Classical Conditioning – Pavlov’s Dogs in the Modern World

The image of Pavlov’s dogs salivating at the sound of a bell is one of psychology’s most famous experiments — but classical conditioning isn’t just history. It remains a foundation for understanding human and animal behaviour today, shaping everything from advertising to education and even our digital habits.

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The Experiment That Started It All

In the early 1900s, Ivan Pavlov, a Russian physiologist, noticed that his dogs began to salivate not only when food was presented but also when they heard the footsteps of the assistant who fed them.

To study this formally, Pavlov paired a neutral stimulus (a bell) with an unconditioned stimulus (food). After several repetitions, the dogs began to salivate at the sound of the bell alone — demonstrating classical conditioning.

Key terms:

• Unconditioned stimulus (UCS): food

• Unconditioned response (UCR): salivation

• Conditioned stimulus (CS): bell

• Conditioned response (CR): salivation to the bell

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How It Applies Today

Classical conditioning still influences behaviour in many modern contexts:

• Advertising: Products are paired with positive imagery or music to evoke emotional responses.

• Education: Students may associate success or anxiety with particular environments, teachers, or subjects.

• Everyday life: Notifications and alerts on phones create conditioned responses — a sound or vibration can trigger anticipation or stress.

• Therapy: Techniques such as systematic desensitisation use conditioning to help individuals overcome phobias.

The principle remains the same — linking two stimuli so that one triggers a learned response to the other.

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Skills Highlight

• Understanding the principles of associative learning

• Applying psychological theory to real-world contexts

• Evaluating classical conditioning in contrast to operant conditioning

• Analysing modern examples of conditioned behaviour

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Why It Works in Teaching

Pavlov’s research is both historical and highly relatable. Students can easily recognise conditioned responses in daily life — from craving certain foods to checking their phones — helping them connect classic experiments to modern psychology.

 

Understanding Algorithms Through Flowcharts

Before writing a single line of code, good programmers learn to think logically — breaking problems down into clear, ordered steps. Flowcharts are one of the best ways to visualise this process. They turn abstract algorithms into simple, visual maps that show exactly how data moves and decisions are made.

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The Concept

An algorithm is a set of instructions that solves a problem or completes a task.
A flowchart represents that algorithm visually, using standard symbols to describe processes, decisions, and flow of control.

Common flowchart symbols:

• Oval: Start or End

• Parallelogram: Input or Output (data entry or display)

• Rectangle: Process (a calculation or action)

• Diamond: Decision (yes/no or true/false)

• Arrows: Direction of flow

Example problem:
Find the largest of two numbers.

The algorithm can be shown as a flowchart:

1. Start

2. Input numbers A and B

3. If A > B, output A

4. Else, output B

5. End

This visual approach makes logic clear even before coding begins.

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The Classroom Application

Students can draw flowcharts on paper or use software such as Lucidchart, draw.io, or Python’s Turtle/Flowgorithm. They then convert their flowcharts into real code — seeing how structured logic translates into Python’s if, while, and for statements.

Typical student tasks include:

• Calculating averages

• Testing divisibility

• Simulating decisions in games or apps

• Building sorting or counting routines

Flowcharts encourage stepwise refinement — simplifying complex problems into smaller, testable parts.

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Skills Highlight

• Understanding algorithm structure and logical sequence

• Translating flowcharts into executable code

• Debugging by tracing flow and decision paths

• Linking computing logic with mathematical reasoning

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Why It Works in Teaching

Flowcharts make computational thinking visible. Students can plan, predict, and debug before touching code, developing a stronger understanding of how algorithms control software, games, and devices in everyday life.

 

Rates of Reaction – The Effect of Concentration Using the PASCO Colourimeter

The rate of a chemical reaction depends on how frequently particles collide with enough energy to react. One of the easiest ways to explore this relationship is with the reaction between sodium thiosulfate and hydrochloric acid, which produces a cloudy sulfur precipitate. Using a PASCO colourimeter, students can measure this change precisely, transforming a classic “disappearing cross” experiment into a fully quantitative study of reaction kinetics.

