Electric current
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
What is Electric Current? An electric current consists of moving electric charges. In metals, current is the flow of free electrons . Electric current The charge passing a point per unit time. I = Q/t Where: I = current (A), Q = charge (C), t = time (s) Units Current is measured in amperes (A) 1 milliampere (mA) = 0.001 A = 10^-3 A Charge is measured in coulombs (C) 1 coulomb = 1 ampere × 1 second (1 C = 1 A s) Electrical Conduction in Metals In a metal conductor: Each atom has one or more loosely held electrons (free electrons) When a p.d.
is applied, free electrons drift from negative to positive terminal This drift of electrons constitutes the electric current FIG 4.2.10: Electron drift in a metal conductor A metal wire connected to a battery (+ terminal on right, − on left).
Inside the wire, positive metal ions shown as fixed circles in a lattice. Free electrons (small negative circles) drift slowly from left to right (− to +). Arrows show electron movement direction opposite to conventional current direction.
Conventional Current vs Electron Flow Conventional Current Electron Flow Direction: positive → negative Direction: negative → positive Used in circuit diagrams Actual movement of electrons Historical convention Physical reality in metals Remember Conventional current flows from + to − .
Electrons flow from − to + (opposite direction). Using Ammeters An ammeter measures electric current. It must be connected: In series with the component + terminal of ammeter to + terminal of supply Analogue Ammeter Digital Ammeter Pointer moves across scale Numerical display May have multiple scales/ranges Select range before measuring Reads d.c.
only (simple type) Can read a.c. and d.c. Direct Current (d.c.) and Alternating Current (a.c.) Direct Current (d.c.) Alternating Current (a.c.) Electrons flow in one direction only Direction of flow reverses regularly Produced by batteries and cells Produced by generators, mains supply Constant or varying magnitude Varies sinusoidally with time FIG 4.2.14-15: Current-time graphs for d.c.
and a.c. Three graphs with current (I) on y-axis and time (t) on x-axis. (a) Steady d.c.: horizontal straight line above zero. (b) Varying d.c.: wavy line that stays above zero. (c) a.c.: smooth sinusoidal wave oscillating above and below zero, showing one complete cycle with positive and negative half-cycles.
Frequency of a.c. The frequency is the number of complete cycles per second, measured in hertz (Hz) . Mains supply frequency = 50 Hz (in most countries). Worked Examples Example 1: Calculating Current Question: A charge of 12 C passes a point in a circuit in 4 s.
Calculate the current. Answer I = Q/t = 12/4 = 3 A Example 2: Calculating Charge Question: A current of 2.5 A flows for 20 s. How much charge passes a point in the circuit? Answer Q = I × t = 2.5 × 20 = 50 C
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