# Questions and answers on Magnetic field and E.M induction for UTME

Question 1

Induced emfs are best explained using{UTME 2013}

A. Lenz’s law B. Ohm’s law C. Faraday’s law D. Coulomb’s law

Solution

Athough both Faraday and Lenz explained the induced emf

Faraday’s law talk about the emf is induced as a result of the rate of change of magnetic flux linkage while

Lenz says the the induced emf move in such a way as to oppose the change producing

Look at the two theory Faraday BEST explained induced emf

C is the correct option

Question 2

A conductor of length 1m moves with a velocity of 50ms-1 at an angle of 300 to the direction of a uniform magnetic

field of flux density 1.5 Wbm-2. What is the e.m.f. induced in the conductor?{UTME 2012}

A. 37.5V B. 50.5V C. 75.0V D. 80.5V

Solution

E = Blv sinQ

E = 1.5 * 1 * 50 sin30

E = 37.5 v

A is the correct answer

Question 3

A particle carrying a charge of 1.0 x10-8C enters a magnetic field at 3.0 x102 ms-1 at right angles to the field. If

the force on this particle is 1.8 x 10-8N, what is the magnitude of the field?{UTME 2012}

A. 6.0 x 10-1T B. 6.0 x 10-2T C. 6.0 x 10-3T D. 6.0 x 10-4T

Solution

F = Bqv

B = F / qv

B = 1.8 x 10-8 / (1.0 x10-8 * 3 x 102)

B = 0.6 x 10-2 = 6.0 x 10-3 T

C is the correct option

Question 4

A step-down transformer has a power output of 50W and efficiency of 80%. If the mains supply voltage is 200V,

calculate of the primary current of the transformer.{UTME 2008}

A. 0.31A B. 3.20A C. 3.40A D. 5.00A

Solution

efficiency = power output / power input

80/100 = 50/power input

50*100 / 80 = power input

power input = 62.5

power input = IpVp

Ip = 62.5 / 200 = 0.31 A

A is the correct option

Question 5

Two long parallel wires X and Y carry currents 3A and 5A respectively. If the force experienced per unit length by X is 5 x 10-5N, the force per unit length experienced by wire Y is{UTME 2007}

A. 3 x 10-5N B. 3 x 10-6N C. 5 x 10-4N D. 5 x 10-5N

Solution

Following the above formula and supporting it with Newton’s third law that states the if object X exert a force on object y, object y will exert the same force on X but in opposite direction.

For two parallel current carrying conductor, regardles of their current, they will exert the same force on each other but in opposite direction

D is the correct answer

Read short note on magnetic field and em induction