OCR Specification focus:
‘Describe practical uses of capacitors for storing and releasing electrical energy.’
Capacitors serve as rapid-response electrical energy stores, delivering bursts of power and smoothing voltage variations across circuits, making them vital in modern electronic applications today.
Capacitors as Energy Stores in Practical Systems
Understanding how capacitors act as temporary energy stores is essential for appreciating their role in electrical and electronic design. A capacitor stores energy in its electric field, created when separated charges accumulate on its plates. Unlike batteries, which rely on chemical processes, capacitors store and release energy quickly, making them ideal for applications requiring rapid charge and discharge cycles.
Although this subsubtopic focuses on uses rather than mathematical treatment, the ability of a capacitor to store energy is linked to its capacitance, its potential difference, and the electric field between its plates. These relationships determine whether a capacitor is suitable for tasks such as smoothing power supply output, providing backup power, or delivering fast pulses in timing circuits.
Energy Storage Behaviour
When a capacitor is connected to a power source, charge accumulates on its plates. As more charge builds up, the potential difference increases, strengthening the electric field in the dielectric. The stored energy can then be released when the capacitor is connected into a circuit requiring a burst of electrical energy. This makes capacitors especially valuable in systems where short-duration but high-impact energy delivery is necessary.
The amount of energy stored depends on device design and operating voltage. As capacitors discharge rapidly, they are not substitutes for long-term energy stores such as batteries, but they serve as complementary components where quick response is needed.
In all these roles, the capacitor is acting as a temporary energy store, rapidly taking in energy from a supply and then releasing it in a controlled way into a circuit.
Diagram of a parallel-plate capacitor showing the plates, dielectric, and geometric factors that determine capacitance. When charged, energy is stored in the electric field between the plates. The figure also includes plate dimensions, which are helpful background but not required in detail by the OCR specification. Source.
EQUATION
—-----------------------------------------------------------------
Energy Stored in a Capacitor (W) = ½ C V²
W = energy stored, measured in joules
C = capacitance, measured in farads
V = potential difference across the capacitor, measured in volts
—-----------------------------------------------------------------
This equation is included because it underpins how engineers select capacitors for different energy-storage applications.
Key Features Making Capacitors Useful as Energy Stores
Rapid Charge and Discharge
Capacitors can charge and discharge far more quickly than chemical cells due to the absence of slow electrochemical reactions. This enables them to:
provide immediate power during brief outages or dips in supply voltage
supply short bursts of energy in timing or pulsing circuits
rapidly stabilise voltage in sensitive electronic systems
High Power Density
While their energy density is low compared with batteries, their power density is high. Power density refers to the rate at which stored energy can be delivered. Capacitors are therefore used when the requirement is not total stored energy but the speed and intensity of delivery.
Long Operational Lifespan
Because capacitors do not rely on chemical reactions, they can undergo millions of charge–discharge cycles without significant degradation, making them reliable components in systems requiring continuous rapid cycling.
Practical Applications of Capacitors as Energy Stores
Smoothing and Stabilising Power Supplies
Many electronic circuits rely on capacitors to maintain a steady voltage by compensating for fluctuations from power sources, particularly rectifiers. Capacitors act as reservoirs of charge that are:
charged when supply voltage temporarily rises
discharged when supply voltage momentarily falls
This ensures a more stable output, which is vital for digital electronics, amplifiers, and microprocessors.
In a basic d.c. power supply, a reservoir capacitor is connected across the rectifier output: it charges up when the rectified voltage rises and then discharges between peaks, smoothing the supply delivered to the load.
Circuit diagram of a full-wave bridge rectifier with a smoothing capacitor across the d.c. output. The capacitor stores energy at voltage peaks and releases it between them to reduce ripple. The diagram also shows the diode bridge, which is wider context beyond the OCR subsubtopic. Source.
Providing Burst Energy in Camera Flash Units
Camera flashes require an intense pulse of light. The capacitor:
gradually charges from the battery
stores energy electrostatically
releases it in a very short, high-power burst to operate the flash tube
Batteries alone cannot supply this burst quickly enough, illustrating why capacitors excel in rapid-delivery applications.
Maintaining Power During Brief Interruptions
Capacitors can act as miniature backup systems. When the mains supply or battery input dips briefly:
the capacitor discharges to sustain the circuit
data or processes are preserved until the main supply recovers
This is commonly used in memory-storage circuits, microcontrollers, and real-time clocks.
