25

1.6 Electric resistance