Electric current, the ampere and circuit symbols
Current is the charge flowing per second, i = Q/t, measured in amperes. Electrons drift from − to +, but conventional current flows from + to − outside the cell.
What electric current is
Electric current is the rate of flow of charge: the charge that passes a cross-section of a conductor each second, i = Q/t. Your NIOS lesson writes current as i; many books use I. A metal wire has a huge number of free electrons moving about at random, so there is no net flow. When a cell is joined across the wire, the electrons drift slowly in one direction. A cell turns chemical energy into electrical energy and keeps a steady potential difference. Electrons leave the negative terminal, go through the wire and return to the positive terminal. By convention, the direction of current is taken from the positive to the negative terminal, opposite to the electron flow. Several cells joined together make a battery. A battery gives direct current (DC), which flows in one direction only.
Units, meters and a caution
- 1 ampere is the current when 1 coulomb of charge passes a cross-section in 1 second: 1 A = 1 C/1 s.
- Small currents are given in milliamperes (1 mA = 10⁻³ A) and microamperes (1 μA = 10⁻⁶ A).
- An ammeter measures current. It is joined in series, so all the current passes through it.
- A voltmeter measures potential difference. It is joined in parallel, across the part.
- A multimeter (AVO meter) can measure current (A), voltage (V) and resistance (O, for ohm).
- Caution: never join the two ends of a battery directly with a bare wire. The current becomes very large, and the wire and battery get hot and are damaged.
Circuit symbols you must draw
| Component | How to draw the symbol | What it does |
|---|---|---|
| Electric cell | A long thin line (+) and a short thick line (−), side by side | Gives a potential difference |
| Battery | Two or more cell symbols joined in a row | Gives a larger potential difference |
| Plug key or switch (open) | Two small brackets ( ) on the wire with an empty gap | Circuit is broken: OFF |
| Plug key or switch (closed) | The same brackets with a dot (•) between them | Circuit is complete: ON |
| Wire joint | Two wires meeting, with a dark dot at the joint | Connects the wires |
| Wires crossing without joining | One wire drawn across the other with no dot | No connection |
| Electric bulb | A circle with a cross inside it | Gives light |
| Resistor of resistance R | A zigzag line | Opposes the current |
| Variable resistance (rheostat) | A zigzag line with an arrow drawn across it | Changes the current |
| Ammeter | A circle with A inside, + and − marked | Measures current |
| Voltmeter | A circle with V inside, + and − marked | Measures potential difference |
| Fuse | A small rectangle with the wire running through it | Melts and breaks the circuit if the current is too large |
Charge, current and electrons
(a) A current of 0.25 A flows through a torch bulb for 4 minutes. How much charge passes, and how many electrons is that? (b) 45 C of charge flows through a fan in 1.5 minutes. Find the current. (e = 1.6 × 10⁻¹⁹ C)
- (a) Given: i = 0.25 A, t = 4 min = 240 s. Formula: Q = i × t.
- Q = 0.25 A × 240 s = 60 C.
- Number of electrons n = Q/e = 60 C ÷ (1.6 × 10⁻¹⁹ C) = 3.75 × 10²⁰.
- (b) Given: Q = 45 C, t = 1.5 min = 90 s. Formula: i = Q/t.
- i = 45 C ÷ 90 s = 0.5 A.
(a) 60 C, or 3.75 × 10²⁰ electrons; (b) 0.5 A
How to draw a simple circuit
- Draw a cell: a long thin line (+) and a short thick line (−). Mark + and −.
- From the + end, draw a wire to a plug key and then to a bulb (a circle with a cross).
- Draw an ammeter (A in a circle) in line with the bulb, and bring the wire back to the − end.
- Draw a voltmeter (V in a circle) on two short wires across the bulb, in parallel.
- Put arrows on the wire from + round to −, and label them conventional current.
A phone charger label such as 5 V, 2 A gives the push (volts) and the flow (amperes). A torch lights the moment you switch it on, though each electron drifts only a fraction of a millimetre a second. All the electrons in the wire start moving together.
1 marks: “Name the instrument used to measure electric current and say how it is joined in a circuit.” An ammeter, joined in series.
2 marks: “A cell is joined to a bulb. In which direction do the electrons move in the wire, and in which direction is the conventional current taken?” Electrons move from the negative terminal through the wire to the positive terminal. Conventional current is taken the opposite way, from positive to negative outside the cell.
2 marks: “A current of 0.2 A flows through a bulb for 10 minutes. Find the charge that passes and the number of electrons. (e = 1.6 × 10⁻¹⁹ C)” Q = it = 0.2 A × 600 s = 120 C. n = Q/e = 120 ÷ (1.6 × 10⁻¹⁹) = 7.5 × 10²⁰ electrons.
i = Q/t counts coulombs per second. Ammeter goes in line (series); voltmeter goes alongside (parallel).