Magnetism

The diagram in Figure 1 shows a transformer used in a phone charger that reduces mains voltage at 240V a.c. A peak voltage of 5 V is required as the output from the transformer. There are 760 turns of wire on the primary winding.

• 
  A) 14 turns

• B) 16 turns 

• C)18 turns

• D) 20 turns

Calculate the energy stored in a secondary coil of inductor with inductance 0.9H passing a current of 2.6A.

• A) 3.042

• B) 5.025

• C) 1.025

• D) 0.025

An engineer takes readings across the terminals of a DC electric motor. They record a supply voltage of 120V and a current of 5A. (a) Calculate the input power of the DC motor.

• A) 500W

• B) 600W

• C) 700W

• D) 800W

A coil of 300 turns is wound around a wooden ring with a mean circumference of 700mm and a cross-sectional area of 600mm². If a current of 4A flows through it, calculate 
Magnetic field strength.

• A) 171.4 A/m

• B) 180 A/m 

• C) 200 A/m

• D) 1.292μWb

A coil of 300 turns is wound around a wooden ring with a mean circumference of 700mm and a cross-sectional area of 600mm². If a current of 4A flows through it, calculate Magnetic field density

• A) 171.4 A/m

• B) 180 A/m 

• C) 210 A/m

• D) 2.15mT

A coil of 300 turns is wound around a wooden ring with a mean circumference of 700mm and a cross-sectional area of 600mm². If a current of 4A flows through it, calculate Total flux

• A) 171.4 A/m

• B) 2.15mT

• C) 1.292μWb

• D) 200 A/m

What is the magnetic flux density in the coil with 120 turns, relative permeability of 200, and current of 1.5A?

• A) 0.30 T

• B) 0.45 T 

• C) 0.40 T

• D) 0.50 T