If a free electron moves through a potential difference of 1 kV, then the energy gained by the electron is given by
- A.1.6 × 10⁻¹⁹ J
- B.1.6 × 10⁻¹⁶ J
- C.1 × 10⁻¹⁹ J
- D.1 × 10⁻¹⁶ J
Correct answer
Explanation
The correct answer is B, 1.6 × 10⁻¹⁶ J. The energy gained is the charge of the electron multiplied by the potential difference it crosses.
Energy W = qV. The electron's charge is 1.6 × 10⁻¹⁹ C and the potential difference is 1 kV, that is 1000 V or 10³ V. So W = 1.6 × 10⁻¹⁹ × 10³ = 1.6 × 10⁻¹⁶ J. In atomic physics this amount is written simply as 1 keV (kilo-electron-volt), since one electron-volt is the energy an electron gains across 1 volt, 1.6 × 10⁻¹⁹ J.
A is wrong because 1.6 × 10⁻¹⁹ J is the energy gained across only 1 volt, one electron-volt.
C is wrong because it drops the electron's charge value of 1.6 and also uses 1 volt instead of 1000 volts.
D is wrong because it has the right power of ten but leaves out the factor 1.6 from the electron's charge.
Exam tip: 1 eV = 1.6 × 10⁻¹⁹ J, so 1 keV = 1.6 × 10⁻¹⁶ J and 1 MeV = 1.6 × 10⁻¹³ J.
Practice Questions
View allThe principle involved in paper chromatography is
- A.Absorption
- B.Partition
- C.Solubility
- D.Volatility
Show answer
Paper chromatography separates substances because each one divides, or partitions, itself differently between two liquid phases. In paper chromatography the stationary phase is the water held in the cellulose fibres of the paper, and the mobile phase is the solvent that rises up the paper by capillary action.
Read the full explanation →Which one of the following phenomena CANNOT be exhibited by sound waves ?
- A.Reflection
- B.Refraction
- C.Interference
- D.Polarisation
Show answer
Sound in air is a longitudinal wave, and only transverse waves can be polarised. Polarisation means restricting the vibrations of a wave to a single plane.
Read the full explanation →Which one of the following is NOT a luminous object ?
- A.Sun
- B.Electric lamp
- C.Candle
- D.Moon
Show answer
The Moon gives out no light of its own; it only reflects sunlight, so it is a non-luminous object. A luminous object produces and emits its own light, while a non-luminous object is seen only because it reflects light falling on it.
Read the full explanation →In experiment #1, a bar magnet is moved towards a conducting wire loop axially, with the magnet's north pole facing the loop. In experiment #2, the same process as in experiment #1 is repeated except that the south pole of the magnet faces the loop. Which one of the following statements is true in this context?
- A.The direction of current in the loop will be of opposite nature in both the experiments.
- B.The direction of current in the loop will be the same in both the experiments.
- C.No current will flow in either of the two experiments.
- D.More current will flow in the loop in experiment #1.
Show answer
Reversing the pole that approaches the loop reverses the change in magnetic flux, so the induced current reverses too. When a magnet moves towards a closed loop, the magnetic flux through the loop changes and an electric current is induced (Faraday's law of electromagnetic induction).
Read the full explanation →Which one of the following statements about classification of plants is correct?
- A.Thallophytes have well differentiated body design
- B.Funaria is a fungus
- C.All Pteridophytes are Phanerogams
- D.Vascular system is not found among Bryophytes
Show answer
Mosses and liverworts have no xylem or phloem, so water moves through them cell to cell. Bryophytes, such as Funaria (a moss) and Marchantia (a liverwort), are called the amphibians of the plant kingdom because they live on land but need water for fertilisation.
Read the full explanation →