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The Experiment

Reaction:

$$\text{Na}_2\text{S}_2\text{O}_3(aq) + 2\text{HCl}(aq) \rightarrow 2\text{NaCl}(aq) + \text{SO}_2(g) + \text{S}(s) + \text{H}_2\text{O}(l)$$

Traditional method:
Students mix sodium thiosulfate and hydrochloric acid, and time how long it takes for a printed cross beneath the flask to disappear as sulfur forms.

With the PASCO colourimeter:

• The colourimeter measures light transmission through the solution at regular intervals.

• As sulfur forms, the solution becomes cloudy, reducing the amount of transmitted light.

• The data are recorded automatically in PASCO Capstone, producing a transmission vs time graph.

By repeating the experiment with different thiosulfate concentrations, students can plot reaction rate against concentration and determine the order of reaction with respect to thiosulfate.

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The Science

As concentration increases, more particles occupy the same volume, leading to more frequent collisions and a faster rate of reaction.

The colourimeter allows students to calculate initial reaction rates objectively, avoiding human error and giving a clear quantitative link between concentration and rate.

$$\text{Rate} = k [\text{Na}_2\text{S}_2\text{O}_3]^n$$

The graph of rate versus concentration reveals whether the reaction is first, second, or zero order with respect to thiosulfate.

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Skills Highlight

• Using PASCO colourimeters for real-time quantitative data

• Measuring reaction rate from absorbance or transmission curves

• Controlling variables: temperature, volume, and acid concentration

• Analysing graphs to interpret reaction order and rate laws

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Why It Works in Teaching

The experiment connects a familiar reaction with advanced data analysis. Students see the transition from a qualitative observation to precise measurement and mathematical modelling — exactly the kind of scientific thinking needed at GCSE and A-Level.

 

Demonstrating and Visualising Electric Fields

Electric fields are often described in theory, but they can also be made visible in the lab. With a simple setup using castor oil, semolina, and electrodes, students can see how invisible forces act between charges — a striking and memorable demonstration of field lines in physics.

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The Experiment

Equipment:

• 90 mm petri dish

• Castor oil (enough to cover the base)

• A pinch of semolina grains

• Two metal electrodes (pins, rods, or plates)

• High-voltage DC power supply (around 1–5 kV, current-limited for safety). We find that a Wimshurst machine works best as we can turn the handle a few times to see the effect

Method:

1. Pour a thin layer of castor oil into the petri dish — this acts as an insulating medium that allows particles to move freely without conducting electricity directly.

2. Sprinkle a light dusting of semolina grains evenly over the surface.

3. Insert the two electrodes in the oil — start with parallel plates for a uniform field.

4. Apply voltage gently and observe the movement of semolina grains.

   • The grains align themselves along the electric field lines, forming a visual map of the field.

Repeat the demonstration with different configurations:

• Parallel plates: uniform straight lines.

• Point and plate: radial pattern showing divergence from a point charge.

• Two points: curved lines showing attraction or repulsion.

• Parallel wires: more complex patterns with symmetrical curves.

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The Science

The semolina grains become polarised in the electric field. One side becomes slightly positively charged, the other negatively charged, so each grain aligns with the direction of the field. The castor oil slows the motion, allowing the pattern to form clearly and remain stable.

This visualisation shows how electric field lines represent direction and strength:

• Lines are closer together where the field is stronger.

• Lines curve smoothly, never crossing.

• The pattern changes shape with each electrode configuration, matching textbook diagrams almost perfectly.

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Skills Highlight

• Constructing experimental setups safely for electrostatics

• Observing and recording field patterns qualitatively

• Linking visual evidence to theoretical field diagrams

• Understanding polarity, potential difference, and charge interaction

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Why It Works in Teaching

Seeing the invisible is always powerful in physics. The semolina-in-oil demonstration makes electric fields concrete, a vivid link between diagrams on the board and the real behaviour of charges in space. Students grasp the geometry and symmetry of electric fields far more effectively when they can see them form before their eyes.

 

Calculus in Context – Finding Maximum Profit

Calculus is often seen as an abstract mathematical tool, but in reality, it’s one of the most powerful methods for solving real business and economic problems. One of the clearest examples is finding maximum profit — where differentiation turns raw data into decision-making power.

