Current — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions

Section 1 of 10Definition of current

Electric current

• Electric current the rate of flow of electric charge, or the charge passing a point per unit time.

Carriers

• Carriers in a metal wire the carriers are free electrons; in a salt solution they are ions.

Unit

• Unit the ampere (A), where 1 A = 1 C s⁻¹.
• Smaller currents use milliamps (1 mA = 10⁻³ A) and microamps (1 µA = 10⁻⁶ A).

Section 2 of 10The equation I = Q / t (Extended)

I

Q / t

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

• I is current (A), Q is charge (C) and t is time (s).
• Rearranged, Q = I t and t = Q / I.

Charge from a graph

• Charge from a graph the area under a current–time graph equals the charge passed.
• For a steady current the area is a rectangle, I × t.
• For a changing current the area can be estimated by counting squares.

Section 3 of 10Conventional current and electron flow

1Conventional current

• Conventional current flows from the positive terminal of the cell, round the circuit, to the negative terminal.

2Electron flow (Extended)

• Electron flow (Extended) electrons are negative, so they drift the opposite way, from negative to positive.

3Circuit symbols

• Circuit symbols arrows on diodes and ammeters follow conventional current.

Section 4 of 10Conduction in a metal wire

1Free electrons

• Free electrons a potential difference across the wire gives the electrons a small net drift towards the positive end.

2Drift speed

• Drift speed the drift is slow, often less than a millimetre per second.

3Instant response

• Instant response a lamp lights at once because the electric field travels through the wire at close to the speed of light and pushes all the electrons together.

4Same current in a thin section

• Same current in a thin section the current is the same along a wire, so the electrons drift faster where it is thinner.

Section 5 of 10Measuring current

Ammeter

• Ammeter measures current and is connected in series so the whole current passes through it.

Low resistance

• Low resistance a good ammeter has very low resistance so it does not change the current.

Analogue ammeter

• Analogue ammeter read from directly in front to avoid parallax error.

Digital ammeter

• Digital ammeter gives a direct reading and may flicker between values.

Zero error

• Zero error check the reading is zero before starting or subtract the zero value.

1

Break the circuit at the point where the current is to be measured.

2

Insert the ammeter in the gap with its + terminal towards the + side of the cell.

3

Close the switch and read the current.

4

Convert any reading in mA to A before using it in equations.

Section 6 of 10Current in a series circuit

Same everywhere

• Same everywhere all the charge entering a component leaves it, so ammeters before and after a lamp read the same.

Energy, not charge

• Energy, not charge a lamp transfers energy from the charge but does not use up the charge.

Parallel circuits

• Parallel circuits the current splits at a junction and the branch currents are re-united further on.

Section 7 of 10Effects of a current

Heating

• Heating a current heats a conductor, as in a kettle element.

Magnetic

• Magnetic a current produces a magnetic field, as in an electromagnet.

Chemical

• Chemical a current causes chemical change, as in electrolysis.

Section 8 of 10More from the specification

More from the specification

Electrostatic Charging by Friction

• When insulating materials like a polythene rod or an acetate rod are rubbed with a dry cloth in an electrostatic experiment, friction transfers electrons;
  • polythene gains electrons to become negatively charged,
• while acetate loses electrons to become positively charged.

Detection of Static Charge Using a Gold-Leaf Electroscope

• When a charged object approaches the conductive metal plate of an electroscope, charge is induced down the stem and gold leaf;
  • like charges repel,
• causing the leaf to diverge from the plate, confirming the presence of static voltage.

Electric Current

• Direct vs Alternating Flow — Electric current I is the rate of flow of electric charge, driven by voltage from a power supply; direct current (DC) flows continuously in one direction,
  • (I = Q / t, in amperes, A)
• whereas alternating current (AC) periodically reverses its direction of flow.

Electroscope

• Instrument with a metal cap, stem and gold leaf used to detect charge.

Insulator

• Material that does not allow current to flow.

Alternating current

• Current that continually reverses direction.

Field lines

• Lines showing the direction and strength of a magnetic field.

Electromotive force

• Energy transferred from other forms to electrical energy per unit charge.

Gold-leaf electroscope

• A device in which a charged gold leaf falls when photoelectrons discharge the plate.

Mains supply

• Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.

Electrical power

• Rate of energy transfer, P = I V.
  What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)

Section 9 of 10Key terms

Term	Meaning
Current	Rate of flow of charge, I = Q / t, in amperes.
Ampere	Current of one coulomb per second.
Conventional current	Current direction from the positive to the negative terminal.
Ammeter	Instrument that measures current, connected in series.
Charge carrier	Particle that carries charge, such as an electron or ion.

Section 10 of 10Exam technique

1

Forgetting to convert minutes to seconds or mA to A before using I = Q / t.

2

Saying electrons move from positive to negative — conventional current does, electrons go from negative to positive.

3

Believing a lamp uses up current — the same current leaves as enters.

4

Connecting an ammeter in parallel with a component — it must be in series.