Visually, how can you distinguish between an ac and dc power supply?

By the use of slow motion camera.

Explanation:

Visually, it is very hard to differentiate between an ac and dc power supply. But Since, we that In Ac supply polarity changes 100 times in a second ( because frequency of ac supply is 50 Hz generally). Whereas, Dc gives a steady power supply. So, in slow motion camera we can easily capture the flickering light tubes which won't happen in case of dc supply.

Alternating current (AC), is the form of electrical energy that consumers use whenever they plug televisions, fans, bulb, AC, and any other appliances into a socket.

Direct Current (DC)

There are many uses of direct current (DC). It can be used to charge battery and large power supplies for motors and electronic system.

Alternating Current is the form whereby electric power is provided for residences and businesses. Alternating current may be produced using a device known as alternator.  

With Alternating current (AC), electrons don’t really flow in AC; they vibrate from positive to negative and back from negative to positive. They really vibrate back and forth. In AC, there is no continuous vibration unlike in DC.

Direct current (DC)

Direct current power in large quantities can be used in production of aluminum. It can also be used in other electrochemical processes.

With Direct current (DC), there is a movement of electron in one direction, that is, from negative (-) to positive (+). It is a constant current that flows continuously until it is switch off or the source of its power runs out.  An example of a DC power is a battery.

They may flow throw a conductor such as wire; they can also flow through insulators, semiconductors or vacuum as in ion beams or electron.  

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a small object of mass 3.66 g and charge -17.6 µc is suspended motionless above the ground when immersed in a uniform electric field perpendicular to the ground. what are the magnitude and direction of the electric field?