Study Notes
Overview
Power, in Physics, is a fundamental concept that quantifies how quickly energy is transferred or work is done. For your OCR GCSE exam, a precise understanding of this topic is essential as it forms a bridge between the principles of mechanics (forces and motion) and electricity (circuits). Examiners frequently test candidates' ability to not only define power accurately but also to select and apply the correct formula from a suite of options depending on the context. Typical questions range from simple one-step calculations to more demanding multi-step problems, often involving efficiency calculations, which are a key area for Higher Tier candidates. Mastering this topic is not just about memorising formulas; it's about developing the analytical skill to deconstruct a problem, identify the known variables, and execute a logical, clearly-communicated solution. This guide will equip you with the knowledge and exam technique to do just that.
Key Concepts
Concept 1: Power as the Rate of Energy Transfer
The single most important concept to grasp is the definition of power. It is not simply 'energy', but the rate at which energy is transferred. Examiners are looking for this specific wording. If you think of two identical cars, one with a 1.0-litre engine and one with a 3.0-litre engine, both can reach 70 mph. However, the car with the more powerful engine will reach that speed much faster. It is converting the chemical energy in its fuel into kinetic energy at a higher rate. This is the essence of power.
Example: A crane lifts a 1000 kg container 15 metres upwards in 30 seconds. Another crane does the same job in 1 minute. The first crane is more powerful because it does the same amount of work (transfers the same amount of gravitational potential energy) in less time.
Concept 2: Mechanical vs. Electrical Power
Power is a universal concept, but in your exam, it will appear in two main flavours: mechanical and electrical. It is crucial to recognise the context of the question to choose the right formula.
- Mechanical Power: This relates to forces and motion. When a force does work (e.g., a person running upstairs, a motor lifting a weight), the power is the work done divided by the time taken.
- Electrical Power: This relates to circuits. When a current flows through a component (e.g., a bulb, a resistor, a motor), it transfers energy. The power is related to the current, voltage, and resistance of the component.
Mathematical/Scientific Relationships
Fluency with the following formulas is non-negotiable. You must know which to use and when.
| Formula | Context | Tier | Status |
|---|---|---|---|
| P = W / t or P = E / t | Mechanical Power (Work Done / Energy Transferred over Time) | Both | Must memorise |
| P = I × V | Electrical Power (Current × Voltage) | Both | Given on formula sheet |
| P = I² × R | Electrical Power (Current² × Resistance) | Both | Given on formula sheet |
| P = V² / R | Electrical Power (Voltage² / Resistance) | Higher Only | Given on formula sheet |
| Efficiency = Useful Power Out / Total Power In | Efficiency Calculations | Both | Must memorise |
Symbol Meanings:
- P: Power, measured in Watts (W)
- W: Work Done, measured in Joules (J)
- E: Energy Transferred, measured in Joules (J)
- t: Time, measured in seconds (s)
- I: Current, measured in Amperes (A)
- V: Potential Difference (Voltage), measured in Volts (V)
- R: Resistance, measured in Ohms (Ω)
Required Practical: Investigating Power
While there isn't a single 'Power' required practical, skills are often tested through investigations into motors or circuits.
Example Investigation: Power of a Small Electric Motor
- Apparatus: Low voltage DC motor, set of masses (e.g., 100g each), metre ruler, stopwatch, retort stand, clamp, boss, thread, power supply, voltmeter, ammeter.
- Method:
- Set up the motor to lift a known mass (e.g., 200g = 0.2kg) by winding a thread around its spindle.
- Measure the height the mass will be lifted in metres (e.g., 0.5m).
- Connect the motor to the power supply with the ammeter in series and the voltmeter in parallel.
- Turn on the power supply and simultaneously start the stopwatch. Stop the watch when the mass reaches the target height.
- Record the time taken (t), the current (I), and the voltage (V).
- Calculate the input power to the motor: P_in = I × V.
- Calculate the useful output power: First, find the work done (Gravitational Potential Energy): E = m × g × h. Then, P_out = E / t.
- Calculate the efficiency of the motor: Efficiency = (P_out / P_in) × 100%.
- Common Errors: Parallax error in reading the ruler or meters; reaction time error with the stopwatch; assuming the motor is 100% efficient.
- Exam Application: Examiners can ask you to describe this method, analyse results from it, identify sources of error, or calculate the efficiency.