⇒ Electric current is a flow of electric charge
⇒ The size of the elctric current, I, is the rate of flow of electric charge, which is given by the equation:
⇒ Electric current is measured in amperes, A, by an ammeter connected in series with components
⇒ The potential difference or voltage, V, between two points in an electric circuit is the work done (energy transferred), W, per coulomb of charge that passes between the points and is given by the equation:
⇒ Potential difference is measured in vols, V, by a voltmeter connected in parallel with components
⇒ The resistance of a component, R, can be found by measuring the current, I, through and potential difference, V, across the component; resistance is defined by the equation:
⇒ Resistance is measured in Ohms, Ω
⇒ The current flowing through, the potenial difference across and the resistance of a component are related by the equation: V = I x R
⇒ The potential difference provided by cells connected in series is the sum of the potential difference of each cell (depending on the direction in which they are connected)
⇒ A lightning bolt is an extreme example of an electric current
⇒ AN electric current is formed when the electrons move. Electric current, I, is the rate of flow of charge:
⇒ The SI unit of current is the ampere (symbol, A), which is nearly always abreviated to amps
⇒ Note here the use of ▵Q and ▵t, rather than Q and t
⇒ Electronic circuits, such as those that control household appliances, operate with much smaller currents (typically milliamps (mA, 10-3 A), and many microelectronic circuits, such as the printed circuit boards inside many computer devices, operate with currents of the order of microamps (μA, 10-6 A)
⇒ Even currents of the order of microamps still involve the movement of about 6 x 1012 electrons per second)
⇒ Not all electric currents involve the flow of electrons
⇒ Charged ions in solution (called an electrolyte) can also flow and create a current
⇒ Car batteries work due to the flow of hydrogen (H+) ions and sulfate (SO42-) iions and can deliver currents of up to 450A for 2.5 seconds
⇒ Both of the ions in the solution contribute to the current, which can be measured externally by an ammeter connected in series with both electrodes
⇒ Tip: in the exam you are assessed on your understanding of the significant figures of the numbers that you use in calculations. In this example, both the pieces of data used in the calculations are to 2 significant figures (sf). As a general rule, you should state your answer to the same number of sf as the least significant piece of data used - 2 sf in this case
⇒ Below shows the variation of the current drawn from a cell wit time
⇒ The current is drawn for a total of 10 seconds, but 5s after switching on A, switch B is closed allowing current to travel through the second identical resistor
⇒ As ▵Q = I x ▵t the charge transferred during the first 5s is the area under the current-time graph from t = 0s to t = 5s
⇒ This graph shows the variation of current with time through part of the timing circuit for a set of traffic lights
⇒ In a similar way, the gradient of a charge-time graph (Q-t) is the current