What are the minimum components needed for an electrical circuit to do work? What device allows us to control whether or not current flows in the circuit?

Hint: For an electrical circuit to do useful work, we will need equipment that acts as a power supply source. Then, we will need equipment that can turn this power supplied to them into useful work by converting electrical energy into other forms of useful energies. And lastly, we need a medium to supply this power from one point in the circuit to another.

Complete answer:
We saw that, we basically need at least three components to construct an electrical circuit that does work.
The first component of our electrical circuit is a power supply equipment. A cell is a device which can be used to provide power in the circuit.
Now, the second component in our electrical circuit is a load that can utilize this power. We can use a heater (which is basically a resistance) to convert this electrical energy into thermal energy.
And, lastly to transfer this power from the cell to the load and to complete the circuit, we will use a conductor. For this purpose, we will use conducting wires in our circuit.
Thus, our three-component circuit is complete and an example of such a circuit is:
         
Here, the power dissipated by the resistance (heater) is equal to:
$\begin{align}
  & \Rightarrow P=\dfrac{{{V}^{2}}}{R} \\
 & \Rightarrow P=\dfrac{{{220}^{2}}}{20} \\
 & \Rightarrow P=2.42kW \\
\end{align}$
Hence, the minimum components needed for an electrical circuit to do work is three, that are, a power supply, a load and a conductor.
A switch is a device that allows us to control whether or not current flows in the circuit.

Note:
Whenever constructing an electrical circuit, one should be very careful of the power load of the appliance used. Providing excessive power to the appliance may heat up and burn the appliance whereas providing less power might affect its performance and gradually affect its shelf life. These are some key thoughts that go into the manufacturing of these appliances too.