Key ideas

What you'll learn

How physicists use a few “big ideas” to explain many different situations.
Why models, fields, differences, proportionality and equations keep appearing across physics.
How to recognise these ideas in unfamiliar exam questions.
How to use them to reason, not just memorise facts.

Physics can look like lots of separate topics: forces, electricity, waves, radioactivity, energy and matter. But underneath, the same ideas appear again and again.

This map shows the main ideas that link the physics course together.

Concept map showing the key ideas in GCSE physics: models, cause and effect, fields, differences driving change, proportionality and mathematical laws

1. Models help us explain things we cannot see directly

A model is a simplified representation of something real. It helps you explain observations and make predictions.

Models are not perfect copies of reality. They are useful because they leave out detail so we can focus on the important pattern.

Definition

Model

A model is a simplified description, diagram or equation that represents a real system and helps us explain or predict what happens.

Examples you meet in GCSE physics include:

the particle model of matter, where solids, liquids and gases are described using tiny particles
the wave model, where light and sound are described using waves
circuit models, such as current flowing around a complete circuit

Particle model example

In the particle model, a gas is made of particles moving randomly. These particles collide with the walls of their container. Each collision produces a tiny force on the wall, and many collisions together cause pressure.

Example

Using the particle model to explain gas pressure

A sealed gas syringe is heated. Explain why the pressure of the gas increases.

The particle model says that gas particles move randomly and collide with the container walls.
Heating transfers energy to the gas particles, so their average kinetic energy increases.
Faster particles collide with the walls more often and with greater force.
Because pressure depends on the force of collisions over an area, the gas pressure increases.

Common Mistake

Treating models as the real thing

A model is not “the truth in miniature”. For example, particles in diagrams are drawn as neat circles, but real atoms and molecules are much more complex.

2. Cause and effect explains why changes happen

A cause is something that makes another thing happen. An effect is the result.

In physics, you often need to connect a cause to an effect using a law or model. For example:

a resultant force causes acceleration
a change in an atomic nucleus can cause radioactive emission
a potential difference can cause current in a circuit

Definition

Resultant force

The resultant force is the overall force on an object after all the forces acting on it have been combined.

If the resultant force on an object is not zero, the object accelerates. Acceleration means the rate of change of velocity.

Key Idea

Forces and motion

A non-zero resultant force causes acceleration. Bigger resultant force means bigger acceleration for the same mass.

Example

Calculating acceleration from resultant force

A trolley has a mass of 2.0 kg. A resultant force of 6.0 N acts on it. Calculate its acceleration.

Use Newton’s second law:

F = ma

Rearrange to make acceleration the subject:

\frac{F}{m}

Substitute the values with units:

\frac{6.0\ \text{N}}{2.0\ \text{kg}} = 3.0\ \text{m/s}^2

Tip

Cause before effect

In explanations, try to write the chain in order: cause → process → effect. For example: “A resultant force acts, so the object accelerates, so its velocity changes.”

3. Fields explain action at a distance

Some forces act even when objects are not touching. This is called action at a distance.

Examples include:

Earth attracting a falling object by gravity
a magnet attracting an iron nail
charged objects attracting or repelling each other

To explain this, physicists use the idea of a field.

Definition

Field

A field is a region where an object experiences a non-contact force. Gravitational, magnetic and electric fields are examples.

A field can be shown using field lines. The closer the field lines are together, the stronger the field.

Gravitational fields

A mass produces a gravitational field around it. Another mass placed in that field experiences a force. Near Earth’s surface, this force is called weight.

Weight is calculated using:

W = m g

where:

$W$ is weight in newtons (N)
$m$ is mass in kilograms (kg)
$g$ is gravitational field strength in newtons per kilogram (N/kg)

Example

Calculating weight in a gravitational field

A 60 kg student stands on Earth, where the gravitational field strength is 9.8 N/kg. Calculate the student’s weight.

Use the equation linking weight, mass and gravitational field strength:

W = m g

Substitute the values:

60\ \text{kg} \times 9.8\ \text{N/kg}

Calculate the weight:

588\ \text{N}

Common Mistake

Confusing mass and weight

Mass is the amount of matter in an object, measured in kilograms. Weight is a force caused by gravity, measured in newtons.

4. Differences drive change

Many physical changes happen because there is a difference between two places or objects.

A difference can create a “push” for change:

a temperature difference causes energy transfer by heating
a pressure difference can cause fluid movement
a potential difference causes charge to move in a circuit

Definition

Potential difference

A potential difference is the energy transferred per unit charge between two points in a circuit. It is measured in volts (V).

