Current in Electric Circuits

Overview

• Electric circuits are used to transfer energy from a battery/power supply to components in the circuit, which then transfer the energy to their surroundings.
• For an electric current to flow, you need:
  • a complete circuit for it to flow around
  • something to push it around the circuit
    • This push might be provided by a cell, battery or power supply.

Main components of a circuit

• Current is the rate at which electric charge passes a point in a circuit.
  • Current flows from the positive terminal of the battery.
  • Throughout a series circuit, the current is the same all the way around.
• A cell is a device that provides an electromotive force (e.m.f.) in a circuit by means of a chemical reaction.
• A battery is two or more electrical cells connected together in series.

Types of current

• Direct current (d.c.) is electric current that flows in the same direction all the time.
• Alternating current (a.c.) is electric current that periodically changes direction.

Conductors

• The wires used to connect up circuits are made up of metal because metals are good conductors of electric current.
  • The metal is usually surrounded by plastic, so that, if two wires touch, the electric current cannot pass directly from one to another (causing a short circuit). It can also prevent electric current from escaping and causing shocks.
  • Remember that metals are good electrical conductors because they contain electrons that can move about freely.
• Plastics (polymers) are good electrical insulators.

What is electric current?

• When a circuit is complete, an electric current flows.
  • Remember that current flows from the positive terminal of the supply, around the circuit, and back to the negative terminal.
• Charge travels around the circuit.
  • The battery or power supply in a circuit provides the push needed to make the current flow. This push is the same force that causes electric charges to attract or repel one another.
• Charge is carried around a circuit by the current; negative charge is carried by electrons.

Measuring electric current

• To measure electric current, we use an ammeter. There are two types:
  • an analogue meter has a needle, which moves across a scale
  • a digital meter gives a direct read-out in figures
• An ammeter is a meter for measuring electric current.
  • It is connected into a circuit in series, between other components in a circuit.
  • Since the current is the same throughout a series circuit, it doesn’t matter where the ammeter is placed.
  • Amperes, also called amps (A) is the SI unit of electric current.
• A galvanometer is a meter for measuring tiny electric current.

Current and electrons

• In a metal, some electrons are free to move about. In copper, there is one conduction electron for each atom of the metal. The atoms, having lost an electron, are positively charged.
• A battery pushes the conduction electrons through the metal. The force is the attraction between unlike charges discussed previously.
• There are two different pictures happening in a circuit (in terms of positive and negative current):
  • Conventional electric current flows from the positive terminal to the negative terminal.
  • Negatively charged electrons leave the negative terminal and flow around to the positive terminal.

Current and charge

• Remember that an ammeter measures the rate at which electric charge flows past a point in a circuit.
• Remember that electric current is defined as the charge passing a point in the circuit per unit of time, usually per second.
• This relationship between current and charge can be written as a formula: $$\text{current (A)}=\frac{\text{charge (C)}}{\text{time (s)}}$$ $$I=\frac{Q}{t}$$