Electric current, circuits and potential difference
Electric current is the rate of flow of charge (I = Q/t). Charge flows only in a closed circuit, pushed by a potential difference (V = W/Q).
What is electric current?
Atoms contain tiny charged particles. Electrons carry a negative charge. In a metal wire, some electrons are free to move. When they drift in one direction, charge flows. Electric current is the amount of charge that flows through a cross-section of a conductor in one second. A conductor is a material that lets charge flow easily, like copper. So I = Q/t, where Q is the charge in coulomb (C) and t is the time in second (s). The SI unit of current is the ampere (A). One ampere is the current when 1 coulomb of charge flows in 1 second. Small currents are measured in milliampere (1 mA = 10⁻³ A) and microampere (1 μA = 10⁻⁶ A).
Conventional current vs electron flow
In circuit diagrams, the arrow for current always shows conventional current.
Electric circuit: parts and symbols
| Component | What it does | How to draw the symbol |
|---|---|---|
| Cell | Gives a potential difference; the source of energy | A long thin line (+) and a short thick line (−), side by side |
| Battery | Two or more cells joined together | Two or more cell symbols in a row |
| Key or switch | Opens or closes the circuit | Two dots with a gap (open) or joined by a line or plug (closed) |
| Resistor | Opposes the flow of current | A zig-zag line or a small rectangle |
| Rheostat (variable resistor) | Changes the resistance, and so the current | A zig-zag line with an arrow across it |
| Ammeter | Measures current; joined in series | A circle with A inside, + and − marked |
| Voltmeter | Measures potential difference; joined in parallel | A circle with V inside, + and − marked |
| Bulb | Glows when current flows | A circle with a cross inside |
| Joined wires | Wires connected at a point | Two lines meeting with a dot at the joint |
Electric potential and potential difference
Water flows from a higher level to a lower level, like water from an overhead tank to your kitchen tap. In the same way, positive charge flows from a point of higher potential to a point of lower potential. An electric circuit is a closed, unbroken path for this flow. If the path is broken, it is an open circuit and no current flows. Electric potential at a point is the work done in moving a unit positive charge from infinity to that point against the electric force. Potential difference between two points is the work done in moving a unit positive charge from one point to the other. So V = W/Q. The SI unit is the volt (V): 1 volt = 1 joule per coulomb. A cell keeps a potential difference across its ends by a chemical reaction. That keeps the charge moving.
Worked example: current, electrons and work
(a) A charge of 30 C flows through a torch bulb in 1 minute. Find the current. (b) How many electrons make up a charge of 1.6 C? (c) How much work is done in moving 5 C of charge across a potential difference of 12 V?
- (a) Given Q = 30 C, t = 1 minute = 60 s. Formula: I = Q/t.
- I = 30 C ÷ 60 s = 0.5 A.
- (b) Formula: Q = ne, so n = Q/e.
- n = 1.6 C ÷ (1.6 × 10⁻¹⁹ C) = 1 × 10¹⁹ electrons.
- (c) Formula: V = W/Q, so W = V × Q.
- W = 12 V × 5 C = 60 J.
(a) 0.5 A (b) 1 × 10¹⁹ electrons (c) 60 J
A mobile phone charger is marked something like 5 V, 2 A. The volts are the push (potential difference); the amperes tell how much charge flows each second into the battery.
2 marks: “Define electric current. State its SI unit.” Rate of flow of charge through a cross-section of a conductor, I = Q/t. SI unit: ampere (A); 1 A = 1 C/s.
2 marks: “Define potential difference and the volt.” Work done to move a unit positive charge between two points, V = W/Q. 1 volt = 1 joule per coulomb.
4 marks: “A current of 0.4 A flows through a wire for 5 minutes. Find the charge that passes and the number of electrons.” Q = It = 0.4 × 300 = 120 C. n = Q/e = 120 ÷ (1.6 × 10⁻¹⁹) = 7.5 × 10²⁰ electrons.
Current is Charge per second (I = Q/t); Voltage is Work per charge (V = W/Q).