Electricity and Magnetism Answers

1 In the circuit shown, each of the cells has an emf of 2V and internal resistance of
0.1Ω
. Find the current in through the
0.5Ω
a) 0.13A
b) 3.45A
c) 1.27A
d) 2.44A

2) Calculate the electric field everywhere outside of a very long rod, radius R and charged to a uniform linear charge density
λ
.

a) E⃗ =λπrϵ0r^
E→=λπrϵ0r^

b) E⃗ =λπϵ0ln1rr^
E→=λπϵ0ln1rr^

c) E⃗ =λ2πrϵ0r^
E→=λ2πrϵ0r^

d) E⃗ =λ2πϵ0ln1rr^
E→=λ2πϵ0ln1rr^

9) Calculate the currents in
3Ω
resistor
Click here to see exhibit
a)0.36A
b) 2.54A
c) 1.42A
d) 3.46A

20) The term 'lost volts' refers to -----------
a) energy per coulomb dissipated in the external (load) resistance
b) terminal potential difference
c) energy per coulomb dissipated in the internal resistance of the source of emf
d) energy per coulomb wasted to heat up the connecting wires

15) A battery has emf 13.2V and internal resistance
24mΩ
. If the load current is 20.0A, find the terminal voltage o f the battery
a) 12.7V
b) 14.5V
c) 16.8V
d) 17.7V

16) Electrical energy is sold by PHCN in units of kilowatt-hour (kWh). The lighting of a house is done with five 60W bulbs which are swithed on for approximately three hours per day. What is the lighting bill for the household over a period of 30days at the rate of N1.20 per kilowatt-hour?
a) N1.50
b) N25.30
c) N32.40
d) N52.20

11) The current I in a conductor as a function of time t is given as
I(t)=5t2−3t+10
where current is in ampres A and t is in seconds s. What quantity of charge moves across a section through the conductor during the interval t=2s to t=5s?
a) 154.4C
b) 193.5C
c) 225.5C
d) 300.0C

12) A nicrome wire is 1.0m long and
1.0mm2
in cross-sectional area. It carries a current of 4.0 A when a potential difference of 2 V is applied between its ends. Calculate the conductivity of the wire.

a) (2MΩm)−1

b) (4kΩm)−1

c) (2mΩm)−1

d) (4Ωm)−1

9) A coil of wire has a resistance of
25.0Ω
at
20oC
and a resistance of
25.1Ω
at 35
oC
What is its temperature coefficient of of resistance?

a) 4.5×10−4/oC

b) 3.5×10−3/oC

c) 2.6×10−4/oC

d) 4.0×10−5/oC

10) A copper wire has resistance of
2.0Ω
at
0oC
and
2.26Ω
at
30oC
. What is its resistance at
50oC
?

a) 2.43Ω

b) 3.34Ω

c) 1.52Ω

d) 5.31Ω

7) A wire with resistance of
8.0Ω
is drawn out through a die so that its new length is three times its original length. Find the resistance of the longer wire assuming that the resistivity and density of the material are unaffected by the drawing process.

a) 72Ω

b) 60Ω

c) 80Ω

d) 45Ω

8) A cirrent flows in a wire of circular cross-section with the free electrons travelling with a mean drift velocity v. If an equal current flows in a wire of the same material but of twice theradius, what is the new mean drift velocity?
a) v/4
b) v/2
c) 2v
d) 4v

5) A 0.40 mm diameter copper wire carries a current of
3μA
. Find the current density in the wire

a) 15.16A/m2

b) 33.32A/m2

c) 26.17A/m2

d) 23.87A/m2

6) Calculate the resistance of 180 m of silver wire having a cross section of
0.33mm2
. The resistivity of silver is
1.6×10−8Ωm

a) 11.2Ω

b) 9.6Ω

c) 14.6Ω

d) 7.5Ω

3) the electrical force on a negative charge located near an isolated positive charge
a) points away from the isolated charge
b) is tangent to a circle centred about the isolated charge
c) points toward the isolated charge
d) is zero
4) The average velocity of the electrons in a piece of wire of cross-sectional area
2.0×10−6m2
is
2.5×10−4m/s
when a steady current of 4.0 A is flowing. Calculate the number of free electrons per unit volume of the material

a) 3×1019

b) 5×1020

c) 2×1023

d) 5×1028

1) Calculate the potential difference between the plates of a parallel plate capacitor so that the gravitational force on a proton would be balanced by the electric field (proton mass =
1.67×10−27kg
, electronic charge
e=1.6×10−19C
, plate separation is 0.5cm. Take
g=9.8m/s2)

a) 1.4×10−6V

b) 2.0×10−6V

c) 1.7×10−6V

d) 3.2×10−6V

2) Select the correct option from the following
a) electric field is a scalar quantity
b) electromotive force is a vector quantity
c) electric current is a scalar quantity
d) eletric potential is a vector quantity

19) Kirchhof's loop rule is statement of conservation of -----------
a)mass
b) energy
c) charge
d) momentum
20)In the space surrounding a wire carrying a steady current, there exists
a) a magnetic field pointing radially outward
b) a magnetic field circling the wire
c) an oscillating magnatic field
d) no magnatic field