Magnetic effect of current, Fleming's rules, motors, generators and transformers.
Oersted found that a current produces a magnetic field. For a straight wire, the field forms concentric circles whose direction follows the right-hand thumb rule: thumb along the current, fingers curl along the field. A solenoid behaves like a bar magnet; wrapped on soft iron it becomes an electromagnet, whose strength grows with current, number of turns and an iron core.
How can the strength of an electromagnet be increased?
Increase the current, increase the number of turns, and use a soft-iron core.
Why is soft iron, not steel, used for the core of an electromagnet?
Soft iron is magnetised easily and loses magnetism when the current stops; steel keeps its magnetism.
A current-carrying conductor in a magnetic field feels a force, F = BIL when the wire is at right angles to the field. Direction: Fleming's left-hand rule — forefinger = field, middle finger = current, thumb = force (motion), all mutually perpendicular. A DC motor turns electrical energy into mechanical energy using this force on a coil, with a split-ring commutator reversing the current every half turn.
Name the three fingers in Fleming's left-hand rule and what each shows.
Forefinger: magnetic field. Middle finger: current. Thumb: force (motion).
Why is a split-ring commutator needed in a DC motor?
It reverses the current in the coil every half rotation so the torque always acts the same way and the coil keeps turning.
A changing magnetic flux through a coil induces an emf (Faraday). Direction: Lenz's law, or Fleming's right-hand rule for a moving conductor. An AC generator turns mechanical into electrical energy. A transformer works only on AC: Vs/Vp = Ns/Np. For an ideal transformer Vp Ip = Vs Is.
A transformer has 100 turns on the primary and 300 on the secondary, with 220 V input. Find the output voltage.
Vs = Vp × Ns/Np = 220 × 300/100 = 660 V (step-up).
A step-down transformer has Vp = 220 V, Np = 1000 and Ns = 50. Find Vs.
Vs = 220 × 50/1000 = 11 V.
In the step-up case, the primary current is 3 A. Find the secondary current (ideal transformer).
Is = Vp Ip / Vs = 220 × 3 / 660 = 1 A. Voltage rises 3 times; current falls to one third.
AC of frequency 50 Hz: how many times per second does the current reverse direction?
Each cycle reverses it twice, so 2 × 50 = 100 times per second.
A transformer takes 2200 W and delivers 2090 W. Its efficiency?
Efficiency = 2090/2200 × 100 = 95 %.
1. Who discovered the magnetic effect of current?
2. State the right-hand thumb rule.
3. State Fleming's left-hand rule.
4. What energy change takes place in a DC motor? In a generator?
5. What is electromagnetic induction?
6. Why can a transformer not work on DC?
7. A transformer has 500 primary turns, 2000 secondary turns, input 110 V. Output voltage?
8. Is the transformer above step-up or step-down?
9. Why is the core of a transformer laminated?
10. Name two ways to increase the emf induced in a generator coil.