Alternating Current and Direct Current — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

Section 1 of 10Direct current (d.c.)

Direct current

Direct current a steady current constantly flowing in the same direction in a circuit, from positive to negative.

Sources

• Sources cells, batteries, solar cells and d.c. power supplies give direct current.

Terminals

• Terminals a d.c. supply has a fixed positive terminal and a fixed negative terminal.

Graph

• Graph current against time is a horizontal line that never crosses the time axis.

Devices

• Devices phones and laptops need d.c., so their chargers convert mains a.c. into d.c.

Section 2 of 10Alternating current (a.c.)

Alternating current

Alternating current a current that continuously changes direction, going back and forth around a circuit.

Terminals

• Terminals an a.c. supply has two identical terminals that change from positive to negative and back again.

Charge carriers

• Charge carriers they move back and forth about a fixed position instead of travelling round the circuit.

Source

• Source a.c. is produced by electrical generators, in which a coil rotates in a magnetic field.

Graph

• Graph current or voltage against time is a sine wave, with positive values for one direction and negative values for the other.

Section 3 of 10Features of an a.c. graph

Features of an a.c. graph

Period T

• Period T the time for one complete cycle, in seconds.

Frequency f

• Frequency f the number of cycles each second, in hertz (Hz), where f = 1 / T.

Peak value (amplitude)

• Peak value (amplitude) the maximum current or voltage reached.

Section 4 of 10Mains electricity

Mains electricity

Type

• Type mains electricity is a.c.

UK mains

• UK mains frequency 50 Hz and a potential difference of about 230 V.

Other countries

• Other countries some use 60 Hz with 110–120 V.

Reversals

• Reversals the current reverses direction 100 times per second at 50 Hz, too fast to see lamps flicker.

Transmission

• Transmission a.c. is used because its voltage can be changed easily and efficiently with a transformer,
• so it can be stepped up for the grid and stepped down for homes.

Section 5 of 10Reading an oscilloscope trace

d.c. source

• d.c. source a flat line displaced from the centre line.

a.c. source

• a.c. source a wave.

Frequency

• Frequency count the divisions for one cycle, multiply by the time-base setting to get T and use f = 1 / T.

Peak voltage

• Peak voltage count the divisions from the centre to the top of the wave and multiply by the Y-gain.
  • (volts per division)

Changing the source

• Changing the source doubling the frequency halves the period and squashes the wave horizontally,
  • whereas doubling the voltage makes the wave taller with the same width.

Section 6 of 10Comparing d.c. and a.c.

Feature	Direct current	Alternating current
Direction	Constant, one direction	Constantly changing
Source	Cells and batteries	Generators (mains)
Terminals	Fixed positive and negative	Two identical terminals
Graph	Horizontal line	Sine wave

Section 7 of 10Which devices use which

d.c. only

• d.c. only electronic circuits and electrolysis need d.c.

Either

• Either heaters and filament lamps work on both.

Diode

• Diode allows current in one direction only, so it can rectify a.c. into d.c.

Transformer

• Transformer it does not work on d.c. because it needs a changing magnetic field.

Section 8 of 10More from the specification

More from the specification

Parallel current force behavior

• Parallel current force behavior conductors carrying electric currents in opposite directions repel each other due to interacting cylindrical magnetic fields.

Direct vs Alternating Current

• Direct current (d.c.) flows continuously in one direction around a circuit, supplied by batteries;
  • alternating current (a.c.) repeatedly reverses direction, supplying electrical energy to mains networks.

Electrostatic Induction and Detection

• Rubbing a polythene rod with a dry cloth charges it negatively by transferring electrons;
  • bringing the rod near the metal plate of a gold-leaf electroscope induces positive charge on the plate and negative charge on the leaves,
• causing leaf deflection.

Electrical Power and Resistance Equations

• In electrical conductors, electrical energy transferred is calculated using the equation E = P·t = I·V·t,
• where electrical resistance R = V / I opposes current flow.

Direct vs Alternating Current

• Direct current (d.c.) flows continuously in one direction around a circuit, supplied by batteries;
  • alternating current (a.c.) repeatedly reverses direction, supplying electrical energy to mains networks.

Electrostatic Induction and Detection

• Rubbing a polythene rod with a dry cloth charges it negatively by transferring electrons;
  • bringing the rod near the metal plate of a gold-leaf electroscope induces positive charge on the plate and negative charge on the leaves,
• causing leaf deflection.

Electrical Power and Resistance Equations

• In electrical conductors, electrical energy transferred is calculated using the equation E = P·t = I·V·t,
• where electrical resistance R = V / I opposes current flow.

Gold-leaf electroscope

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

Resistance

• Opposition to current, R = V / I, in ohms (Ω).

Electroscope

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

Conductor

• Material that allows current to flow.

Insulator

• Material that does not allow current to flow.

Component

• Part of a vector along one axis.

Energy transferred

• W = V Q, in joules.
  What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)

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.

Section 9 of 10Key terms

Term	Meaning
Direct current (d.c.)	Steady current in one direction only.
Alternating current (a.c.)	Current that continuously changes direction.
Frequency	Number of cycles per second, measured in hertz.
Period	Time for one complete cycle.
Mains electricity	Domestic a.c. supply, 50 Hz and about 230 V in the UK.

Section 10 of 10Exam technique

1

Describing a.c. as current that flows both ways at once — it reverses direction repeatedly, not simultaneously.

2

Confusing period and frequency — T is seconds per cycle, f is cycles per second.

3

Saying a battery gives a.c. — batteries give d.c.

4

Drawing a d.c. graph that crosses the time axis — d.c. keeps one direction.