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The Concept

Profit depends on revenue and cost:

Profit=Revenue−Cost

If revenue and cost each depend on the number of units sold ($x$), then profit is a function of $x$. The goal is to find the value of $x$ that gives the greatest profit.

By differentiating the profit function with respect to $x$, we can find where the slope = 0, meaning profit stops increasing — the maximum point.

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Example

Suppose a company’s profit function is:

$$P(x)=-2x^2+40x-100$$

Differentiate to find the turning point:

$$\frac{dP}{dx}=-4x+40$$

Set this to zero to find the maximum:

$$-4x+40=0\Rightarrow x=10$$

Substitute back into the original equation:

$$P(10) = -2(10)^2 + 40(10) - 100 = 100$$

So the maximum profit is £100, when 10 units are sold.

The second derivative, $\frac{d^2P}{dx^2} = -4$, is negative — confirming a maximum point.

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The Real-World Connection

This simple process mirrors how businesses use data:

• If sales grow too slowly, revenue won’t cover costs.

• If production expands too far, costs rise faster than income.

• The sweet spot — found through calculus — gives the best balance of output and efficiency.

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Skills Highlight

• Differentiating quadratic and polynomial functions

• Using the first and second derivatives to locate maxima and minima

• Interpreting results in economic and practical contexts

• Applying mathematical reasoning to real decision-making

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Why It Works in Teaching

Linking calculus to business and economics transforms it from pure theory into something purposeful. Students see that differentiation isn’t just about curves — it’s about optimisation, helping to make real-world decisions about efficiency, profit, and performance.

 

Conservation of Momentum in Two Dimensions

The law of conservation of momentum states that in a closed system, the total momentum before and after a collision remains the same, provided no external forces act. Most students first encounter this concept in one dimension, but momentum becomes much more interesting when collisions occur at angles. Using an air hockey table or an air puck system, students can observe momentum conservation in two dimensions and see the theory unfold frame by frame.

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The Experiment

An air hockey table or an air track with gliders provides a near-frictionless surface. Two pucks are set on a collision path, either head-on or at an angle. A top-down video camera records the collision.

Using software such as PASCO Capstone, Tracker, or Logger Pro, students can:

1. Track each puck’s motion before and after impact.

2. Draw velocity vectors showing direction and magnitude.

3. Split momentum into x- and y-components and calculate totals before and after the collision.

Total momentum before = Total momentum after

$$m_1u_{1x} + m_2u_{2x} = m_1v_{1x} + m_2v_{2x}$$ $$m_1u_{1y} + m_2u_{2y} = m_1v_{1y} + m_2v_{2y}$$

The results show that even when the pucks scatter in different directions, the total momentum in both axes remains constant.

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The Science

Collisions can be elastic (kinetic energy conserved) or inelastic (some energy lost as heat or deformation). However, momentum is always conserved.
The vector approach shows that momentum is not just about speed but direction — making it essential for understanding real-world physics such as vehicle collisions, snooker impacts, or atomic interactions.

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Skills Highlight

• Recording and analysing motion using video tracking

• Decomposing vectors into x- and y-components

• Verifying conservation laws experimentally

• Linking abstract vector mathematics to physical evidence

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Why It Works in Teaching

The combination of air hockey, sensors, and video analysis makes an abstract law tangible. Students can see how momentum balances in both directions, not through equations alone but through real motion, geometry, and evidence.

 

Investigating Photosynthetic Pigments with a Pasco Spectrometer

Photosynthesis depends on a range of pigments that capture light energy from different parts of the spectrum. While chlorophyll dominates, other pigments, such as carotenoids and xanthophylls, also contribute, extending the range of light that plants can use. Using a PASCO spectrometer and coloured filters, students can investigate how different wavelengths affect light absorption — and discover why plants aren’t simply “green.”

