Which one of the following devices is non-ohmic ?
- A.Conducting copper coil
- B.Electric heating coil
- C.Semi conductor diode
- D.Rheostat
Correct answer
Explanation
The correct answer is C, Semi conductor diode. A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it.
An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature. A p-n junction diode behaves very differently. In forward bias almost no current flows until the voltage passes a threshold of about 0.7 V for silicon, after which the current rises steeply. In reverse bias only a tiny current flows until breakdown. Its current-voltage graph is therefore curved and one-sided, which is exactly why diodes are useful as rectifiers that turn alternating current into direct current. Other non-ohmic devices include transistors, thermistors and electrolytes.
A is wrong: a copper coil is a metallic conductor and follows Ohm's law at constant temperature.
B is wrong: a heating coil of nichrome is also a metallic, ohmic resistor.
D is wrong: a rheostat is a variable wire resistor and is ohmic.
Exam tip: metallic conductors are ohmic; diodes, transistors and thermistors are non-ohmic.
Practice Questions
View allConsider the following animals :
1. Hedgehog
2. Marmot
3. Pangolin
To reduce the chance of being captured by predators, which of the above organisms rolls up/roll up and protects/protect its/their vulnerable parts?
- A.1 and 2
- B.2 only
- C.3 only
- D.1 and 3
Show answer
Hedgehogs and pangolins curl into a tight ball when threatened, covering the soft belly and face with spines or scales; marmots do not.
Read the full explanation →Which one of the following is used in preparing a natural mosquito repellent?
- A.Congress grass
- B.Elephant grass
- C.Lemongrass
- D.Nut grass
Show answer
Lemongrass and its close relative citronella grass yield aromatic oils that keep mosquitoes away, so they are the base of many natural repellents.
Read the full explanation →What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)
- A.286 u
- B.106 u
- C.83 u
- D.53 u
Show answer
Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u. Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound.
Read the full explanation →Which one of the following minerals is used as a fuel in nuclear power stations ?
- A.Bauxite
- B.Quartz
- C.Feldspar
- D.Pitchblende
Show answer
Pitchblende, or uraninite, is the chief ore of uranium, the fuel used in most nuclear power stations. Pitchblende is a heavy, black mineral made mostly of uranium oxide.
Read the full explanation →The raw materials used for the manufacture of Portland cement are
- A.Lime, silica and sulphur dioxide
- B.Lime, silica and carbon dioxide
- C.Lime, silica and alumina
- D.Lime, silica and boric acid
Show answer
Portland cement is made by heating a mix of limestone, which supplies lime, with clay, which supplies silica and alumina.
Read the full explanation →