Unit notes and practice questions
Everything here is for unit 4 of the Grade 12 Physics textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
The cards filed under this chapter on the Grade 12 Physics page. Open one at a time.
When you use it
Use to calculate the electromotive force induced in a coil by a changing magnetic flux.
Watch out
The negative sign represents Lenz law, indicating that the induced emf opposes the change in flux.
Drafted from Grade 12 Physics, pages 118-142, then checked twice before it went up
When you use it
Use to calculate the magnetic field strength at a given distance from a long straight current-carrying wire.
Watch out
The distance r must be in meters, not centimeters or millimeters.
Drafted from Grade 12 Physics, pages 118-142, then checked twice before it went up
When you use it
Use to determine the total magnetic flux passing through a surface placed in a magnetic field.
Watch out
The angle theta is between the magnetic field and the area normal vector, not the surface plane.
Drafted from Grade 12 Physics, pages 118-142, then checked twice before it went up
When you use it
Use when given the angle between the magnetic field and the surface of a loop rather than its normal line.
Watch out
If the problem gives an angle with the surface plane, you must use 90 minus that angle for theta in the flux equation.
Drafted from Grade 12 Physics, pages 118-142, then checked twice before it went up
When you use it
Use to relate primary and secondary voltages with the number of turns in an alternating current transformer.
Watch out
Transformers operate only with alternating current and cannot step up or step down direct current.
Drafted from Grade 12 Physics, pages 118-142, then checked twice before it went up
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
What happens if a permanent bar magnet is cut into two equal halves?
Answer: B Each piece becomes a new magnet with both a north pole and a south pole.
Why
Magnetic poles are always found in pairs. No matter how many times a permanent magnet is cut into two, each piece always has a north and a south pole.
Question 2
What is the SI unit of magnetic flux?
Answer: C Weber (Wb)
Why
The SI unit of magnetic flux is the Weber (Wb), whereas the SI unit of magnetic field is Tesla (T).
Question 3
How does an electromagnet differ from a permanent magnet?
Answer: A An electromagnet loses its magnetism when the electric current is turned off.
Why
An electromagnet generates a magnetic field only when an electric current is supplied to it, and it loses its magnetism when the current is switched off.
Question 4
Which of the following is a correct difference between electric field lines and magnetic field lines?
Answer: C Magnetic field lines form continuous closed loops without beginning or end.
Why
Unlike electric field lines (which start on positive charges and end on negative charges), magnetic field lines form continuous closed loops without a starting or ending point.
Question 5
A student in Hawassa observes that a long straight wire carrying a steady current produces a magnetic field of 0.8 T at a distance of 0.5 cm from the wire. What is the magnetic field strength at a distance of 1.0 cm from the same wire?
Answer: C 0.4 T
Why
The magnetic field B around a long straight wire is inversely proportional to the distance r (B = μ0I / (2πr)). When distance doubles from 0.5 cm to 1.0 cm, the magnetic field is halved: 0.8 T / 2 = 0.4 T.
Question 6
Which statement correctly describes the Earth's magnetic poles relative to its geographic poles?
Answer: A The Earth's North magnetic pole corresponds to the South Geographic Pole.
Why
The Earth behaves like a giant bar magnet inside, where the North magnetic pole corresponds to the South Geographic Pole and vice versa.
Question 7
A rectangular card with dimensions 3 cm by 2 cm is held perpendicular to a uniform magnetic field of 0.1 T. What is the magnetic flux passing through the card?
Answer: A 6.0 × 10<sup>-5</sup> Wb
Why
Area A = 0.03 m × 0.02 m = 6.0 × 10-4 m2. Since the plane is perpendicular to the field, θ = 0° (cos 0° = 1). Therefore, ΦB = B × A = 0.1 T × 6.0 × 10-4 m2 = 6.0 × 10-5 Wb.
Question 8
Lenz's law, which determines the direction of an induced current, is a consequence of which fundamental principle?
Answer: B Conservation of energy
Why
Lenz's law states that the induced current opposes the change that creates it, which directly follows from the principle of conservation of energy (and Newton's third law).
Question 9
A coil consisting of 500 turns develops an average induced electromotive force (emf) of 60 V. Over what time interval must a magnetic flux change of 0.06 Wb occur across the coil?
Answer: B 0.50 s
Why
Using Faraday's law: |ε| = N × (ΔΦB / Δt). Rearranging for Δt gives Δt = (N × ΔΦB) / |ε| = (500 × 0.06 Wb) / 60 V = 30 / 60 = 0.50 s.
Question 10
In an electrical workshop in Addis Ababa, a technician uses a transformer with 500 primary loops and 5000 secondary loops. If the primary alternating input voltage is 220 V, what is the output voltage from the secondary coil?
Answer: C 2200 V
Why
Using the transformer formula: Vs / Vp = Ns / Np. Thus, Vs = Vp × (Ns / Np) = 220 V × (5000 / 500) = 220 V × 10 = 2200 V.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 4 runs from page 118 to page 142 of the Ministry of Education edition.
Ministry of Education edition, English, 186 pages
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