The direction of change is usually from higher to lower:

thermal energy transfers from hotter to colder objects
fluids move from higher pressure to lower pressure
conventional current flows from higher potential to lower potential in a complete circuit

Example

Predicting the direction of change

A metal spoon at 20 °C is placed in hot water at 80 °C. Explain the direction of energy transfer.

Compare the temperatures: the water is hotter than the spoon.
Apply the rule that energy transfers by heating from a hotter object to a colder object.
Energy transfers from the hot water to the cooler spoon until they reach the same temperature, if no energy is lost to the surroundings.

Key Idea

No difference, no drive

If there is no difference in temperature, pressure or potential, there is no overall driving effect for that change.

5. Proportionality shows simple relationships

Two variables are directly proportional if one increases by the same factor as the other.

For example, if doubling the mass doubles the weight, then weight is directly proportional to mass.

Definition

Directly proportional

Two quantities are directly proportional if their ratio stays constant. A graph of directly proportional quantities is a straight line through the origin.

The general form is:

y = k x

where $k$ is the constant of proportionality.

Examples include:

weight and mass: $W = m g$
force and extension of a spring, as long as the limit of proportionality is not exceeded: $F = k e$

Example

Testing proportionality from data

A spring has an extension of 0.040 m when a force of 2.0 N is applied, and an extension of 0.100 m when a force of 5.0 N is applied. Decide whether force is directly proportional to extension.

Calculate the ratio for the first result:

\frac{F}{e} = \frac{2.0\ \text{N}}{0.040\ \text{m}} = 50\ \text{N/m}

Calculate the ratio for the second result:

\frac{F}{e} = \frac{5.0\ \text{N}}{0.100\ \text{m}} = 50\ \text{N/m}

The ratio is the same both times, so force is directly proportional to extension for these results.

Common Mistake

Straight line does not always mean proportional

A straight-line graph only shows direct proportionality if it passes through the origin. A straight line that misses the origin shows a linear relationship, but not direct proportionality.

6. Physics laws are written mathematically

A physical law is a relationship that describes a pattern in nature. In GCSE physics, laws are often written as equations.

Equations are powerful because they let you:

calculate unknown quantities
predict what will happen if something changes
compare results using units
draw and interpret graphs

Definition

Physical law

A physical law is a mathematical relationship that describes how physical quantities are linked, based on repeated observations and evidence.

For example, Ohm’s law links potential difference, current and resistance:

V = I R

where:

$V$ is potential difference in volts (V)
$I$ is current in amperes (A)
$R$ is resistance in ohms (Ω)

Example

Using a mathematical law

A resistor has a resistance of 4.0 Ω and a current of 2.5 A flows through it. Calculate the potential difference across it.

Choose the equation linking potential difference, current and resistance:

V = I R

Substitute the values:

2.5\ \text{A} \times 4.0\ \Omega

Calculate the potential difference:

10\ \text{V}

Tip

Equations are models too

An equation is a compact model. It tells you which quantities matter, how they are linked, and what should happen if one quantity changes.

Bringing the ideas together

These key ideas are not separate topics. They overlap.

For example, a spring experiment uses several ideas at once:

the spring is modelled as obeying Hooke’s law
the applied force causes an extension
force and extension may be directly proportional
the relationship is written mathematically as $F = k e$
the evidence can be shown on a graph

That is why exam questions often feel “unfamiliar”: they may be testing whether you can spot the underlying key idea in a new context.

Exam technique

In the exam

Identify the big idea first: is the question about a model, a cause and effect chain, a field, a difference, proportionality, or an equation?
For explanation questions, write the sequence clearly: cause → physical process → final effect.
For calculations, write the equation, substitute with units, then calculate and check that the unit matches the quantity.

Self review

Check yourself

What is the difference between mass and weight, and how does a gravitational field link them?
How can you tell from a graph whether two quantities are directly proportional?
Give one example where a difference causes a physical change.

What you'll learn

How physicists use a few “big ideas” to explain many different situations.
Why models, fields, differences, proportionality and equations keep appearing across physics.
How to recognise these ideas in unfamiliar exam questions.
How to use them to reason, not just memorise facts.

Physics can look like lots of separate topics: forces, electricity, waves, radioactivity, energy and matter. But underneath, the same ideas appear again and again.

This map shows the main ideas that link the physics course together.

Concept map showing the key ideas in GCSE physics: models, cause and effect, fields, differences driving change, proportionality and mathematical laws

1. Models help us explain things we cannot see directly

A model is a simplified representation of something real. It helps you explain observations and make predictions.

