Ohms law.
V = I * R or I = V/R or I = V/R
Things can get complicated if values change, you may then need to resort to calculus but this is rarely necessary.
Consider a power supply rated at 10V and 500mA. This means that the supply will give out 10V and will be happy providing up to 500mA output.
Connect a 100R resistor across the output, the current will be 100mA and the supply will hardly notice it.
Connect another 100R across the output, the current will now be 200mA and the supply is beginning to work.
Connect three more 100R resistors, the current is now 500mA and the supply is fully loaded. It will probably get warm from the resistance in its innards but should not break into a sweat.
Connect some more resistors and the supply will be overloaded, it will try to provide enough current to maintain 10V but may not manage it. The excess current will heat the supply and it may fail. A fuse is intended to melt if the current is too high so limiting the heat in more expensive components. Transitors make fast fuses!
If you wish your circuitry to last, do not ask for more than the rated current or supply more than the rated voltage
V = I * R or I = V/R or I = V/R
Things can get complicated if values change, you may then need to resort to calculus but this is rarely necessary.
Consider a power supply rated at 10V and 500mA. This means that the supply will give out 10V and will be happy providing up to 500mA output.
Connect a 100R resistor across the output, the current will be 100mA and the supply will hardly notice it.
Connect another 100R across the output, the current will now be 200mA and the supply is beginning to work.
Connect three more 100R resistors, the current is now 500mA and the supply is fully loaded. It will probably get warm from the resistance in its innards but should not break into a sweat.
Connect some more resistors and the supply will be overloaded, it will try to provide enough current to maintain 10V but may not manage it. The excess current will heat the supply and it may fail. A fuse is intended to melt if the current is too high so limiting the heat in more expensive components. Transitors make fast fuses!
If you wish your circuitry to last, do not ask for more than the rated current or supply more than the rated voltage