Direct potential difference
Definition
Direct potential difference
A potential difference whose direction never changes, so one terminal always stays positive and the other always stays negative.
Definition
Direct current (d.c.)
A current in which charge flows around the circuit in one direction only.
- Cells, batteries and solar cells all produce a direct potential difference.
- One terminal of a cell is always positive and the other is always negative, so the potential difference always acts in the same direction.
- A direct potential difference drives a direct current (d.c.), in which charge flows around the circuit in one direction only.
- The size of a direct potential difference can change, for example as a battery runs flat, but its direction never reverses.
- Direction, not size, is therefore what makes a supply direct.
- Phones, laptops, torches and other portable devices all run on d.c. from their batteries.
Alternating potential difference
Definition
Alternating potential difference
A potential difference that repeatedly reverses its direction, so each terminal switches between positive and negative many times each second.
Definition
Alternating current (a.c.)
A current that repeatedly reverses its direction of flow around the circuit.
- The generators in power stations produce an alternating potential difference, and this is what the mains supply delivers to homes.
- The two terminals of an alternating supply repeatedly swap over, so each one is positive for a moment and then negative.
- An alternating potential difference drives an alternating current (a.c.), in which the charge flows one way, then the opposite way, again and again.
- During each cycle, the potential difference rises from zero to a maximum in one direction, falls back to zero, rises to a maximum in the opposite direction and returns to zero.
- An alternating potential difference is therefore changing in both size and direction all the time.
- The UK mains supply completes 505050 of these cycles every second.
- The national grid uses a.c. because transformers need an alternating supply to change the potential difference.
- Devices that need d.c. but plug into the mains, such as phone chargers, contain circuits that convert the alternating supply into a direct one.
Showing the difference on a graph
Definition
Data logger
An electronic device that automatically records readings from a sensor at regular time intervals.
- The clearest way to see the difference between the two supplies is a graph of potential difference against time, called a trace.
- A trace can be recorded with a voltage sensor connected to a data logger, which takes hundreds of readings each second and plots them on a screen.
- The vertical axis shows potential difference in volts, the horizontal axis shows time, and a horizontal zero line marks 0 V0\ \text{V}0 V.
- Points above the zero line show the potential difference acting in one direction, and points below it show the potential difference acting in the opposite direction.
- A steady d.c. supply gives a flat horizontal line that stays on one side of the zero line for the whole trace.
- An a.c. supply gives a repeating wave that crosses the zero line again and again, showing that its direction keeps reversing.
- The height of the wave above the zero line is the peak potential difference, the largest value the supply reaches.
- The time taken for one complete wave is the period of the supply, and the number of complete waves each second is its frequency.
Practical
Comparing a.c. and d.c. traces
- Aim: to use a data logger to record and compare the potential difference against time traces of a direct supply and an alternating supply.
- Apparatus: a data logger with a voltage sensor, a laptop or display screen, a fresh 1.5 V1.5\ \text{V}1.5 V cell in a holder, a low-voltage laboratory power supply with an a.c. output, a 2.5 V2.5\ \text{V}2.5 V filament lamp in a holder, a switch and connecting leads with crocodile clips.
- Method, the d.c. trace:
- Connect the cell, switch and lamp in series, then clip the two leads of the voltage sensor across the lamp, red lead to the side joined to the positive terminal.
- Set the data logger to record at a high rate, such as 100010001000 readings per second, for a total time of 0.1 s0.1\ \text{s}0.1 s.
- Close the switch, start logging and watch the lamp glow steadily while the trace builds on the screen.
- Save the trace, open the switch and disconnect the cell.
- Method, the a.c. trace:
- Replace the cell with the a.c. output terminals of the laboratory power supply, set to its lowest setting of about 2 V2\ \text{V}2 V.
- Keep the voltage sensor across the lamp, switch on the supply and close the switch.
- Record a second trace over the same 0.1 s0.1\ \text{s}0.1 s at the same logging rate.
- Display both traces on the same axes, with the same scales, so that they can be compared directly.
- Results: the d.c. trace is a flat line at about 1.5 V1.5\ \text{V}1.5 V that never crosses 0 V0\ \text{V}0 V, while the a.c. trace is a smooth wave that swings above and below 0 V0\ \text{V}0 V, completing 555 full cycles in 0.1 s0.1\ \text{s}0.1 s.
- Maths: 555 cycles in 0.1 s0.1\ \text{s}0.1 s gives a period of 0.15=0.02 s\dfrac{0.1}{5} = 0.02\ \text{s}50.1=0.02 s and a frequency of 50.1=50 Hz\dfrac{5}{0.1} = 50\ \text{Hz}0.15=50 Hz, matching the frequency of the mains.
- Watch out: a logging rate that is too low records only a few points per cycle and turns the smooth a.c. wave into a jagged or misleading shape, and reversing the sensor leads on the d.c. supply moves the flat line below zero without changing the supply.
- Safety: use only the low-voltage outputs of the power supply, never connect the voltage sensor to a mains socket, and switch off the supply before changing any connections.
Example
Identifying two supplies from their traces
- Trace A is a horizontal line at 6 V6\ \text{V}6 V for the whole 0.1 s0.1\ \text{s}0.1 s, so the potential difference never changes direction and supply A is d.c.
- Trace B rises to 4 V4\ \text{V}4 V, falls through zero to −4 V-4\ \text{V}−4 V and repeats, so the potential difference reverses and supply B is a.c.
- Trace B shows 555 complete cycles in 0.1 s0.1\ \text{s}0.1 s, so each cycle lasts 0.02 s0.02\ \text{s}0.02 s.
- The peak potential difference of supply B is 4 V4\ \text{V}4 V, which is read from the highest point of the wave.
Common Mistake
- Do not say that the difference is only that a.c. "changes" while d.c. "stays the same", because the key difference is that a.c. reverses direction and d.c. does not.
- Do not decide a supply is a.c. because its trace is not flat, since a d.c. supply can vary in size without ever crossing the zero line.
- Do not confuse a trace of potential difference with a trace of current, because a voltage sensor measures potential difference.
Exam technique
Explaining direct and alternating supplies
- State the direction behaviour of each supply: a direct potential difference acts in one direction only, while an alternating one repeatedly reverses direction.
- Link each potential difference to the current it drives, using the terms direct current and alternating current.
- When you are given a trace, refer to the zero line: a trace that crosses it shows a.c., and a trace that stays on one side shows d.c.
- Give one source of each, such as a battery for d.c. and the mains for a.c.
Self review
- What is the difference between a direct potential difference and an alternating potential difference?
- Name one source of d.c. and one source of a.c.
- Describe the trace given by a steady d.c. supply on a potential difference against time graph.
- How does an a.c. trace show that the potential difference reverses direction?
- What is the frequency of an a.c. trace that shows 101010 complete cycles in 0.2 s0.2\ \text{s}0.2 s?
