Direct potential difference: a supply that pushes one way only
Definition
Direct potential difference
A potential difference that acts in one direction only.
- A cell or a battery supplies a direct potential difference.
- Its terminals keep the same polarity, so one terminal stays positive and the other stays negative.
- Because the direction of the potential difference never changes, the current in the connected circuit flows one way only.
- On a graph of potential difference against time, a steady direct supply is a horizontal line that stays on the same side of zero.
Alternating potential difference: a supply that keeps reversing
Definition
Alternating potential difference
A potential difference that repeatedly changes direction.
- The polarity of an alternating supply continually reverses, so each terminal is positive and then negative in turn.
- The current it drives therefore changes direction over and over.
- On a graph of potential difference against time, an alternating supply swings between positive and negative values and crosses zero each time it reverses.
- The one idea that separates the two supplies is direction: a direct potential difference acts in one direction only, while an alternating potential difference keeps reversing.
Direct and alternating current
Definition
Direct current (dc)
A current that flows in one direction only.
Definition
Alternating current (ac)
A current that repeatedly changes direction.
- A potential difference is what drives a current around a circuit, so the current always flows in the direction the potential difference pushes it.
- A direct potential difference therefore drives a direct current (dc), which flows in one direction only.
- An alternating potential difference drives an alternating current (ac), which reverses every time the potential difference reverses.
- The current also follows the size of the potential difference, so as an alternating potential difference rises, falls and reverses, the current through a resistor rises, falls and reverses with it.
- This is why a cell is often called a dc supply and the mains an ac supply: each supply is named after the type of current it drives.
Exam technique
- To explain the difference, compare both supplies in one sentence: a direct potential difference acts in one direction only, whereas an alternating potential difference repeatedly changes direction.
- Writing only that one is dc and the other is ac does not explain anything, so it earns nothing.
The UK mains: about 230 V at 50 Hz
Definition
Frequency
The number of complete cycles each second, measured in hertz (Hz\text{Hz}Hz).
- Mains electricity is supplied to homes and other buildings as an alternating potential difference.
- In the United Kingdom the domestic mains supply is about 230 V230\ \text{V}230 V.
- It has a frequency of 50 Hz50\ \text{Hz}50 Hz, which means the supply completes 50 full cycles every second.
- During each cycle the potential difference changes in one direction and then back again.
Example
The UK mains supply has a frequency of 50 Hz50\ \text{Hz}50 Hz. Calculate the number of complete cycles in 0.20 s0.20\ \text{s}0.20 s.
- Start with the relationship: number of cycles=frequency×time\text{number of cycles} = \text{frequency} \times \text{time}number of cycles=frequency×time.
- Substitute the values: number of cycles=50 Hz×0.20 s\text{number of cycles} = 50\ \text{Hz} \times 0.20\ \text{s}number of cycles=50 Hz×0.20 s.
- Since 1 Hz1\ \text{Hz}1 Hz is one cycle per second, number of cycles=10\text{number of cycles} = 10number of cycles=10.
- The mains completes 10 full cycles in 0.20 s0.20\ \text{s}0.20 s.
Common Mistake
- Do not confuse the two numbers: 230 V230\ \text{V}230 V is the potential difference of the supply, while 50 Hz50\ \text{Hz}50 Hz is how often it completes a cycle.
- An alternating supply does not simply switch on and off; it repeatedly reverses direction.
Self review
- What type of potential difference does a cell provide?
- How does an alternating potential difference differ from a direct one?
- What type of current does an alternating potential difference drive, and why?
- What type of supply is UK mains electricity, and what is its approximate potential difference?
- What is the UK mains frequency, and what does that number mean?
