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A-Level Magnetic flux, the induction laws of Faraday and Lenz, and power transformers
What the A-Level syllabus expects for Magnetic flux, the induction laws of Faraday and Lenz, and power transformers, and how to practise it.
What the syllabus expects
- Define magnetic flux as the flux density multiplied by the area held perpendicular to that flux density
- Understand the concept of magnetic flux linkage and work with it
- Apply Φ = BA and NΦ = NBA in problems
Scope: N is the number of turns - Conclude from suitable induction experiments that a changing magnetic flux can drive an e.m.f.
- Conclude from those experiments that the induced e.m.f. is directed so as to fight the change that produces it
- Identify from those experiments what controls how large the induced e.m.f. turns out to be
- Put Lenz's law together with Faraday's law of electromagnetic induction to solve problems
- Explain a few everyday applications of electromagnetic induction
- Understand how a basic iron-core transformer works, and use Ns/Np = Vs/Vp = Ip/Is for the ideal case in problems
How it's examined
Questions on this topic most often ask you to find, identify, state. About 6% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (2 marks)
State Faraday's law of electromagnetic induction.
Show the worked answer
Faraday's law of electromagnetic induction: the magnitude of the induced e.m.f. is directly proportional to (equal to) the rate of change of magnetic flux linkage through the circuit.
Example 2 (2 marks)
Consider the four level rotor blades turning within the Earth's magnetic field. Give the potential difference between the far tips of two blades that point in exactly opposite directions, and justify your answer.
Show the worked answer
Consider two diametrically opposite blades of the same rotor turning in the Earth's field. Each blade acts as a rod sweeping through the field, so a motional emf is induced between the hub and each tip. For the two opposite blades, both the velocity v and the radial direction (dl from hub to tip) reverse together, so the integral of (v x B).dl from hub to tip has the same sign and magnitude for each blade. Hence both tips reach the same potential relative to the hub. The potential difference between the two far tips is therefore the difference of two equal potentials = 0.
Example 3 (2 marks)
For an oscillating LC circuit (C = 2.0 μF charged initially to 40 V, L = 8.0 mH): at the moment the capacitor becomes completely discharged, the circuit current reaches its maximum value. Work out this maximum current in the circuit.
Show the worked answer
Energy is conserved in the LC circuit. When the capacitor is fully discharged all its initial electrical energy is stored in the inductor as magnetic energy, and the current is maximum. (1/2) C V² = (1/2) L I_max² I_max = V sqrt(C/L) = 40 x sqrt(2.0x10⁻⁶ / 8.0x10⁻³) = 40 x sqrt(2.5x10⁻⁴) = 40 x 0.01581 = 0.63 A.
More worked questions on this topic
- For that ideal transformer (2700 : 450 turns), the primary e.m.f. reads E = 220 sin(100πt) volt (3 marks)
- The 0.72 m diameter disc in the 0.017 T field cuts about 6.9 × 10⁻³ Wb per revolution and spins (3 marks)
- Coupled to a vehicle's wheels, a massive aluminium disc 0.72 m across spins in a 0.017 T field (3 marks)
More A-Level H2 Physics topics
Physical quantities, units, measurement uncertainty and vector basics · Types of force, turning effects and conditions for equilibrium · Kinematics, uniformly accelerated motion, momentum and Newton's laws · Energy stores and transfers, work, kinetic and potential energy, fields, power and efficiency · Falling freely, the gravitational potential energy of a uniform field, and how air resistance changes the motion · Impulse and the conservation of momentum and energy · all of A-Level H2 Physics