You can change the value of the voltage of the battery using the slider in the left top.
You can change the position of the slider on the shear resistor effectively dividing the resistor in a left part and a right part with two changing values. The sum of the two is of course equal to the total resistance of R_2. The values of the two resistors can be modified by making the resistors bigger or smaller. The lengths can be changed on the left, the diameters on the right. R_2 can be visually divided into two parts by clicking on the slider or ingoing wire and reset again by clicking on the ingoing wire.
You can put some Ammeters in the circuit by clicking on the concerning wires. A Voltmeter can be put over R_1. These meters can show you the values, if you click on the battery. First some potentials and currents are shown, by colors and arrows. Watch the color change if you click the switch to close or open it. In the top left a checkbox is now displayed to show you the values of the potentials and the meters. If you click the battery again almost everything is reset to the start situation.

Find out what ratio of resistors (R_1 / R_2) is needed to maintain a required Voltage of 6 V within 1% on R_1, when the battery has a Voltage of 30 V.
What is the ratio of the two parts of R_2 (R_l / R_R)?
When we call the power dissipated on R_1 the useful power, what is the efficiency of this circuit?