Electric current
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
What is Electric Current?
- An electric current is a flow of electric charge
- in metals, current is the flow of free electrons
Defining Current
Extended Only
- Electric current is the charge passing a point per unit time
- Formula:
- I = (Q/t)
- Where:
- I = current (A)
- Q = charge (C)
- t = time (s)
- Units:
- current is measured in amperes (A); 1 mA = 0.001 A = 10^-3 A
- charge is measured in coulombs (C); 1 C = 1 A × 1 s
Worked Example: Calculating Current
Museli measures a charge of 12 C passing a point in a circuit in 4 s, and wants to find the current.
- Step 1: write down the equation. I = (Q/t)
- Step 2: substitute the values. I = (12/4)
- Step 3: calculate the answer. I = 3 A
Worked Example: Calculating Charge
Muumbe has a current of 2.5 A flowing for 20 s, and wants to find the charge that passes a point in the circuit.
- Step 1: rearrange the equation. Q = I × t
- Step 2: substitute the values. Q = 2.5 × 20
- Step 3: calculate the answer. Q = 50 C
Electrical Conduction in Metals
- Each atom in a metal has one or more loosely held (free) electrons
- Applying a p.d. across the metal makes the free electrons drift from the negative terminal towards the positive terminal
- This drift of electrons is what makes up the electric current in a metal
Free electrons drift from the negative terminal towards the positive terminal
Conventional Current and Electron Flow
Extended Only
- Conventional current flows from positive to negative
- this is the direction used in circuit diagrams, and is a historical convention
- The actual flow of free electrons is from negative to positive
- this is the physical reality of what happens inside a metal
Using Ammeters
- An ammeter measures electric current
- It must be connected:
- in series with the component being tested
- with its + terminal connected towards the + terminal of the supply
| Analogue Ammeter | Digital Ammeter |
|---|---|
| Pointer moves across a scale | Numerical display |
| May have multiple scales/ranges to choose from | Select the range before measuring |
| Simple types read d.c. only | Can read both 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 and the mains supply |
| Constant or varying magnitude | Varies sinusoidally with time |
Steady d.c., varying d.c., and a.c. all shown as current-time graphs
- The frequency of a.c. is the number of complete cycles per second, measured in hertz (Hz)
- mains supply frequency is 50 Hz in most countries
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