The electric current I in a circuit varies directly as the voltage V. When 21V are applied, the current is 4A. What is the current when 18 volts are applied

4 * 18/21

To find the current when 18 volts are applied, we can use the concept of direct variation. Direct variation means that two quantities are related by a constant ratio. In this case, the electric current (I) and voltage (V) are directly proportional, so we can set up the following equation:

I = kV

where k is the constant of variation.

To find the value of k, we can use the given information. When 21 volts are applied, the current is 4A. Plugging these values into the equation, we have:

4 = k * 21

To solve for k, we divide both sides of the equation by 21:

4/21 = k

Now we have the value of k, which is approximately 0.1905.

Next, we can use this value of k to find the current when 18 volts are applied. Plugging the values into the equation, we have:

I = 0.1905 * 18

Calculating this, we find that the current is approximately 3.43A when 18 volts are applied.