Models are not perfect copies of reality. They are useful because they leave out detail so we can focus on the important pattern.

Definition

Model

A model is a simplified description, diagram or equation that represents a real system and helps us explain or predict what happens.

Examples you meet in GCSE physics include:

the particle model of matter, where solids, liquids and gases are described using tiny particles
the wave model, where light and sound are described using waves
circuit models, such as current flowing around a complete circuit

Particle model example

Example

Using the particle model to explain gas pressure

A sealed gas syringe is heated. Explain why the pressure of the gas increases.

The particle model says that gas particles move randomly and collide with the container walls.
Heating transfers energy to the gas particles, so their average kinetic energy increases.
Faster particles collide with the walls more often and with greater force.
Because pressure depends on the force of collisions over an area, the gas pressure increases.

Common Mistake

Treating models as the real thing

A model is not “the truth in miniature”. For example, particles in diagrams are drawn as neat circles, but real atoms and molecules are much more complex.

2. Cause and effect explains why changes happen

A cause is something that makes another thing happen. An effect is the result.

In physics, you often need to connect a cause to an effect using a law or model. For example:

a resultant force causes acceleration
a change in an atomic nucleus can cause radioactive emission
a potential difference can cause current in a circuit

Definition

Resultant force

The resultant force is the overall force on an object after all the forces acting on it have been combined.

If the resultant force on an object is not zero, the object accelerates. Acceleration means the rate of change of velocity.

Key Idea

Forces and motion

A non-zero resultant force causes acceleration. Bigger resultant force means bigger acceleration for the same mass.

Example

Calculating acceleration from resultant force

A trolley has a mass of 2.0 kg. A resultant force of 6.0 N acts on it. Calculate its acceleration.

Use Newton’s second law:

F = ma

Rearrange to make acceleration the subject:

\frac{F}{m}

Substitute the values with units:

\frac{6.0\ \text{N}}{2.0\ \text{kg}} = 3.0\ \text{m/s}^2

Tip

Cause before effect

In explanations, try to write the chain in order: cause → process → effect. For example: “A resultant force acts, so the object accelerates, so its velocity changes.”

3. Fields explain action at a distance

Some forces act even when objects are not touching. This is called action at a distance.

Examples include:

Earth attracting a falling object by gravity
a magnet attracting an iron nail
charged objects attracting or repelling each other

To explain this, physicists use the idea of a field.

Definition

Field

A field is a region where an object experiences a non-contact force. Gravitational, magnetic and electric fields are examples.

A field can be shown using field lines. The closer the field lines are together, the stronger the field.

Gravitational fields

A mass produces a gravitational field around it. Another mass placed in that field experiences a force. Near Earth’s surface, this force is called weight.

Weight is calculated using:

W = m g

where:

$W$ is weight in newtons (N)
$m$ is mass in kilograms (kg)
$g$ is gravitational field strength in newtons per kilogram (N/kg)

Example

Calculating weight in a gravitational field

A 60 kg student stands on Earth, where the gravitational field strength is 9.8 N/kg. Calculate the student’s weight.

Use the equation linking weight, mass and gravitational field strength:

W = m g

Substitute the values:

60\ \text{kg} \times 9.8\ \text{N/kg}

Calculate the weight:

588\ \text{N}

Common Mistake

Confusing mass and weight

Mass is the amount of matter in an object, measured in kilograms. Weight is a force caused by gravity, measured in newtons.

4. Differences drive change

Many physical changes happen because there is a difference between two places or objects.

A difference can create a “push” for change:

a temperature difference causes energy transfer by heating
a pressure difference can cause fluid movement
a potential difference causes charge to move in a circuit

Definition

Potential difference

A potential difference is the energy transferred per unit charge between two points in a circuit. It is measured in volts (V).

The direction of change is usually from higher to lower:

thermal energy transfers from hotter to colder objects
fluids move from higher pressure to lower pressure
conventional current flows from higher potential to lower potential in a complete circuit

Example

Predicting the direction of change

A metal spoon at 20 °C is placed in hot water at 80 °C. Explain the direction of energy transfer.

Compare the temperatures: the water is hotter than the spoon.
Apply the rule that energy transfers by heating from a hotter object to a colder object.
Energy transfers from the hot water to the cooler spoon until they reach the same temperature, if no energy is lost to the surroundings.

Key Idea

No difference, no drive

If there is no difference in temperature, pressure or potential, there is no overall driving effect for that change.

