Practical skills Revision Notes

Setting up the experiment for speed and acceleration

To investigate the motion of a trolley down a ramp, you'll need a ramp, a trolley, a ruler or tape measure, and a stopwatch. Set up the ramp at a gentle incline. Mark several distinct distances along the ramp from the release point. The trolley should be released from rest at the top of the ramp. Crucially, the independent variable is usually the angle of the ramp or the distance travelled, while the dependent variable is the time taken or the speed/acceleration calculated. Control variables include the mass of the trolley, the condition of the ramp's surface, and the starting point on the ramp. Consistency in releasing the trolley is vital for reliable results.

Measuring distance and time

Distance is measured using a ruler or tape measure from the starting point to a marked finish line along the ramp. Time is measured using a stopwatch. A common method involves one person releasing the trolley and calling 'start' as it crosses the starting line, while another person starts the stopwatch. As the trolley crosses a predetermined finish line, the second person calls 'stop' and records the time. For better accuracy, several finish lines can be marked, and the time taken to reach each can be recorded. This allows for calculating average speed over multiple intervals. Repeating each measurement three to five times and calculating an average time for each distance helps reduce random errors and improve the reliability of the results.

Calculating speed and acceleration

Once distance and time measurements are collected, speed can be calculated using the formula: speed = distance / time. If you measure times for different distances, you can calculate the average speed for each segment. To determine acceleration, you can plot a graph of distance against time squared (d vs t²) or velocity against time (v vs t). For a trolley accelerating uniformly from rest, the relationship between distance and time is given by d = 0.5 * a * t², where 'a' is acceleration. If you plot distance against the square of time (t²), the graph should be a straight line through the origin, and its gradient will be half the acceleration (0.5a). A more advanced method uses light gates to measure instantaneous velocities at different points, allowing for a direct plot of velocity against time, where the gradient gives acceleration.

Evaluating results and sources of error

After collecting and calculating, evaluate your results. Look for anomalous data points that deviate significantly from the trend. These might indicate measurement errors and should be re-examined or disregarded if a clear reason for error is found. Common sources of error in this experiment include: reaction time errors when using a stopwatch (random error), parallax error when reading the ruler, friction between the trolley and the ramp/wheels, and air resistance. Systematic errors can arise from inaccurately calibrated measuring equipment or consistently releasing the trolley from slightly different positions. Repeating measurements and averaging helps reduce the impact of random errors, while systematic errors require careful setup and calibration.

Setting up the Circuit for I-V Characteristics

To investigate the current-potential difference (I-V) characteristics of a component, you need a circuit that allows you to vary the potential difference across it and measure both the potential difference and the current.

1. Connect the component (e.g., resistor, filament lamp, or diode) in series with an ammeter to measure the current flowing through it. The ammeter must always be placed in series.
2. Connect a voltmeter in parallel across the component to measure the potential difference across it. The voltmeter must always be placed in parallel.
3. Include a power supply (e.g., a battery pack or a variable power supply) and a variable resistor (rheostat) in series with the component. The variable resistor allows you to change the potential difference across the component and thus the current flowing through it.
4. Remember to identify safety precautions, such as handling electrical equipment carefully and ensuring connections are secure to prevent shorts.

Collecting Data and Plotting Graphs

Once the circuit is set up, you can collect your data.

1. Vary the resistance of the variable resistor to change the current and potential difference across the component. Start with a low potential difference and gradually increase it, taking readings at regular intervals.
2. For each setting, record the potential difference (V) from the voltmeter and the current (I) from the ammeter in a table. It is good practice to repeat measurements to calculate an average and identify anomalous results.
3. Plot a graph with potential difference (V) on the y-axis and current (I) on the x-axis. This is conventionally known as a V-I graph. Alternatively, current on the y-axis and potential difference on the x-axis (I-V graph) is also acceptable.
4. Draw a smooth curve or straight line of best fit through your plotted points. The shape of this graph reveals the component's characteristic.

Interpreting I-V Graphs for Different Components

The shape of the current-potential difference graph tells you how the resistance of a component changes.

• Ohmic Resistor: An ohmic resistor (at constant temperature) will show a straight line passing through the origin. This indicates that current is directly proportional to potential difference (V ∝ I), and its resistance is constant. It obeys Ohm's Law.
• Filament Lamp: The graph for a filament lamp is a curve that starts steep but becomes less steep as the potential difference and current increase. This means its resistance increases as it gets hotter. It does not obey Ohm's Law.
• Diode: A diode's graph typically shows very low current (or almost zero) when the potential difference is applied in one direction (reverse bias). In the opposite direction (forward bias), very little current flows until a certain 'threshold voltage' is reached, after which the current increases sharply with a small increase in potential difference. This indicates very high resistance in reverse bias and low resistance in forward bias above the threshold.