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The Experiment

Students set up a PASCO light sensor with a white light source and a series of coloured filters (red, blue, and green).
They:

1. Measure the light intensity passing through a pigment extract or leaf sample at each wavelength.

2. Record how much light is absorbed (low transmission) or reflected (high transmission).

3. Plot a spectral absorption graph, showing how pigment extracts respond to different colours of light.

Alternatively, a PASCO spectrometer can be used to collect continuous absorption data across the visible spectrum.

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The Science

Each pigment absorbs specific wavelengths of light due to the arrangement of its electrons.

• Chlorophyll a absorbs mainly red and blue light, reflecting green.

• Chlorophyll b, carotenoids, and xanthophylls absorb in slightly different regions, broadening the plant’s overall light-harvesting ability.

By comparing absorption and photosynthesis rates, students can link pigment properties to plant adaptation and efficiency in different environments.

Plants contain a variety of photosynthetic pigments, primarily

chlorophylls and carotenoids (which include carotenes and xanthophylls), that allow them to absorb a broader range of light wavelengths for photosynthesis. These different pigments can be separated and identified using chromatography.

Plants with Different Photosynthetic Pigments

While most green plants contain the same primary pigments, the relative abundance and specific types can vary, particularly across different plant and algal groups: 

Different plants to test could include:

• Green leaves (e.g., spinach, grass) for typical chlorophylls and carotenoids.
• Red or purple leaves (e.g., red cabbage, some Ficus benjamina cultivars) to observe anthocyanins (though these are not photosynthetic pigments, they co-exist).
• Brown algae (seaweed) contain chlorophyll c and fucoxanthin.
• Carrots or corn for high amounts of carotenes and xanthophylls, respectively.

Testing Photosynthetic Pigments 

The standard method for separating and identifying these pigments is chromatography (paper or thin-layer chromatography, TLC), often followed by spectrophotometry. 

Materials 

• Leaf samples (e.g., spinach, a red leaf variety)
• Pestle and mortar
• Acetone (organic solvent)
• Chromatography paper or TLC plate
• Chromatography solvent (e.g., a mixture of petroleum ether, acetone, and trichloromethane)
• Capillary tube
• Pencil and ruler
• Beaker or test tube with a cover
• 
  

Procedure (Thin-Layer Chromatography) 

Extract the pigments: Grind a piece of leaf tissue in a mortar and pestle with a small amount of acetone to break open the cells and dissolve the pigments.

1. Spot the plate: Draw a pencil line near the bottom of a TLC plate. Use a capillary tube to repeatedly spot the pigment extract onto the line, allowing each spot to dry before applying the next, to create a concentrated spot.
2. Develop the chromatogram: Place the plate in a beaker containing a shallow layer of chromatography solvent, ensuring the solvent level is below the pencil line. Seal the container to saturate the atmosphere with solvent vapour.
3. Separate the pigments: Allow the solvent (mobile phase) to move up the plate by capillary action. Different pigments travel at different speeds because they vary in size and solubility in the mobile phase compared to their affinity for the stationary phase (the plate material).
4. Analyse the results: Once the solvent has nearly reached the top, remove the plate and immediately mark the solvent front with a pencil. You will see colored spots (bands) at different heights.
5. Identify pigments:
   • Colour: Carotenes (orange) travel furthest, followed by xanthophylls (yellow), chlorophyll a (blue-green), and chlorophyll b (yellow-green).
   • Rf value: Calculate the retention factor (Rf) for each pigment using the formula:
     
     

     $R_f=\frac{\text{distance travelled by pigment}}{\text{distance travelled by solvent}}$
   • Compare the calculated Rf values to known standards for identification.
6. Further testing: The individual pigment bands can be scraped off the TLC plate, dissolved in a suitable solvent (e.g., alcohol), and analysed using a spectrophotometer to determine their specific light absorption spectrum. This confirms which wavelengths each pigment absorbs most effectively. 

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Skills Highlight

• Using spectrometers to measure light absorption.

• Plotting and interpreting graphs of intensity vs wavelength.

• Relating pigment chemistry to photosynthetic efficiency.

• Understanding experimental design and controlled variables.

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Why It Works in Teaching

This investigation turns colour into data. Students can see the relationship between wavelength, absorption, and plant adaptation — a clear, visual link between physics and biology that strengthens understanding of photosynthesis.