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# Tag Archives: diameter

## How to determine the diameter of a rod using thread an meter scale?

You are given a sufficiently long piece of thread, a meter stick and a thin round shape iron rod. Explain how would you measure the diameter of iron rod by using given thread and meter stick.

Yosef asked

Answer:

Wind the thread over the rod closely a number of times say 20 and mark the end point. Now unwind it and measure the length of the thread used to wind over the rod.

This length = n x 2πr, where n is the no. of windings and r is the radius of the rod.

Therefore, the diameter of the rod = 2r = The length of wire used for ‘n’ windings /nπ

## How wheel + Tyre diameter affects a vehicle’s speed

I have been told a larger wheel/tyre decrease acceleration because its harder to turn the wheel making the car rev slower.

But the same person tells me it will increase the top speed.

My question is if the car has a max rev of 8000rpm but only reaches 7000rpm in its highest gear would a smaller wheel/tyre not make it easier for the car to rev higher thus possible increasing the top speed by using a smaller wheel?

(Garry asked)

**Answer**:

**top speed**other conditions remaining the same.

## Wire Gauge and Resistance problem

I am using 18 **gauge** nicrome wire having ohm of 17.3 , 220 volt. I want to use 10 gauge wire how to decide the length?

Padmanava asked

**Answer**:

18 gauge corresponds to a diameter of 1.22 mm

Therefore radius **r1**= 1.22/2=0.61 mm=**0.00061m**

10 gauge means diameter is 3.25 mm

Radius **r2** = 3.25/2=1.625mm=**0.001625m**

Sine the resistance needs to be the same, we can use the formula

Which gives

As resistivity is same.

Substitute the values and you will get the answer.

## Current electricity – Numerical

A 10m length of aluminium wire has a diameter of 1.5mm. It carries a current of 12A. Find (i) the current density. (ii) the drift velocity (iii) the electric field in the wire. aluminium has approximately 10^29 free electrons per m^3

## A numerical problem on Drift velocity

Two conducting wires X and Y of same diameter but different materials are joined in series across a battery . if the number density of electrons in X is twice that in Y . Find the ratio of drift velocity of electrons in the two wires.

(Adithya Singh asked this)