AC, DC and Mains Electricity
In this lesson we learn about types of current:
AC and DC
How to calculate missing values
How to identify approaches to questions
Click the image to enlarge it.
Practice Questions
Question 1:
What do the abbreviations 'AC' and 'DC' stand for in the context of electricity?
Answer:
AC stands for Alternating Current.
DC stands for Direct Current.
Question 2:
Describe what is meant by 'Direct Current' (DC). How does the current behave in a DC circuit?
Answer:
Direct Current (DC) is an electric current that flows in only one direction. The flow of charge is constant and steady, meaning the current remains at a constant value (or varies, but always in the same direction).
Question 3:
Sketch a current-time graph for a steady Direct Current (DC). Label the axes.
Answer:
`Current (I) / A
^
|
| -----------------
| |
| |
+---------------------> Time (t) / s
0`
Answer Walkthrough:
Key features:
- Horizontal line: Represents constant current.
- Above zero: Represents current flowing in one direction.
- Axes labeled: Current ($I$) on the y-axis, Time ($t$) on the x-axis.
Question 4:
Give two examples of common sources of Direct Current (DC) electricity.
Answer:
Two common sources of DC electricity are:
- Batteries (e.g., AA batteries, car batteries).
- Solar cells / Solar panels. (Other acceptable answers: DC power supplies, phone chargers/laptops (though they convert AC to DC)).
Question 5:
Describe what is meant by 'Alternating Current' (AC). How does the current behave in an AC circuit?
Answer:
Alternating Current (AC) is an electric current that repeatedly reverses its direction of flow. The current flows back and forth, constantly changing its magnitude and direction over time.
Question 6:
Sketch a current-time graph for Alternating Current (AC). Label the axes and indicate what 'frequency' refers to on the graph.
Answer:
`Current (I) / A
^
| / \ / \
| / \ / \
| / \ / \
+---+-------+---+-------+-----> Time (t) / s
| \ / \ /
| \ / \ /
| \ / \ /
v \ / \ /`
Explanation of Frequency: Frequency refers to the number of complete cycles (one full oscillation from positive to negative and back to positive current) that occur per second. On the graph, one cycle is from one peak to the next peak, or one trough to the next trough, or from one zero crossing to the next corresponding zero crossing after one complete oscillation.
Answer Walkthrough:
Key features:
- Sinusoidal wave: Represents the constantly changing magnitude and direction.
- Crosses the time axis: Shows reversal of direction.
- Axes labeled: Current (I) on the y-axis, Time (t) on the x-axis.
- Frequency explanation: Links the visual representation to the definition of frequency.
Question 7:
Give two examples of common sources of Alternating Current (AC) electricity.
Answer:
Two common sources of AC electricity are:
- Mains electricity (the electricity supplied to homes and businesses).
- Generators / Power stations. (Other acceptable answers: AC power supplies).
Question 8:
What is the typical frequency of the mains electricity supplied to homes in the UK?
Answer:
The typical frequency of mains electricity in the UK is 50 Hz (Hertz).
Question 9:
Why is AC primarily used for transmitting electricity over long distances from power stations to homes?
Answer:
AC is primarily used for transmitting electricity over long distances because its voltage can be easily changed (stepped up or stepped down) using transformers. Stepping up the voltage for transmission significantly reduces the current for a given amount of power ($P=IV$), which in turn minimises energy loss due to heating in the transmission cables ($P_{loss}=I^2R$).
Question 10:
Most electronic devices (like mobile phones or laptops) use DC internally, but they plug into AC mains sockets. Explain how this is possible.
Answer:
This is possible because these devices use an AC-to-DC adapter (often called a 'charger' or 'power brick'). This adapter contains electronic components (including a transformer to step down the voltage and a rectifier to convert AC to DC) that transform the high-voltage AC from the mains into the low-voltage DC required by the device.
Question 11:
Consider a simple circuit with a battery and a light bulb. Sketch the current-time graph for the current flowing through the bulb.
Answer:
`Current (I) / A
^
|
| -----------------
| |
| |
+---------------------> Time (t) / s
0`
Answer Walkthrough:
Since a battery provides DC, the graph will be a steady horizontal line above the time axis.
Question 12:
If you were to plug a simple resistive heater directly into a mains socket, describe the current flowing through the heater.
Answer:
The current flowing through the heater would be Alternating Current (AC). This means its direction would reverse 50 times per second (in the UK), and its magnitude would continuously change from a maximum in one direction, through zero, to a maximum in the opposite direction, and back again.
Question 13:
In terms of electron movement, how would you describe the difference between DC and AC?
Answer:
- In DC, electrons flow steadily in one continuous direction around the circuit.
- In AC, electrons don't flow continuously in one direction; instead, they oscillate back and forth, repeatedly changing their direction of movement within the wire.
Question 14:
Could a transformer be used to step up the voltage of a DC power supply? Explain why or why not.
Answer:
No, a standard transformer cannot be used to step up the voltage of a DC power supply.
Explanation: Transformers work on the principle of electromagnetic induction, which requires a changing magnetic field. Direct current produces a constant magnetic field (not a changing one). Therefore, it cannot induce a voltage in the secondary coil of a transformer.
Question 15:
Imagine you have an unknown power supply and you connect an oscilloscope to it to view its current-time graph. If you see a fluctuating wave that regularly crosses the time axis, what type of current is the power supply providing? Justify your answer.
Answer:
The power supply is providing Alternating Current (AC).
Justification: The graph showing a fluctuating wave that regularly crosses the time axis means that the current is constantly changing in magnitude and periodically reversing its direction (flowing positive then negative). This is the defining characteristic of AC.