An instrument can measure the electric current called an ammeter. To measure current ammeter must connect in series with the circuit whose current is to be measured.

The measurement of current through the resistor by using an ammeter is shown in the below figure.

The electric current can also be measured using a galvanometer. The galvanometer gives both the direction and the magnitude of the electric current.

The current can be measured by detecting the magnetic field associated with the current without breaking the circuit. There are various instruments used to measure the current without breaking the circuit.

• Hall effect current sensor transducers
• Current transformer (CT) (Only measure AC)
• Clamp-on meter
• Shunt resistors
• Magneto resistive field sensors

We have all seen that a 60 watt electric lamp provides less than 100 watts electric light. When we pay for electricity consumption, we are actually paying for charges of electricity for a certain period of time. Electrical calculation is therefore very important in analyzing an electrical circuit or network.

Suppose, an element supplies or consumes an energy of dw joules for a time of dt second, then power of the element can be represented as,

This equation can also be rewritten as,

Hence, from the as the expression of voltage and current in the equation are instantaneous, the power is also instantaneous. The expressed power is time-varying.

So, the power of a circuit element is the product of voltage across the element and current through it.

As we have already mentioned, the circuit element can absorb or deplete energy. We represent the absorption of energy by placing a positive (+) sign on the energy output. Similarly, we put a dot (-) in it when representing the force brought in by a circle.

There is a simple relationship between the direction of the current power, the magnitude of the electrical power and the strength of the circuit element. We call this relationship a sign meeting that you did not do. When a current enters an object through its terminal voltage polarity, we set the positive (+) sign before the output voltage and current. This means that the element takes or consumes energy from the electrical circuit. On the other hand, when a current through an element leaves its terminal voltage polarity, we set a negative (-) mark before the current and current voltage output. This means that the element brings or energizes the electrical circuit.

Source: electrical4u