Practical skills Revision Notes

Method for Investigating Spring Extension

To investigate spring behaviour, set up a clamp stand with a G-clamp and attach a spring. Measure the original length of the spring using a ruler or metre rule – this is crucial for calculating extension. Add known masses to the spring hanger, one at a time. After adding each mass, allow the spring to settle and then measure its new length. Record both the mass (and convert to force in Newtons, F = mg where g is approximately 9.8 N/kg or 10 N/kg for simplicity in GCSE) and the corresponding extended length. Calculate the extension by subtracting the original length from each measured length. Repeat measurements and calculate averages to improve reliability. Plot a graph of force (y-axis) against extension (x-axis).

Variables and Safety Considerations

In this investigation, the independent variable is the applied force, which you change by adding different masses. The dependent variable is the spring's extension, the measurement you record as a result of the changing force. Control variables include using the same spring throughout the experiment, maintaining a constant temperature (as temperature can affect spring properties), and ensuring the ruler is positioned consistently for each measurement. Safety is paramount: secure the clamp stand firmly to the bench using a G-clamp to prevent it from toppling. Ensure masses are added carefully to the spring hanger to avoid them falling and causing injury. Wearing eye protection is recommended in case a spring breaks or detaches.

Analysing Results and Hooke's Law

When you plot a graph of force against extension, for an elastic spring, you should observe a straight line passing through the origin, which demonstrates Hooke's Law. This law states that force is directly proportional to extension (F = kx), where 'k' is the spring constant. The gradient of this straight line represents the spring constant, indicating the stiffness of the spring. If the graph starts to curve or deviates from a straight line, it indicates that the spring has exceeded its elastic limit. Beyond this point, the spring will be permanently deformed and will not return to its original length when the load is removed. It is important to note the elastic limit to avoid damaging the spring during the experiment.

Calculating Force and Extension, and Identifying Limits

To calculate the force applied to the spring, use the formula F = mg, where 'm' is the mass in kilograms and 'g' is the gravitational field strength (approximately 9.8 N/kg on Earth). Always convert mass from grams to kilograms before calculation if necessary. The extension is calculated as the stretched length minus the original length. For example, if a spring is 10 cm unstretched and 12 cm when a mass is added, the extension is 2 cm. Identifying the elastic limit is crucial: it's the point beyond which the spring no longer obeys Hooke's Law and will not return to its original shape. This can be observed on the force-extension graph where the line deviates from linearity.

Circuit Construction and Measurement

When investigating circuit component characteristics, you will build a circuit to measure current and potential difference. An ammeter measures current and must always be connected in series with the component you are testing, so the current flows through it. A voltmeter measures potential difference (voltage) and must always be connected in parallel across the component, to measure the 'push' of charge across that component. A power supply provides the potential difference, and a variable resistor can be used to change the potential difference supplied to the component systematically. Ensure all connections are secure for accurate readings.

Identifying Variables

In these investigations, the independent variable is typically the potential difference (voltage) supplied to the circuit or across the component, which you control and change. The dependent variables are the current flowing through the component and the potential difference measured across it. It is crucial to keep control variables constant to ensure a fair test. For example, the temperature of the component should ideally be kept constant, as resistance can change with temperature. Using the same specific component for all readings is also a control variable.

Experimental Method (PAG P6)

1. Set up a series circuit containing a power supply, ammeter, and the component under investigation (e.g., a resistor). Add a variable resistor in series if you need to control the current/potential difference.
2. Connect a voltmeter in parallel across the component whose characteristics you are investigating.
3. Record the ammeter and voltmeter readings.
4. Adjust the variable resistor (or change the number of cells in the power supply) to change the potential difference across the component.
5. Take several pairs of readings (current and potential difference) across a range of values.
6. Repeat the experiment and calculate average readings to improve reliability.
7. Plot a graph of current (I) against potential difference (V) or P.D. (V) against Current (I) to show the component's characteristic.

Safety Precautions and Data Analysis

Always use low voltages to avoid overheating the components or causing electric shock. Ensure circuits are correctly wired before switching on the power supply to prevent short circuits. Check for loose connections. When analysing data, plot a graph of current (y-axis) against potential difference (x-axis). Look for patterns: for an ohmic resistor, the graph is a straight line through the origin (resistance is constant). For a filament lamp, the graph curves as resistance increases with temperature. For a diode, current flows in one direction only above a certain threshold potential difference.

