If you put cold water into a blender and mix it rapidly for several minutes, the water will become warm. From where does the additional thermal energy come?

Thermal energy, often referred to as heat, is simply the internal energy present in a system due to the movement of its particles.
When dealing with water in a blender, the increase in thermal energy is a result of external forces acting on the system. In simple terms, thermal energy makes things warm or hot.
You might notice that your hands feel warm when you rub them together. This happens because frictional force converts the kinetic energy from your hand motions into thermal energy.
In the case of the blender, something similar happens, but on a different scale. As the blender's blades spin rapidly.
they generate a frictional force with the water.
This friction produces thermal energy, which warms up the water.

Electrical energy is the energy that results from the movement of electric charges. It can easily be converted into other forms of energy, such as thermal energy, mechanical energy, or even light energy.
In the context of a blender, electrical energy is what powers the appliance.
When you plug the blender into an electrical outlet and turn it on, electricity flows through the motor.
The motor then converts electrical energy into mechanical energy, spinning the blades.
This transformation is fundamental to understanding how the blender works.
Without this essential conversion, the blades wouldn’t move, and no heat would be generated.
So, the initial source of energy in this process is electrical energy from your power source.

Frictional force is a force that resists the motion of two surfaces sliding against each other. It is highly relevant in everyday activities, including the use of a blender.
As the blender blades spin quickly, they move through the water molecules, creating friction. This friction is crucial for the conversion of energy.
The friction between the blades and water generates heat, which is then transferred to the water, increasing its temperature.
Without frictional force, there would be no heat generated to warm up the water.
While friction is often considered a hindrance, in this case, it is essential for generating thermal energy through the conversion of mechanical energy.
Remember that friction can be both useful and a nuisance, depending on the context. In mechanical devices like the blender, controlled friction is harnessed constructively to produce desired outcomes.