Question

A heater is being designed that uses a coil of nichrome wire (ρ = 100x10^-8 Ω·m) to generate 100 W using a voltage of V=220V. The wire is 2000 m long. What cross sectional area should the wire have?

None of the above

1.7e-2 m²

7.23e-4 m²

4.13e-6 m²

2.06e-3 m²

What is the magnification of a converging lens with a focal length of 7.5 cm when an object is held 5 cm from it?

3.00

2.14

1.36

5.00

None of the above

A proton is moving 5m/s upwards and is 2cm to the right of a wire which is carrying 3 A of current downwards. What is the direction of the force experienced by the proton?

Out of the page

None of the above

To the right

To the left

Into the page

A 5 kg object has a net charge of -78 mC and is in an external electric field. What is the direction and magnitude of the electric field required to cancel the force due to gravity (gravitational field points down)?

628.20 N/C downwards

64.10 upwards

64.10 downwards

None of the above

628.20 N/C upwards

Two protons are placed 3mm apart and fixed in place. If one proton is then set free, what is the maximum velocity it will reach?

None of the above

38.9 m/s

2.32e2 m/s

7.58e8 m/s

9.6 m/s

An uncharged capacitor is connected in series with a battery and a resistor. In terms of the time constant τ, how long does it take the capacitor to charge to 1/3 its max charge?

0.205τ

1.10τ

3τ

None of the above

0.405τ

A 20 turn circular loop with diameter 3 cm is placed in a magnetic field of strength 7 T such that the loop’s area vector is parallel to the magnetic field. The magnetic field is reduced to 0 T over some time and this generates a constant voltage of 5 V. How long did it take for the magnetic field to reduce to 0?

3.23 s

15.32 s

0.0198 s

0.0389 s

None of the above

There are two charges q₁ = 35 μC and q₂ = 50 μC placed 0.5 m apart and fixed in place. If q₂ has a mass of 3 g, what is the magnitude of the acceleration it will experience the moment it is free to move? Note: q₂ is not in any gravitational field

9.8 m/s²

None of the above

21e3 m/s²

10.5e3 m/s²

3.38e2 m/s²

Answer 1

Multiple questions posted, so answering only the first question:

Power P=v2/r, where v is voltage and r is resistance.

So,r=v2/P

Also,r=l/A, where r is resistance, is resistivity, l is length and A is area.

So,v2/P = l/A

=>A=lP/(v2)

Here, = 100x10-8 Ω·m, l = 2000 m , P=100 W, v=220 V.

So, A=100x10-8*2000*100/(220*220) = 4.13*10-6 m2

So, third option is correct.