Physics: Reflection and Refraction, Work, Power and Efficiency and The National Grid

Describe the law of reflection and how to measure angles.

The angle of incidence equals the angle of reflection. Both must be measured in degrees from the normal line (90 degrees to the surface).

Explain ‘what happens’ to light as it enters a more dense medium like glass.

The light slows down and bends towards the normal. The wavelength decreases but the frequency stays the same.

Explain refraction using wavefronts.

• One side of the wavefront hits the boundary and slows down before the other side.

• This difference in speed causes the wave to change direction.

State the equation for Work Done and the unit check for distance.

Work Done = Force x Distance. Unit Check: Distance must be in metres (m). If given cm, divide by 100.

Units for Efficiency.

Efficiency has no units, it is a decimal or percentage.

Why can efficiency never be 100 percent or greater?

Some energy is always dissipated or wasted as heat to the surroundings, usually due to friction.

Why is electricity transmitted at very high voltages?

High voltage allows for a low current. Low current reduces heating in the wires, which prevents energy loss and increases efficiency.

What is the function of a step-up transformer?

It increases the voltage and decreases the current for efficient long-distance transmission.

What is the function of a step-down transformer?

It decreases the voltage to a safe level of 230V for use in homes and factories.

What are the units for Power, Current, and Resistance in the power loss equation?

Power is in Watts (W), Current is in Amps (A), and Resistance is in Ohms. Remember: Power loss = Current squared x Resistance.

How do you find the Total Input Energy if the question gives you several different energy values?

Total Input is the sum of ALL energy outputs (Useful + Wasted) or the specific energy put into the device. Useful is always the specific energy the device is designed for.

What must you do if the efficiency is given as a percentage (e.g. 40%) before using it in a calculation?

Divide it by 100 to turn it into a decimal (e.g. 0.4) before putting it into the equation.

Why is the current kept very low in National Grid transmission cables?

• To reduce power loss.

• Power loss is proportional to the square of the current.

• A low current means significantly less energy is dissipated as heat, making transmission more efficient.

Explain the relationship between current and power loss in a wire.

• If the current is doubled, the power loss increases by four times because current is squared in the power loss equation.

• This is why reducing current is the best way to stop energy being wasted as heat.

What should you do if the exam gives you the angle between the light ray and the mirror surface?

Subtract that angle from 90 to find the correct angle measured from the normal.

The energy turns into heat energy. Improve this sentence

The energy is dissipated into thermal energy.

When should you use the term friction to explain energy loss?

Use friction for mechanical systems with moving parts, like a motor or gears

When should you not use the terms friction to explain energy loss?

Electrical systems, like a lightbulb or transmission wires. Use resistance instead.