Practicals

Question 1

PAG 3.1 - Determining the resistivity of a metal

What is the method for determining the resistivity of a wire?

Answer 1

1. Determine the diameter of the metal wire by taking seevral measurements at different points across it using a screw gauge micrometer.
2. Set up the circuit: Connect a length of metal wire to a power supply in series with an ammeter and a variable resistor. Attach a voltmeter in parallel across the wire.
3. Adjust the power supply or potential divider so that the reading on the voltmeter is 3.0 V.
4. Note the reading on the ammeter. This must be kept constant throughout the experiment.
5. Record the reading on both the ammeter and voltmeter for a range of different lengths of the metal wire. (Remember, the ammeter reading should always be the same.)
6. Calculate resistance: Use Ohm’s Law R = V / I to calculate the resistance R of the wire for each pair of voltage V and current readings.
7. Plot a graph of resistance R against the length L of the wire. The gradient of the line of best fit gives 𝜌 / A (where 𝜌 is the resistivity and A is the cross-sectional area).
8. Determine resistivity: Multiply the gradient by the cross-sectional area A to find the resistivity 𝜌.

Question 2

PAG 3.1 - Determining the resistivity of a metal

What instrument is used to measure the cross-sectional area of a wire?

Answer 2

A micrometer

Question 3

PAG 3.1 - Determining the resistivity of a metal

Should the ammeter be attached in series or in parallel with the wire?

Answer 3

In series.

Question 4

PAG 3.1 - Determining the resistivity of a metal

Should the voltmeter be attached in series or in parallel with the wire?

Answer 4

In parallel.

Question 5

PAG 3.1 - Determining the resistivity of a metal

What are two key sources of error for this practical and how can they be minimized?

Answer 5

1. Measurement of the wire diameter: The diameter might not be uniform along the entire length of the wire, leading to inaccuracies in the calculated cross-sectional area. This can be minimized by taking multiple measurements along the wire and using the average.
2. Temperature effects: The resistance of the metal wire can change with temperature. To minimize this, allow the wire to cool between measurements by switching the power supply off.

Question 6

PAG 3.1 - Determining the resistivity of a metal

What is resistivity, and how does it differ from resistance?

Answer 6

Resistivity (𝜌) is a material property indicating how strongly a material opposes current flow, measured in Ωm. Resistance (R) depends on both the resistivity and the dimensions of the material, calculated by R = 𝜌 X (L / A).

Question 7

PAG 3.2 - Investigating electrical characteristics

What is the method for investigating electrical characteristics?

Answer 7

1. Set up the circuit: Connect the component (e.g., resistor, diode, or filament lamp) in series with an ammeter and a variable power supply. Connect a voltmeter in parallel across the component.
2. Measure and record: Vary the power supply to change the voltage across the component. For each voltage, record the corresponding current from the ammeter.
3. Plot graphs: Plot a graph of current (I) against voltage (V) for each component. Analyze the I-V characteristics to determine the behavior (e.g., ohmic, non-ohmic).

Question 8

PAG 3.2 - Investigating electrical characteristics

What is the purpose of investigating electrical characteristics?

Answer 8

The purpose is to determine the I-V characteristics of different electrical components (e.g., resistor, diode, filament lamp) to understand their behavior in circuits, such as identifying whether they are ohmic or non-ohmic conductors.

Question 9

PAG 3.2 - Investigating electrical characteristics

What are ohmic and non-ohmic conductors?

Answer 9

Ohmic conductors have a linear I-V relationship, meaning their resistance remains constant (e.g., resistors). Non-ohmic conductors have a non-linear I-V relationship, meaning their resistance changes with voltage or current (e.g., diodes, filament lamps).

Question 10

PAG 3.2 - Investigating electrical characteristics

What are the key sources of error in investigating electrical characteristics, and how can they be minimized?

Answer 10

1. Systematic errors: If instruments aren’t calibrated properly then each reading may be inaccirate by a particular amount. Make sure the instruments are properly calibrated beforer starting the prcatical.
2. Instrument accuracy: Use digital meters with fine resolution to reduce reading errors.
3. Heating effects: Allow components to cool between measurements to avoid resistance changes due to heating.

Question 11

PAG 3.3 - Determining a cell’s internal resistance

What is the method for determining the internal resistance of a cell?

Answer 11

1. Set up a circuit with a cell, ammeter and variable resistor in series, and a voltmeter in parallel.
2. Vary the resistance using the variable resistor and measure the voltage and current (take several readings).
3. Plot a V-I graph. V = -rI + ε corresponds to y=mx+c. The internal resistance = -m (-gradient).

Question 12

PAG 3.3 - Determining a cell’s internal resistance

What should you plot on the x and y axis of your V-I graph?

Answer 12

I on the Y-axis and V on the X-axis.

Question 13

PAG 3.3 - Determining a cell’s internal resistance

How can you find the e.m.f. from the graph?

Answer 13

From the y intercept.

Question 14

PAG 3.3 - Determining a cell’s internal resistance

How can ε = V + Ir be rearranged so that it corresponds to y=mx+c with a V-I graph?

Answer 14

ε = V + Ir
V = ε - Ir
V = -rI + ε
y = mx + c

Question 15

PAG 3.3 - Maximum Power of a Cell

What is the method for determining the maximum power of a cell?

Answer 15

1. Measure e.m.f.: With no load connected, measure the potential difference (e.m.f.) across the cell terminals using a voltmeter.
2. Add a load: Connect a lamp or variable resistor as a load. Record the potential difference across the cell and the current flowing through the circuit.
3. Vary the load: Change the load resistance and record the corresponding potential difference and current for several values.
4. Calculate power: Calculate the power delivered to the load using P=IV, where I is the current and V is the potential difference.
5. Plot graph: Plot a graph of power P against load resistance R to identify the resistance at which maximum power is delivered.
6. Determine maximum power: Find the peak value on the power vs. resistance graph to determine the maximum power available from the cell.

Question 16

PAG 3.3 - Maximum Power of a Cell

What is the maximum power theorem?

Answer 16

heoretical maximum power transfer occurs when the load resistance 𝑅 equals the internal resistance 𝑟 of the cell.

Question 17

PAG 3.3 - Maximum Power of a Cell

What is the difference between internal resistance and load resistance?

Answer 17

The load resistance (also known as external resistance) is the total resistance of the components in an external electric circuit. On the other hand, internal resistance is the resistance within the power source that resists current flow.

Question 18

PAG 3.3 - Maximum Power of a Cell

What is a key source of error in determining the maximum power of a cell, and how can it be minimized?

Answer 18

Temperature effects: Minimize heating of the cell and components to prevent changes in resistance. This may mean disconnecting the cell from the circuit and taking a short break in-between readings.

Question 19

PAG 4.1 - Investigating resistance

What different circuit component(s) can be used to measure the resistance of a component?

Answer 19

An ohmeter. You can also use an ammeter connected in series and a voltmeter connected in parallel and then use V=IR to find the resistance.

Question 20

PAG 4.3 - Potential divider circuits with non-ohmic devices

What’s the method to create a sensor with a potential divider circuits and a non-ohmic device?

Answer 20

1. Set up the potential divider: Connect the non-ohmic device (e.g., thermistor, LDR) in series with a fixed resistor R. This forms a potential divider circuit.
2. Connect the power supply: Attach a suitable power supply across the series combination of the non-ohmic device and the fixed resistor.
3. Measure the output voltage: Connect a voltmeter across the non-ohmic device to measure the output voltage V out. This output voltage changes with the resistance of the non-ohmic device.