What is the net force experienced by a bar magnet placed in a uniform magnetic field?
- A.Zero
- B.Depends upon length of the magnet
- C.Never zero
- D.Depends upon temperature
Correct answer
Explanation
The correct answer is A, Zero. In a uniform field the two poles of the magnet feel equal and opposite forces, which cancel each other.
A bar magnet has a north pole and a south pole of equal strength m. In a uniform field B, the north pole is pushed with a force mB along the field and the south pole with the same force mB against it. The net force is therefore zero, so the magnet does not move bodily. However, the two forces act along different lines, so they form a couple and produce a torque τ = MB sin θ, where M is the magnetic moment. This torque turns the magnet until it lines up with the field, which is how a compass needle works.
B is wrong: the length changes the magnetic moment and hence the torque, not the net force.
C is wrong: in a uniform field the net force is always zero; a net force appears only in a non-uniform field.
D is wrong: temperature can weaken a magnet, but the forces on its poles still cancel.
Exam tip: uniform field gives zero net force but a torque; a non-uniform field gives both force and torque.
Practice Questions
View allSyngamy results in formation of
- A.haploid zygote
- B.diploid zygote
- C.non-motile male gametes
- D.motile male gametes
Show answer
Syngamy is the fusion of a male and a female gamete, and since each gamete is haploid, their union gives a diploid zygote.
Read the full explanation →Human insulin molecule is composed of one α-chain having 21 amino acids and one β-chain having 30 amino acids. How many functional insulin genes occur in adult humans?
- A.One
- B.Two
- C.Three
- D.Four
Show answer
Humans have a single functional insulin gene, INS, on the short arm of chromosome 11, and both chains of insulin come from it.
Read the full explanation →Which one of the following is the value of one nanometer?
- A.10⁻⁷ cm
- B.10⁻⁶ cm
- C.10⁻⁴ cm
- D.10⁻³ cm
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One nanometre is 10⁻⁹ metre, and since one metre is 100 (10²) centimetres, it equals 10⁻⁷ centimetre. The prefix nano means one billionth.
Read the full explanation →Which of the following is the general formula for saturated hydrocarbons?
- A.CₙH₂ₙ₊₂
- B.CₙH₂ₙ₋₂
- C.CₙH₂ₙ₊₁
- D.CₙH₂ₙ₋₁
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Saturated open-chain hydrocarbons are the alkanes, and every alkane fits the formula CₙH₂ₙ₊₂. In a saturated hydrocarbon every carbon atom is joined to others only by single bonds, so each carbon holds the largest possible number of hydrogen atoms.
Read the full explanation →Which one of the following is not an epidemic disease?
- A.Cholera
- B.Malaria
- C.Smallpox
- D.Elephantiasis
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Elephantiasis, or lymphatic filariasis, is an endemic disease: it stays steadily present in certain regions and develops slowly over years, rather than breaking out suddenly in a large number of people.
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