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Dabour HubLEARN · PRACTICE · EXPLOREEdexcel International GCSE Science Double Award (Modular) — Physics Unit 1
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Unit 2 · Lesson 2.1.1

Charge and Current

Define electric charge and current, use Q = It and describe conventional current in circuits.

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Electric charge

All matter contains charged particles.

Protons carry positive charge and electrons carry negative charge; neutrons are neutral.
Coulombs (C)
Charge is measured in coulombs (C).
The charge of one electron is about −1.6 × 10−19 C, so a coulomb is the charge of a very large number of electrons (about 6 × 1018).
Like charges repel and unlike charges attract.
In solid metals the charge carriers that can move are free (delocalised) electrons.
Previous lesson

What is current?

Electric current is the rate of flow of charge.

What is current?
In a metal wire it is the drift of free electrons; in an electrolyte it is the movement of ions.
Current is measured in amperes (A), where 1 A means 1 coulomb of charge passing a point each second.
Smaller units are the milliampere (mA = 10−3 A) and microampere (µA = 10−6 A).

Because the current is the rate of flow of charge, a larger current means more charge passes each second.

What is current?
A torch bulb may carry about 0.3 A, a phone charger a few amperes, and a kettle about 10 A.
The current in a human body that can be dangerous is only a few tens of milliamperes, which is why safety rules for electricity are strict.
In a series circuit there is only one path, so the same current passes through every component, however different the components are.

The equation

Q = I t

What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)

Q is charge in coulombs (C), I is current in amperes (A) and t is time in seconds (s). Rearranged, I = Q/t and t = Q/I. Always convert minutes to seconds and mA to A before substituting. For a current of 2.0 A for one minute, Q = 2.0 × 60 = 120 C.

Conventional current and electron flow

Before electrons were discovered, current was defined as flowing from the positive terminal of a cell, round the circuit, to the negative terminal. This conventional current direction is still used on circuit diagrams. The electrons in a metal actually move the opposite way, from negative to positive. In circuit questions always use the conventional direction unless asked about electrons.

The direction arrow on a circuit symbol such as a diode or an LED points the way conventional current flows.

Conventional current and electron flow
Remembering that electrons are negative and move from the negative terminal explains why electron flow and conventional current are opposite.
The speed at which individual electrons drift is very slow, often fractions of a millimetre per second,
  • but the electric field travels through the whole wire almost instantly,
  • which is why a lamp comes on straight away when a switch is closed.

Circuits and measuring current

A current needs a complete, conducting loop and a source of energy such as a cell or battery.

Ammeter
An ammeter measures current and must be connected in series with the component, so that the same current passes through it.
A good ammeter has very low resistance so that it does not alter the current.
The current is the same at all points in a series loop,
  • because charge cannot be created or destroyed or pile up at one point.
    • (see the series and parallel lessons)

When reading an ammeter, check the scale and range, and note the units. A digital ammeter may display in mA. If the reading is negative, the meter has been connected the wrong way round, but this does not damage the circuit.

Cells, batteries and direct current

A cell supplies direct current (d.c.), where the current always flows one way.

Cells, batteries and direct current
A battery is two or more cells connected together.
The mains supply is alternating current (a.c.), which reverses direction many times a second; this is dealt with in a later topic.
Circuit symbols for cell, battery, switch, lamp, ammeter and resistor should be learnt,
  • and you should be able to draw simple series and parallel circuits using a ruler.

Worked examples

Worked example 1. A current of 0.50 A flows through a lamp for 2.0 minutes. How much charge passes?

t = 2.0 × 60 = 120 s.

Q = It = 0.50 × 120 = 60 C.

Worked example 2. A phone charger delivers a charge of 7200 C in 1.0 hour. Find the average current.

t = 3600 s.

I = Q/t = 7200/3600 = 2.0 A.

Worked example 3. How long does it take for 45 C to pass through a wire carrying 15 mA?

I = 15 mA = 0.015 A.

t = Q/I = 45/0.015 = 3000 s = 50 minutes.

Key terms

TermMeaning
ChargeProperty of matter causing electric forces, measured in coulombs.
CurrentRate of flow of charge, in amperes.
AmpereOne coulomb per second.
Conventional currentDirection from positive to negative terminal.
AmmeterMeter placed in series that measures current.
CoulombUnit of charge, 1 A × 1 s.

Avoid these mistakes

1
Using minutes instead of seconds in Q = It.
2
Forgetting to convert mA to A.
3
Saying current is "used up" by a component: charge continues round the circuit.
4
Placing the ammeter in parallel.
5
Saying conventional current goes from negative to positive.
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