Variables and Evaluation (PAG P6)

For this practical, the independent variable is the potential difference (V) across the component. This is the variable you intentionally change using the variable resistor. The dependent variable is the current (I) flowing through the component, which you measure using the ammeter. Control variables include the temperature of the component (especially for ohmic resistors, as resistance changes with temperature), and ensuring the connections are identical for each component.

To evaluate the results, consider sources of error. For instance, ammeters and voltmeters have a certain precision, leading to reading uncertainty. Fluctuations in the power supply can affect readings. Anomalous results should be identified and investigated (e.g., by re-taking the measurement). The accuracy of the graph depends on the number of readings taken and how well the line of best fit is drawn. Percentage error could be calculated for resistance if comparing to a known value.

Investigating Force and Extension

To investigate the relationship between force and extension in a spring, suspend a spring from a retort stand. Measure the spring's initial length using a ruler. Add known masses (e.g., 100g at a time) to the spring. For each mass added, record the total mass and the new length of the spring. Calculate the force exerted by the masses using F = mg, where g is approximately 9.8 N/kg. Calculate the extension by subtracting the initial length from the current length. Repeat this process until the spring's extension is no longer proportional to the applied force, indicating the limit of proportionality has been exceeded.

Identifying the Limit of Proportionality

The limit of proportionality is the point beyond which the extension of a spring is no longer directly proportional to the applied force. If you plot a graph of force against extension, the limit of proportionality is the point where the graph deviates from a straight line passing through the origin. Before this limit, the spring obeys Hooke's Law, meaning force is directly proportional to extension. After this limit, the spring may be permanently deformed, meaning it will not return to its original length when the force is removed. Accurate data collection is crucial for correctly identifying this point.

Variables and Safety in the Experiment

In the force-extension experiment, the independent variable is the applied force (changed by adding masses), and the dependent variable is the extension of the spring (measured). Control variables include the type of spring used, the temperature of the room, and the method of hanging the masses. Safety precautions are important: wear eye protection to guard against snapping springs or falling weights. Ensure the retort stand is stable and clamp it to the bench if necessary. Use a padded catch tray below the masses to prevent injury or damage if they fall. Do not overload the spring beyond its elastic limit to prevent permanent damage.

Analysing Results and Graphing

After collecting data, plot a graph with force (in Newtons) on the y-axis and extension (in metres) on the x-axis. For the initial part of the loading, the graph should be a straight line passing through the origin, indicating direct proportionality (Hooke's Law). The gradient of this straight line is the spring constant. The point where the graph starts to curve downwards, showing that proportionally larger forces are needed for smaller extensions, or, more commonly, where the extension increases disproportionately for further increases in force, marks the limit of proportionality. Beyond this point, the spring is likely to be permanently deformed.

Understanding Specific Heat Capacity (c)

Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. It is measured in J/kg°C. Materials with high specific heat capacity, like water, require more energy to change their temperature. This property is important in many applications, from cooking to industrial processes. In this practical, we are aiming to measure this value for a given material by observing its temperature change when a known amount of electrical energy is supplied to it. The formula for energy change is ΔE = m × c × Δθ, where ΔE is the change in thermal energy, m is the mass, c is the specific heat capacity, and Δθ is the change in temperature.

Experimental Setup and Variables

To measure specific heat capacity using electrical heating, you typically use an immersion heater, a power supply, ammeter, voltmeter, thermometer, and a balance. For a liquid, it's placed in an insulated beaker; for a solid, a block with holes for the heater and thermometer is used. The independent variable is the electrical energy supplied, controlled by the power supply and time. The dependent variable is the temperature change of the material. Control variables include the mass of the material, the initial temperature, and ensuring consistent insulation to minimise heat loss. Measurements needed are: mass of material (m), initial temperature (θ₁), final temperature (θ₂), voltage (V), current (I), and time (t).

Methodology and Calculations

1. Measure the mass (m) of the material. 2. Set up the circuit with the immersion heater, ammeter (in series), and voltmeter (in parallel) connected to the power supply. 3. Place the heater and thermometer in the material. 4. Record the initial temperature (θ₁) of the material. 5. Turn on the power supply and record the voltage (V) and current (I) readings. 6. Start a timer. 7. After a suitable time (t), switch off the power supply and record the highest temperature reached (θ₂). 8. Calculate the temperature change Δθ = θ₂ - θ₁. 9. Calculate the electrical energy supplied: E = V x I x t. 10. Assume all electrical energy is converted to thermal energy in the material: E = m x c x Δθ. 11. Rearrange to find specific heat capacity: c = (V x I x t) / (m x Δθ).

Sources of Error and Improvements

The main source of inaccuracy is heat loss to the surroundings. This can be reduced by insulating the beaker/block with cotton wool or felt, using a lid, and reflecting surfaces. Other errors include inaccurate thermometer readings, an ammeter/voltmeter not quite precise, or inconsistencies in timing. Stirring the liquid ensures uniform temperature, reducing errors from uneven heating. Repeating the experiment and calculating an average helps improve the reliability of results. Using a data logger for temperature and time can improve precision and reduce human error in reading instruments.

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.