Question

A rheostat is used in an electrical circuit
To change the resistance of the circuit
To change the potential difference
To change the emf
To change the current through a particular instrument.

Answer 1

Hint: This problem can be solved by knowledge about the simple Circuit. Resistance opposes the flow of charge and it is the ratio of potential difference and current inside the volume of conductor.
Rheostat is varying resistance
So resistance depend on the
Length of the wire \[\left( l \right)\]
Cross section area of conductor \[\left( A \right)\]

$R = \rho \frac{l}{A}$
Relation between Resistance \[\left( R \right)\], current \[\left( I \right)\] and potential difference \[\left( V \right)\]
$V = IR$
Here \[V = \] Potential difference at the end of device
\[I = \] Current flowing through the volume of conductor
\[R = \] Resistance of conductor
Relation of resistance with heat energy \[\left( H \right)\] [Joule's law]
$H = {I^2}Rt$

Complete step by step answer:
First let us draw a diagram to understand the problem better.

Symbol of rheostats
 

Rheostat is made by many coils .They every coil has resistance. There is no issue of positive terminal or negative terminal.
Terminal A and B are used to connect the input which marks the entry point of current.
Terminal C is the output point for the current through rheostat.
A coil of resistive wire is wound on the ceramic core which is non metallic material.
A sliding contact with upper part ceramic or any non metal and lower portion with metal contacts.
The position of the sliding contact represents the resistance in the circuit. So when we slide from left to right then resistance increases in the circuit. Result current will reduce. As our requirement we can allow the current inside the circuit
Hence work of rheostat
Rheostat is a variable resistance which is used in the circuit to control the current.
Resistance can vary in a circuit without the interruption so as to control the current.

Note:
This problem can be solved by knowledge about the simple Circuit. You should have deep knowledge about components of electric circuits. You should know the relations of resistance with other physical quantities.