Final answer:
To calculate the magnetic flux density at a point 20 cm from a wire, the Biot-Savart law is used after determining the current in the wire, which involves first finding the wire's resistance and then using Ohm's law.
Step-by-step explanation:
To calculate the magnetic flux density at a point 20 cm away from the center of a wire, we can apply the Biot-Savart law, which states that the magnetic field (B) due to a current (I) in a long straight wire is given by B = µ0I / (2πr), where µ0 is the permeability of free space (4π x 10-7 T·m/A), I is the current, and r is the distance from the wire to the point of interest. First, we must find the current flowing through the wire by using Ohm's law on the battery and the wire's resistance (V = IR + Ir, where V is the battery's e.m.f., IR is the voltage drop across the external wire, and Ir is the voltage drop across the battery's internal resistance).
Given: e.m.f. (V) = 8 V, internal resistance (r) = 1 Ohm, the length of the wire (L) = 10 cm = 0.1 m, cross-sectional area (A) = 3 x 10-8 m², and resistivity (ρ) = 4.5 x 10-6 Ω·m. The resistance of the wire (R) can be calculated using R = ρL / A. Once the current (I) is determined, we can calculate the magnetic flux density at the required distance, which is a normal distance (r) 20 cm = 0.2 m away from the center of the wire.