Core Concepts in Physics: Energy, Forces, and Circuits
Energy and Work Done
Defining Work, Power, and Energy
- Work Done: Energy transferred when a force moves an object over a distance.
- Power: The rate at which energy is transferred (how quickly).
- Energy Stores: Kinetic, gravitational potential, elastic potential, thermal, chemical, and nuclear.
- Energy Transfers: Occur via mechanical work, electrical work, heating, and radiation.
Efficiency in Energy Transfer
The higher the efficiency, the less energy is wasted (often as heat or sound).
Note: No machine is 100% efficient (except ideal transformers, theoretically).
Practical: Measuring Power Output
Climbing Stairs Experiment
To measure power output while climbing stairs, you must:
- Measure weight (mass × gravitational field strength).
- Measure the total height of the stairs.
- Measure the time taken.
Calculate work done and then power. Control variables include using the same stairs, maintaining the same height, and repeating trials for reliability.
Forces, Moments, and Pressure
Defining Forces and Resultant Force
A force is a push or pull that can change an object’s speed, direction, or shape.
The Resultant Force is the overall force acting on an object, accounting for the direction of all individual forces.
Moments and Levers
A Moment is the turning effect of a force about a pivot.
- A larger force or a greater distance from the pivot results in a larger moment.
- Levers reduce the force needed by increasing the distance from the pivot point.
Pressure in Fluids and Upthrust
- Pressure in liquids increases with depth.
- Pressure depends on the fluid density and gravitational field strength.
- Upthrust: The upward force exerted by a fluid on a submerged object.
- An object floats if the upthrust equals its weight.
Practical: Investigating Moments and Levers
Use a metre rule, weights, and a pivot. Balance the rule using weights placed at different distances. This experiment verifies the Principle of Moments: the total clockwise moment equals the total anticlockwise moment.
Electricity and Circuit Analysis
Fundamental Electrical Terms
- Current: The flow of electric charge, measured in amperes (A).
- Potential Difference (Voltage): The force that pushes current around a circuit.
- Resistance: The property that opposes current flow. Higher resistance results in lower current for the same voltage.
Series vs. Parallel Circuits
Series Circuits
- Only one path for current flow.
- Current is the same everywhere.
- Voltage is shared across components.
Parallel Circuits
- Multiple paths for current flow.
- Voltage is the same across each branch.
- Current splits between the branches.
Mains Electricity and Safety
UK mains electricity is 230 V AC (alternating current) at 50 Hz. Batteries provide DC (direct current).
Standard Plug Wiring
- Live Wire (Brown): Carries the high voltage.
- Neutral Wire (Blue): Completes the circuit.
- Earth Wire (Green/Yellow): Safety feature that prevents electric shock.
Fuses and circuit breakers are essential safety devices that cut off the current if a fault occurs.
Practical: Investigating Resistance
Use voltmeters and ammeters to measure current and voltage in both series and parallel configurations. Vary components (resistors, bulbs) and record measurements to analyze resistance behavior.
Static Electricity
Charge Build-up and Interaction
Static charge builds up when electrons are transferred by friction (e.g., rubbing a balloon on hair).
- Like charges repel; opposite charges attract.
- Insulators hold charge; conductors allow electrons to move freely.
Static Electricity Hazards
Sparks caused by static discharge can ignite flammable gases or damage sensitive electronics. Planes and fuel pipes must be earthed to prevent dangerous electrostatic discharges.
Applications of Static Charge
- Paint Sprayers: Charged paint droplets repel each other, ensuring the paint spreads evenly across the surface.
- Photocopiers: Static charge is used to attract toner particles precisely to the required areas on the paper.
Practical: Electrostatic Attraction
Rub rods with different cloths and bring them near each other or small pieces of paper. Observe the resulting attraction, repulsion, and discharge effects.
Magnetism and the Motor Effect
Fundamentals of Magnetism
- Magnetic field lines travel from the North pole to the South pole.
- The magnetic field is strongest at the poles.
- Electromagnets: Solenoids (coils of wire) carrying a current generate a magnetic field.
- To increase the strength of an electromagnet: use more coils, increase the current, or add an iron core.
The Motor Effect
A current-carrying wire placed within a magnetic field experiences a force.
Fleming’s Left-Hand Rule determines the direction of this force (this rule applies specifically when the wire is at a right angle to the field).
Practical: Force on a Current-Carrying Wire
Set up a wire within a magnetic field and pass an electric current through it. Observe the resulting movement (the wire “jumps”). Change the current or the magnetic field strength to analyze the resulting change in force.
Electromagnetic Induction and Power Transmission
The Generator Effect
Moving a magnet inside a coil (or moving a coil within a magnetic field) induces a voltage across the coil. Faster motion results in a larger induced voltage. This principle is the basis of how electrical generators operate.
Transformers and Voltage Control
Transformers are used to change the voltage within the National Grid. They consist of coils wrapped around an iron core.
- Step-Up Transformers: Increase voltage (used for efficient power transmission).
- Step-Down Transformers: Decrease voltage (used for safe home and industrial use).
The National Grid
The National Grid transmits power at high voltage, which results in a low current. This minimizes energy loss as heat in the cables, making power transmission highly efficient. Transformers are crucial for this process.
Practical: Generator Demonstration
Spin a coil between magnets and observe the induced current using a voltmeter. Vary the speed of rotation or the number of coils to test how these factors affect the induced voltage.
The Particle Model of Matter
States of Matter
- Solids: Particles vibrate around fixed positions.
- Liquids: Particles move freely but remain in contact.
- Gases: Particles move quickly and randomly, with large separation.
Internal Energy and Changes of State
Heating a substance increases its internal energy (the sum of kinetic and potential energy).
Temperature rises unless a change of state occurs. During a change of state (e.g., melting or boiling), the temperature remains constant because the energy input is used to break intermolecular bonds instead of increasing particle kinetic energy.
Cooling involves energy loss, causing the temperature to drop or freezing to occur.
Gas Behavior and Pressure
- Higher temperature means faster particles, resulting in higher pressure.
- Reducing the volume of a gas (while keeping the temperature constant) increases the pressure.
Practical: Heating Curve Analysis
Measure the temperature of a substance while heating it from solid to liquid and then to gas (melting and boiling). The flat sections observed on the heating curve indicate that a change of state is taking place.
Density, Pressure, and Atmospheric Effects
Defining Density
Density is the amount of mass contained within a given volume. A more compact substance has a higher density.
Pressure in Fluids
Pressure in fluids increases with both depth and fluid density. This explains why structures like submarines must be designed to withstand extremely high pressures.
Floating and Sinking
- An object floats if its weight is equal to the upthrust exerted by the fluid.
- If an object’s density is greater than the liquid’s density, the object sinks.
Atmospheric Pressure
Air pressure decreases with altitude because there are fewer air particles above you pressing down. This principle is utilized in instruments like barometers and altimeters.
Practical: Determining Density of an Irregular Object
Use the displacement method (water displacement) to accurately find the object’s volume. Calculate density using the measured mass and volume (Density = Mass / Volume). Finally, compare the floating behavior of different materials.