5. Proportionality shows simple relationships

Two variables are directly proportional if one increases by the same factor as the other.

For example, if doubling the mass doubles the weight, then weight is directly proportional to mass.

Definition

Directly proportional

Two quantities are directly proportional if their ratio stays constant. A graph of directly proportional quantities is a straight line through the origin.

The general form is:

y = k x

where $k$ is the constant of proportionality.

Examples include:

weight and mass: $W = m g$
force and extension of a spring, as long as the limit of proportionality is not exceeded: $F = k e$

Example

Testing proportionality from data

A spring has an extension of 0.040 m when a force of 2.0 N is applied, and an extension of 0.100 m when a force of 5.0 N is applied. Decide whether force is directly proportional to extension.

Calculate the ratio for the first result:

\frac{F}{e} = \frac{2.0\ \text{N}}{0.040\ \text{m}} = 50\ \text{N/m}

Calculate the ratio for the second result:

\frac{F}{e} = \frac{5.0\ \text{N}}{0.100\ \text{m}} = 50\ \text{N/m}

The ratio is the same both times, so force is directly proportional to extension for these results.

Common Mistake

Straight line does not always mean proportional

A straight-line graph only shows direct proportionality if it passes through the origin. A straight line that misses the origin shows a linear relationship, but not direct proportionality.

6. Physics laws are written mathematically

A physical law is a relationship that describes a pattern in nature. In GCSE physics, laws are often written as equations.

Equations are powerful because they let you:

calculate unknown quantities
predict what will happen if something changes
compare results using units
draw and interpret graphs

Definition

Physical law

A physical law is a mathematical relationship that describes how physical quantities are linked, based on repeated observations and evidence.

For example, Ohm’s law links potential difference, current and resistance:

V = I R

where:

$V$ is potential difference in volts (V)
$I$ is current in amperes (A)
$R$ is resistance in ohms (Ω)

Example

Using a mathematical law

A resistor has a resistance of 4.0 Ω and a current of 2.5 A flows through it. Calculate the potential difference across it.

Choose the equation linking potential difference, current and resistance:

V = I R

Substitute the values:

2.5\ \text{A} \times 4.0\ \Omega

Calculate the potential difference:

10\ \text{V}

Tip

Equations are models too

An equation is a compact model. It tells you which quantities matter, how they are linked, and what should happen if one quantity changes.

Bringing the ideas together

These key ideas are not separate topics. They overlap.

For example, a spring experiment uses several ideas at once:

the spring is modelled as obeying Hooke’s law
the applied force causes an extension
force and extension may be directly proportional
the relationship is written mathematically as $F = k e$
the evidence can be shown on a graph

That is why exam questions often feel “unfamiliar”: they may be testing whether you can spot the underlying key idea in a new context.

Exam technique

In the exam

Identify the big idea first: is the question about a model, a cause and effect chain, a field, a difference, proportionality, or an equation?
For explanation questions, write the sequence clearly: cause → physical process → final effect.
For calculations, write the equation, substitute with units, then calculate and check that the unit matches the quantity.

Self review

Check yourself

What is the difference between mass and weight, and how does a gravitational field link them?
How can you tell from a graph whether two quantities are directly proportional?
Give one example where a difference causes a physical change.

What you'll learn

1. Models help us explain things we cannot see directly

Particle model example

2. Cause and effect explains why changes happen

3. Fields explain action at a distance

Gravitational fields

4. Differences drive change

5. Proportionality shows simple relationships

6. Physics laws are written mathematically

Bringing the ideas together

Review Key ideas by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Key ideas

What you'll learn

1. Models help us explain things we cannot see directly

Particle model example

2. Cause and effect explains why changes happen

3. Fields explain action at a distance

Gravitational fields

4. Differences drive change

5. Proportionality shows simple relationships

6. Physics laws are written mathematically

Bringing the ideas together

Review Key ideas by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Key ideas

What you'll learn

1. Models help us explain things we cannot see directly

Particle model example

2. Cause and effect explains why changes happen

3. Fields explain action at a distance

Gravitational fields

4. Differences drive change

5. Proportionality shows simple relationships

6. Physics laws are written mathematically

Bringing the ideas together

Review Key ideas by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Key ideas Revision Guide

Key ideas

What you'll learn

1. Models help us explain things we cannot see directly

Particle model example

2. Cause and effect explains why changes happen

3. Fields explain action at a distance

Gravitational fields

4. Differences drive change

5. Proportionality shows simple relationships

6. Physics laws are written mathematically

Bringing the ideas together

Review Key ideas by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Key ideas Revision Guide