[填空题]
1.A student investigates the maghavlm v7jk:o18;j *wp netic field produced by a coil by convwmk9ebim- u,3 4yd:r4w . raestructing a b,4ewu: dami 43rk.w9evmr-ycircuit with a cell of emf $12V$ with non-negligible internal resistance, a variable resistor, a coil, and connecting wires with negligible resistance.
The length of the copper wire used for constructing the coil is $9.4m$.
（1）Explain why the potential difference across the cell is less than $12V$ when the cell is connected to the circuit.
（2）The resistance of the variable resistor $R$ and the internal resistance of the battery $r$ are $2.5\,Ω$ and $0.025\,Ω$, respectively. The potential difference across the coil is $8.2V$. Calculate the resistance per metre of the coil. R= $Ωm^{−1}$
（3）A compass is placed on top of the wire at point $P$, as shown in the following diagram. Deduce the direction of the magnetic field in the location of the compass.
2.The resistance of the variable resistor is gradually decreased to zero. On the axes below, sketch the variation of potential difference across the battery $V$ with the current passing through the coil $I$ as the variable resistor is moved through its full range.
