Electric current
currentMoving electric charges, eg electrons moving through a metal wire. is the rate of flow of chargeProperty of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative charge.. In metal wires, electronSubatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. move and cause a current. The following conditions are needed for an electric currentThe movement of electrically charged particles, for example, electrons moving through a wire or ions moving through a solution. to flow:
- a source of potential differenceThe potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them., such as a batteryA chemical supply of electrical energy. For example, common battery voltages include 1.5 V and 9 V., cell or power pack
- a closed circuitA closed loop through which current moves - from a power source, through a series of components, and back into the power source., which provides a complete path for the charges to move through
Originally, current was defined as the flow of charges from positive to negative. Scientists later discovered that current is actually the flow of electrons, from negative to positive. The original definition is now referred to as 鈥榗onventional current鈥, to avoid confusion with the newer definition of current.
Learn more on electrical charge and current in this podcast
Listen to the full series on 大象传媒 Sounds.
Calculating current
To calculate current, use the equation:
charge flow = current 脳 time
This is when:
- charge flow is measured in coulombs (C)
- current is measured in amperes (amps) (A)
- time is measured in seconds (s)
Each electron in a circuit carries a very small charge but there are many billions of electrons present. Many everyday currents for small household appliances will be measured in milliamps, mA: 1,000 mA = 1 A.
Example calculation
A current of 60 mA flows through a lamp for half an hour. Calculate the charge transferred.
60 mA = 60 梅 1,000 = 0.060 A
0.5 hours = 30 minutes
= 30 脳 60 = 1,800 s
charge flow = current 脳 time
= 0.060 脳 1,800
= 108 C
Question
A charge of 5.0 C is transferred through a wire in 20 s. Calculate the current that flows in the wire.
First rearrange the equation to find current:
charge flow = current 脳 time
\(current = \frac{charge~flow}{time}\)
\(current = \frac{5.0}{20}\)
\(= 0.25~A\)