Final answer:
To control the brightness of a light bulb or the rotation speed of a motor, one can use a variable resistor, like a rheostat or potentiometer, a dimmer for lights, or a motor controller or variable frequency drive for motors. These devices adjust the voltage and current in the circuit, impacting the power consumed by the bulb or motor. The dimming of lights when a large appliance is turned on is due to the IR drop across the circuit.
Step-by-step explanation:
To control the brightness of a light bulb or the spin of a motor in a circuit, you can manipulate the current flow or the voltage applied across these devices because the power consumed by them, which relates to brightness or speed, is a product of voltage and current. One common method for doing this is through the use of a rheostat or a potentiometer, which are types of variable resistors. As you adjust the resistance in the circuit with these devices, you effectively change the voltage drop across the bulb or motor and thus control their performance.
Dimmers are devices specifically designed to control the brightness of light bulbs by altering the voltage reaching the bulb. For motors, a motor controller or a variable frequency drive (VFD) can be used to regulate the spin by changing the frequency of the power supplied to the motor, in addition to or instead of changing the voltage or current.
The phenomenon of lights dimming when a heavy appliance comes on is due to a large current draw by the appliance's motor, which causes a significant IR drop (voltage drop due to resistance in the wires) and hence reduces the voltage across the light bulb. This illustrates the relationship between current, resistance, and voltage in a circuit, which is governed by Ohm's Law.