This topic accounts for approximately 16% of your exam marks.
stable
Very High
Stable16%
Ohm's Law calculations and I-V characteristic graphs are among the most reliably tested question types.
What an electric current is
Electric current is the rate of flow of electric charge around a circuit
Current is measured in amperes, symbol A (often shortened to "amps")
One ampere is the current that flows when one coulomb of charge passes a point in one second, so 1 A = 1 C/s
For current to flow, there must be a complete (unbroken) conducting loop with a source of voltage in it, such as a cell or battery
Charge carriers in different materials
In a metallic conductor, the moving charges are negatively charged electrons; the heavy positive metal ions are fixed in a lattice and only the outermost electrons drift through it
In a liquid electrolyte or ionised gas, the moving charges include both positive and negative ions, which drift in opposite directions
Conventional current vs electron flow
Long before electrons were discovered, scientists agreed that current flows from the positive terminal of a cell, around the external circuit, and back to the negative terminal. This historical choice is called the direction, and it is the direction shown by every arrow on every circuit diagram in exam questions
In a metal wire, the electrons actually drift the other way, from the negative terminal, through the wire, back to the positive terminal
Both descriptions refer to exactly the same physical situation; they just measure the flow in opposite senses. In exam answers, always use the conventional direction unless asked for electron flow explicitly
Measuring current
An ammeter is the instrument that measures current
An ammeter must be wired in series with the component whose current is being measured, because the same current that flows through the component must also flow through the ammeter
An ideal ammeter has zero , so inserting it does not affect the circuit it is measuring
The charge equation
Q = I × t
where:
Q = charge that has passed (coulombs, C)
I = current (amps, A)
t = time the current has been flowing (s)
Rearrangements:
I = Q / t (current is charge per second)
t = Q / I
Convert any milliamp values into amps before substituting: 1 mA = 1 × 10⁻³ A. A current of 250 mA, for example, is 0.250 A
Worked example
Calculating charge from current and time
A torch bulb carries a current of 0.40 A for 30 s. Calculate the charge that passes through the bulb.