Measuring Wave Speed, Frequency and Wavelength

To measure the speed, frequency, and wavelength of waves, a ripple tank is a common apparatus for water waves. For wave speed, measure the distance a wave crest travels (d) and the time taken (t), then calculate v = d/t. To find frequency, count the number of waves (N) passing a point in a given time (T), then f = N/T. Wavelength can be measured by placing a ruler across several wave crests and dividing the total distance by the number of wavelengths. An average across multiple waves reduces measurement uncertainty. The wave equation links these: v = f × λ. For sound waves, microphones and data loggers can measure frequency, and the speed can be determined by measuring the time taken for a sound to travel a known distance.

Investigating Reflection

Reflection can be investigated using a ripple tank for water waves or a ray box for light. In a ripple tank, a barrier is placed in the path of straight waves. Observe how the waves bounce off the barrier, noting that the angle of incidence equals the angle of reflection. For light, a ray box projects a narrow beam of light onto a plane mirror. Draw the incident and reflected rays, and the normal (a line perpendicular to the mirror surface where the ray hits). Measure the angles of incidence and reflection using a protractor. Ensure the room is darkened for clear observation of light rays.

Investigating Refraction

Refraction can also be studied in a ripple tank or with a ray box. In a ripple tank, place a piece of glass or plastic (a 'refractor') in the tank to create a shallower region. As straight waves pass from the deep (faster) to shallow (slower) water, their wavelength decreases and their direction changes if they hit the boundary at an angle. For light, use a ray box to shine a light ray through a transparent block made of glass or plastic. Observe the light bending as it enters and exits the block. Draw the path of the ray, the normal lines at each surface, and measure the angles of incidence and refraction using a protractor. The change in speed causes the change in direction.

Safety and Experimental Setup Considerations

When working with a ripple tank, ensure electrical equipment (like the vibrator motor and lamp) is safely positioned away from water spills to prevent electric shock. Keep water levels consistent for reliable results. For ray box experiments, avoid looking directly into the light source. All measurements should be repeated multiple times and averaged to reduce random errors. When drawing diagrams for reflection and refraction, always include the normal line as a crucial reference point for measuring angles accurately. Ensure protractors are used carefully to minimize measurement uncertainty in angles.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Method for Measuring Speed

To measure the average speed of a trolley, set up a ramp at a fixed angle. Mark a starting point and several distances along the ramp (e.g., 20 cm, 40 cm, 60 cm). Release the trolley from the starting point and use a stopwatch to measure the time it takes to reach each marked distance. Alternatively, use two light gates positioned a known distance apart; the data logger connected to the light gates will record the time taken for the trolley to pass between them. Average speed is calculated by dividing the total distance by the total time. Repeat measurements for each distance and calculate an average time to improve reliability.

Method for Measuring Acceleration

To measure acceleration, you need to know the change in speed over a period of time. Using light gates is particularly effective here. Set up two light gates a known distance apart. The first light gate measures the initial speed of the trolley as it passes through it (distance covered by trolley / time to pass through gate). The second light gate measures the final speed. The data logger records the time interval between the two light gates. Acceleration can then be calculated using the formula: acceleration = (final speed - initial speed) / time taken between gates. Ensure the ramp angle remains constant and the trolley is released from the same point each time.

Variables and Controls

The independent variable is the distance travelled by the trolley down the ramp. The dependent variables are the time taken and consequently, the calculated speed and acceleration. Key control variables include the angle of the ramp (use a protractor to set and maintain it), the mass of the trolley (do not add or remove weights during the experiment), and the starting point on the ramp. Releasing the trolley in the same way (e.g., from rest) each time is also important. Keeping these variables constant ensures that any changes observed in speed or acceleration are due to the changing distance and not other factors.

Data Analysis and Evaluation

After collecting distance and time data, plot a distance-time graph. For an accelerating trolley, this will be a curve, indicating increasing speed. The gradient of the tangent to the curve at any point gives the instantaneous speed. To find acceleration, you might plot a speed-time graph; the gradient of this graph represents acceleration. Evaluate results by considering sources of error, such as reaction time when using a stopwatch, friction in the trolley wheels, and inaccuracies in measuring distances. Repeating measurements helps reduce random errors. Anomalous results should be identified and re-measured if possible, or excluded from averages if clearly incorrect.