Experiment E3 Flashcards
What is the aim of the experiment? (3)
a) To acquaint you with an important bridge circuit, - the Wheatstone bridge.
b) To determine the resistance of several lengths of wire using a slide-wire bridge, a variant of the Wheatstone bridge.
c) To determine the resistance per unit length and hence the resistivity of the wire.
Theory
(i) Resistivity
Resistance has been defined in experiment E1. The resistance (R) of a length of wire of uniform cross-section is proportional to the length (L) of the wire and inversely proportional to the cross-sectional area (A). This can be expressed by the equation:
R = p (L/A)
The balance condition is given by:
P/Q = S/X
Proof
i. At balance no ____ flows between b and d, and so the current (I1) in S and X must be the same, as must the current (I2) in P and Q.
ii. b and d must be at the same _____ since no current flows through bd.
iii. Then Vab = Vad and ___=____ , and so,
I1S= and I1X=
current
potential
Vbc = Vdc
I2P
I2Q
▬▬► If the slide-wire has a uniform cross-sectional area Asw and resistivity ρsw, show that:
Rad/Rdc = l1/l2]
where l1 and l2 are the lengths ad and dc.
▬▬► If S is a known and X an unknown resistance, write down the equation connecting X, S, l1 and l2.
▬▬► The balance point should preferably be in the middle third of the slide wire. Why?
So the relative erros from L1 and L2 must be the smallest.
▬▬► How can this be achieved?
The ratio of P/Q could be altered to make X equal (or close to equal) to S.
For each length interchange the wire and the standard resistance, so that S is on the right and X is on
the left and determine the resistance again.
▬▬► Give the reasons for this procedure. (2)
- In case the slide-wire is not completely uniform
- In case the R at the ends of the slide wire are not equal.
▬▬► Should you show the origin on the graph you will plot? Explain.
Yes, X/L and X will = 0 at the same point on the straight-line graph. Therefore, (0;0) must be shown.
