6.1.1Generating and Distributing Electricity and Household Use
Direct and Alternating Current — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 10 parts Lesson 2 of 8
Part 1 of 8Direct 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.
Part 2 of 8Alternating 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.
Part 3 of 8Features 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.
Part 4 of 8Mains 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.
Part 5 of 8Reading 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.
Part 6 of 8Comparing 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 |
Part 7 of 8Which 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.
Part 8 of 8More 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.
Key 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. |
Avoid these mistakes
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.
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