Energy Storage in Electric Vehicles and Regenerative Braking
Some electric and hybrid vehicles employ high-capacity capacitors (such as supercapacitors) to store energy recovered during braking. They are beneficial because they:
accept charge from regenerative braking extremely quickly
deliver short bursts of power during acceleration
reduce strain on the vehicle’s main battery system
Although standard capacitors in A-Level contexts are smaller and simpler, the underlying principle is identical: rapid uptake and release of stored energy.
Emergency Power for Actuators and Trigger Systems
In safety and industrial equipment, capacitors can deliver fast pulses to:
trigger emergency release mechanisms
operate solenoids or actuators during sudden power loss
Their reliability and rapid energy delivery make them ideal for critical systems that must respond instantly.
Capacitors in Audio and Communication Circuits
In high-frequency or audio systems, capacitors help deliver rapid, controlled bursts of energy to maintain signal fidelity. They support functions such as:
protecting components from sudden changes in voltage
helping maintain consistent signal amplitude
supporting filter networks requiring precise energy behaviour
Design Considerations When Using Capacitors as Energy Stores
Engineers must evaluate:
Capacitance value, determining how much charge and energy can be stored
Voltage rating, ensuring safe operation without dielectric breakdown
Equivalent series resistance (ESR), influencing how quickly energy can be released
Physical size and type, balancing capacitance, durability, and response time
These considerations ensure the capacitor performs effectively in its chosen application, whether providing stable power, rapid pulses, or temporary backup energy.
FAQ
The dielectric increases the capacitor’s ability to store charge for a given potential difference by reducing the electric field between the plates. This increases capacitance, allowing more energy to be stored.
Different dielectric materials also vary in breakdown strength. A material with a higher breakdown voltage allows the capacitor to operate safely at higher potential differences, increasing the maximum energy it can store without failure.
Electrolytic capacitors offer much higher capacitance values in a compact form, making them suitable for storing larger amounts of energy.
Ceramic capacitors respond extremely quickly but usually have far smaller capacitances, so their energy-storage capability is limited. They are therefore used for fast high-frequency tasks rather than bulk energy storage.
The main limiting factor is the capacitor’s equivalent series resistance (ESR). A lower ESR allows faster current flow during discharge.
Other constraints include:
the resistance of the surrounding circuit
the maximum safe current the capacitor can deliver
heating effects during very rapid discharge
These factors determine whether the capacitor can provide a short, intense pulse or a slower release.
The amount of energy a capacitor stores increases with the square of the voltage across it. Operating at higher voltages produces a much larger energy store for a given capacitance.
Higher voltage ratings also improve reliability. They ensure the electric field inside the dielectric does not exceed its breakdown strength during normal operation or transient spikes.
This combination is useful when a system requires both long-term energy supply and rapid bursts of power. The battery provides sustained output, while the capacitor delivers instantaneous energy when demand spikes.
Examples include:
electronic devices that experience short but sharp current surges
automotive systems where acceleration needs sudden power boosts
memory backup circuits where the capacitor bridges brief voltage dips before the battery fully stabilises the supply
Practice Questions
Question 1 (2 marks)
State two practical uses of capacitors as energy stores in electrical or electronic systems.
Question 1 (2 marks)
Award 1 mark for each correct use, up to 2 marks.
Accept any two of:
Providing burst energy for a camera flash.
Smoothing voltage in a power supply.
Supplying brief backup power during short interruptions.
Delivering rapid pulses in timing or triggering circuits.
Storing energy in regenerative braking systems (e.g., supercapacitors in vehicles).
Question 2 (5 marks)
A capacitor is used as an energy store in the power supply of an electronic device.
(a) Explain how the capacitor stores and releases energy during operation of the device.
(b) Discuss why a capacitor, rather than a battery, is used for smoothing the output of a rectifier.
Your answer should refer to the behaviour of the capacitor during charging and discharging.
Question 2 (5 marks)
(a) Maximum 3 marks
1 mark: Energy stored in the capacitor as charge builds on the plates / an electric field forms in the dielectric.
1 mark: Capacitor releases energy by discharging into the circuit when required.
1 mark: During discharge, the stored electrical energy is delivered rapidly to the load.
(b) Maximum 2 marks
1 mark: Capacitors charge and discharge very quickly, unlike batteries which rely on slower chemical processes.
1 mark: Quick charge–discharge behaviour allows capacitors to smooth fluctuations in rectified voltage by storing energy at voltage peaks and supplying it between peaks.
