Power & Efficiency (OCR GCSE Physics A (Gateway)): Flashcards

False.

A material with high thermal conductivity heats up faster because energy is transferred more rapidly by conduction through it.

Fibreglass has a much lower thermal conductivity than the roof material, so it slows the rate of energy transfer by conduction. The air trapped between the glass fibres also acts as an insulator. Using several thick layers further reduces the rate of cooling and energy loss from the building.

In thermal conduction, energy is always transferred from hot to cold regions.

Thermal conductivity is a property of a material that describes how readily it transfers energy by conduction. A higher thermal conductivity means a higher rate of energy transfer across the material.

True.

Cavity wall insulation is made from blown mineral fibre filled with gas, which has low thermal conductivity. This lowers the rate of conduction from the warm interior through the walls to the outside.

Vibrating particles in a hot region have more energy in their kinetic store. They collide with neighbouring particles, transferring energy to those particles' kinetic stores. This process continues particle by particle, carrying energy through the material from hot to cold without the particles themselves moving from place to place.

The aim is to investigate the effectiveness of different materials as thermal insulators and to explore factors that may affect a material's thermal insulation properties, such as material type and thickness.

True.

The best insulator slows the rate of energy loss most effectively, so the water cools more slowly. On a temperature–time graph, this produces the shallowest (least steep) curve.

At the start, the water is at a high temperature, creating a large temperature difference between the water and the room. A greater temperature difference drives a higher rate of energy transfer, so the water cools quickly. As the temperature falls and the difference decreases, the rate of cooling slows.

First, ensure the starting temperature of the water is identical for each material, as water cools rapidly at high temperatures and inconsistency would affect results. Second, wrap insulation around the sides of the beaker as well as the top, because heat escaping through the sides is a systematic error in the basic setup.

False.

Parallax error is a random error. It occurs inconsistently depending on the angle of viewing. It can be reduced by reading the thermometer at eye level every time.

In the investigating insulation experiment, temperature should be recorded every 2 minutes for 20 minutes, or until the water reaches room temperature.

Plot a graph of temperature against time and draw a curve of best fit for each material on the same axes. The curve that takes the longest to fall (the shallowest) represents the best insulator. The curves typically fall steeply at first then level off as the water approaches room temperature.

Efficiency is the ratio of useful energy output from a system to its total energy input. It measures how much of the energy transferred is used for the intended purpose, with the remainder being wasted.

False.

Efficiency has no units. It is a ratio of energy output to energy input (or power output to power input), and the units cancel. It can be expressed as a decimal between 0 and 1, or as a percentage between 0 and 100%.

Useful output energy = 300 − 85 = 215 J

Efficiency = 215 / 300 × 100% = 71.7% (approximately 72%)

The motor transfers about 72% of the input energy usefully, with 28% wasted.

Power is the rate of energy transfer (energy per unit time). Since both useful output and total input are divided by the same time interval, the time cancels and the ratio of useful power output to total power input gives the same result as the energy ratio. This makes the power equation useful when energy values are not directly given.

True.

An efficiency of 0.85 means 85% of the input energy is transferred usefully. The remaining 15% is wasted (dissipated to the surroundings). Efficiency as a decimal plus wasted fraction always sums to 1.

Efficiency can be expressed as a decimal (between 0 and 1) or as a percentage (between 0 and 100%).

A system with low efficiency transfers most of its input energy in ways that are not useful — the energy is wasted, typically dissipated to the surroundings as heat, sound or light. Only a small fraction of the total input energy reaches the intended output store.

The efficiency of a device is improved by reducing wasted energy transfers.

True.

Lubrication reduces friction between moving parts, which reduces unwanted energy transfers by heating to the surroundings.

Friction causes unwanted energy transfers by heating to the machinery and surroundings. This means less energy is available for the useful output of the device, reducing its efficiency.

Electrical resistance in wires causes unwanted energy transfers by heating to the wires and surroundings.

Streamlining is the process of shaping a moving object to reduce air resistance, which decreases unwanted energy transfers by heating to the object and surroundings.

True.

Reducing the current lowers the energy wasted by heating in the wires, improving the overall efficiency of the circuit.

Friction can be reduced by adding bearings to prevent components from directly rubbing together, or by lubricating the moving parts.

Complete the table showing sources of wasted energy and one method to reduce each.

A streamlined posture reduces air resistance, which reduces the unwanted energy transferred by heating to the surroundings. More of the cyclist's energy input goes to useful kinetic energy, improving efficiency.

A useful energy transfer is one that moves energy to the intended store, such as the thermal store of water in a kettle being heated by the heating element.

True.

Both transfer 3600 kJ. The amount of energy transferred depends on both the power of the appliance and the time it is switched on.

The useful transfer is energy moving to the thermal store of the surrounding air by heating. The wasted transfer is energy radiated as light (infrared/visible radiation) to the surroundings, which does not directly heat the room.

The amount of energy an appliance transfers depends on how long it is switched on for and the power of the appliance.

Dissipated energy is energy that is spread out into the surroundings in a less useful form, such as wasted heat or sound, and cannot easily be recovered.

False.

Most of the energy is transferred to the thermal store of the surroundings by heating. Only a small fraction is transferred as visible light, making filament bulbs inefficient.

A washing machine contains an electric motor that converts electrical energy from the mains into kinetic energy. The motor rotates the drum to wash clothes, making kinetic energy the useful output.

Complete the table for energy transfers in a kettle.

The electric motor transfers energy from the mains to the kinetic store of the motor, which compresses a refrigerant chemical into a liquid. This process removes thermal energy from inside the refrigerator, keeping its contents cold.

The power rating of an appliance is the energy transferred per unit time, measured in watts (W). A higher power rating means more energy is transferred each second.

False.

A 2000 W kettle transfers 2000 J per second, not per minute. Power is energy transferred per unit time, and the unit is watts (W = J/s).

The label shows the potential difference required (e.g. 230 V in the UK), the frequency of the supply (e.g. 50 Hz), and the power rating in watts.

The higher the power rating of an appliance, the more energy it transfers per second.

The watt is the unit of power. One watt means one joule of energy is transferred per second (1 W = 1 J/s).

True.

If the voltage is much higher than the rated value, the appliance may fuse, overheat, or catch fire, causing permanent damage.

Different countries use different mains voltages. Plugging an appliance into a supply with a higher voltage than its rated value can cause it to overheat, fuse, or catch fire, making it dangerous and likely to be permanently damaged.

Complete the table with the approximate power rating for each appliance.

An appliance with a higher power rating transfers more energy per second. So for the same time switched on, a higher-rated appliance transfers more total energy than a lower-rated one.