PHY C19: Potential Difference & Power

The next concept we need to explore is potential difference.

  • Potential Difference
    • V = W/Q
    • Assumptions in circuits
  • Electrical Power
    • P = VI
    • Power ratings

Let’s go!


What is potential difference?
Work done (energy transferred) per unit charge.

V = W/Q

If you remember my post on electric potential, you know this is equal to the difference in potential between 2 points in an electric field.

Just to recap: charge carriers do work as they travel from a high potential to a lower potential, changing electrical potential energy into another form of energy.

You can measure potential difference as a VOLTAGE.
Its units are Volts (V).

1 Volt = potential difference between 2 points where 1 Joule of electrical potential energy is converted into other forms per Coulomb of charge travelling between the 2 points.

Rearranging V = W/Q, you can find the work done in a circuit if you know the potential difference & the charge:

W = VQ

What are the assumptions about potential difference as discussed in circuits?
In the following topics, we will talk about potential difference across the ends of a DEVICE (any electrical device: a bulb, a resistor, etc). Unless stated otherwise, we will assume that wires have NO RESISTANCE – so there is no potential difference across a wire. When a device has resistance, the charge carriers will transform their potential energy for another form: heat (see my post on resistance).

This transformation of energy is what we utilise in electrical devices. However, the RATE of energy transformed is also important, bringing us to the next section:


What is electrical power?
We’ve seen how power is defined:
Power = work done / time
OR (since work done = amount of energy transformed),
Power = amount of energy transformed / time
P = W/t

Since W= VQ,
Across an electrical device:
P = VQ/t

But, remember that I = Q/t:
P = VI

Just like other forms of power, electrical power is also measured in WATTS (W).

Power ratings on devices
If you take a look at some electrical appliances, a “power rating” is usually listed on their labels. This tells you the power the device has – the amount of energy it transforms every second. For example, a bulb rated 100W transforms 100 J of electrical energy into light & heat every second. A higher power rating does not necessarily mean a device is better – you will have to check its EFFICIENCY to know that. However, if efficiency is ignored, a 100W bulb will produce more light than a 10W bulb of the same voltage, for example.

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