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Faraday, Lenz and mutual induction

الأسبوع 8 · الوحدة الأولى، الفصل الثالث «الحث الكهرومغناطيسي» (١): تجربة فاراداي والقوة الدافعة المستحثة وقانون فاراداي وقاعدة لنز وقاعدة اليد اليمنى لفلمنج، والحث المتبادل بين ملفين · 3 دروس
2

Lenz’s rule and Fleming’s right-hand rule

Physics Sec 3 Term 1, Chapter 3: pp. 60–67
GoalDecide the direction of an induced current with Lenz’s rule and with Fleming’s right-hand rule.
New words
Lenz’s rule · the induced current flows in the direction whose magnetic effect opposes the change that causes itFleming’s right-hand rule · for a straight wire moving across a field: forefinger = field, thumb = motion, middle finger = induced currentopposes the change · the induced field pushes back against the motion or the change of flux that makes it; it does not oppose the magnet itself
Explanation

Lenz’s rule: the induced current flows in the direction whose own magnetic field opposes the change of flux that produces it. That is the meaning of the minus sign in −N ΔΦ ÷ Δt. When a north pole is pushed towards a coil, the near face of the coil becomes a north pole and repels the magnet; when the north pole is pulled away, the near face becomes a south pole and attracts it back. Either way you must do work, and that work becomes the electrical energy, so energy is conserved. Use the screw rule of Week 4 to find the current: looking at a face, a clockwise current makes it a south pole and an anticlockwise current makes it a north pole. Fleming’s right-hand rule gives the same answer more quickly for a straight wire moving across a field: forefinger along the field, thumb along the motion, middle finger along the induced current, all three at right angles. Use the left hand for a force on a current (motor) and the right hand for an induced current (generator).

Magnet and motionNear face of the coilCurrent seen from the magnetForce on the magnet
N pole approachingNanticlockwiserepels
N pole leavingSclockwiseattracts
S pole approachingSclockwiserepels
S pole leavingNanticlockwiseattracts
Worked examples
An S pole of a magnet is pushed towards a coil face. By Lenz’s rule the face must repel it, so the near face becomes an S pole. A clockwise current, seen from the magnet’s side, makes a face an S pole, so the induced current is clockwise as seen from the magnet.
An N pole is pulled away from a coil face. The coil must try to keep the flux, so the near face attracts the leaving magnet and becomes an S pole. The induced current is clockwise as seen from the magnet’s side. (Compare with the first example: a different pole and a different motion, the same current.)
Motion (thumb)Field (forefinger)Current (middle finger)
eastnorthup
upwestsouth
A straight wire moves east through a field that points north. Right-hand rule: forefinger north, thumb east, so the middle finger points up. The induced current flows up along the wire.
Class activity

Use your right hand. Hold the forefinger pointing north and the thumb pointing east, with the middle finger at right angles to both. Which way does the middle finger point? Then turn the thumb to point up with the forefinger still north: where does the middle finger point now? Check both answers with the table above and with a compass.

Practice
1
An N pole is pushed towards a coil face. Which pole does the near face become, and is the induced current clockwise or anticlockwise as seen from the magnet’s side?
2
An S pole is pulled out of a coil. Which pole does the near face become and which way is the current seen from the magnet’s side?
3
A wire moves up through a field that points west. In which direction does the induced current flow?
4
A wire moves north through a field that points down. In which direction does the induced current flow?