11.1. Electric Current and Circuits

We are familiar with air current and water current. We know that flowing water constitute water current in rivers. Similarly, if the electric charge flows through a conductor (for example, through a metallic wire), we say that there is an electric current in the conductor. In a torch, we know that the cells (or a battery, when placed in proper order) provide flow of charges or an electric current through the torch bulb to glow.

Electric current is akin to a flow of water in a river. Just as water moves in a current, an electric current flows when charged particles move through a conductor like a wire. When you turn on a torch, it essentially completes a circuit, allowing the electric current from the battery to flow to the bulb, causing it to light up.

Imagine filling a bucket with water from a hose. The flow of water through the hose represents electric current, and the bucket is like a device (like a torch) that uses that current to perform work, such as lighting up.

A continuous and closed path of an electric current is called an electric circuit. Now, if the circuit is broken anywhere (or the switch of the torch is turned off), the current stops flowing and the bulb does not glow.

An electric circuit forms a path for the current to flow, much like a racetrack for cars. If there's a break in that track (like a cut wire or a turned-off switch), the cars (or current) can't complete the circuit, just as cars would stop if the track were interrupted.

Think of a light bulb in a room. When you flip the switch off, it's like closing a door that prevents anyone from moving through a hallway—no one can get through, so the light goes out.

How do we express electric current? Electric current is expressed by the amount of charge flowing through a particular area in unit time. In circuits using metallic wires, electrons constitute the flow of charges. However, electrons were not known at the time when the phenomenon of electricity was first observed.

Electric current is measured by the amount of electric charge that flows through a conductor over time, typically in coulombs per second, or amperes (A). While we now know that it’s primarily electrons moving, early definitions didn’t account for this as electrons were not discovered then.

Consider a water pipe. The amount of water passing a certain point in a second represents how much water is flowing—just like the current tells us how much electric charge is moving through a wire.

The SI unit of electric charge is coulomb (C), which is equivalent to the charge contained in nearly 6 × 10^18 electrons. The electric current is expressed by a unit called ampere (A). One ampere is constituted by the flow of one coulomb of charge per second, that is, 1 A = 1 C/1 s.

Electric charge is quantified in coulombs, representing a large number of electrons. When we talk about current, one ampere means that one coulomb of charge moves past a point in a circuit each second. This is a vital measurement that helps in designing electrical devices.

Picture a moving conveyor belt in a factory. If one box (or coulomb of charge) arrives at the exit every second, the speed of the conveyor belt represents one ampere of current.

An instrument called ammeter measures electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured. A schematic diagram of a typical electric circuit comprising a cell, an electric bulb, an ammeter and a plug key is illustrated.

An ammeter is a specific tool used to measure electric current, and it must be connected in series within the circuit to accurately measure the current flowing through it. This means that all the current must pass through the ammeter for it to provide a reading.

Think of a traffic camera positioned on a single-lane road; it only measures the cars (current) that actually drive past it (the current flowing through the circuit). If the road were wider (parallel), it wouldn’t capture every car accurately.

Example: A current of 0.5 A is drawn by a filament of an electric bulb for 10 minutes. Find the amount of electric charge that flows through the circuit.

To calculate the charge flowing through the circuit, use the equation Q = I × t, where I is current (in amperes), and t is time (in seconds). In this case, a 0.5 A current over 600 seconds (10 minutes) results in a total charge of 300 coulombs.

Imagine you are filling a water tank (the charge). If a hose (current) flows at a steady rate for a certain time (minutes), you can easily calculate how much water (charge) fills the tank during that period.

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