Required practicals electricity topic

Resistance of the Length of a Wire at a Constant Temperature

Variables:

Independent variable = Length of resistance wire, L . Dependent variable = Resistance, R . Control variables: Potential difference of the power supply, Temperature of the wire.

Method:

1. Set up the apparatus by connecting two crocodile clips to the thin resistance wire a distance of 10 cm apart and setting the power supply to 1.5 V

2. Connect the wire, using the clips, to the rest of the circuit

3. Record the potential difference from the voltmeter and current from the ammeter

4. Move the clips in 10 cm intervals further apart

5. Take new measurements from the voltmeter and ammeter for each length reading

6. Continue until the crocodile clips are a length of 1 m apart.

7. Calculate the resistance of each length of wire using the equation:

   Resistance = P.d ÷ current

8. Plot a graph of resistance (on the y-axis) against length (on the x-axis) and draw a line of best fit. The graph should be a straight line through the origin with a positive correlation

This means that the longer the piece of wire, the higher the resistance - DIRECTLY PROPORTIONAL

Combinations of Resistors in Series and Parallel

The aim of this experiment is to investigate how combinations of resistors in series and parallel affect the total resistance in electrical circuits

Independent variable = Number of resistors. Dependent variable = Total resistance, R. Control variables: Potential difference of the power supply, Temperature of the resistors.

Method:

1. Connect the circuit shown in figure 1 with a battery of 4 V, first with one resistor (R1) with the voltmeter connected in parallel and ammeter in series

2. Close the switch and record the reading on the voltmeter and ammeter

3. Repeat steps 1 and 2 for just the second resistor (R2)

4. Open the switch and add connect both R1 and R2 in series as shown in figure 2, connecting the voltmeter in parallel to both resistors

5. Close the switch and record the new readings on the voltmeter and ammeter

6. Open the switch and arrange R1 and R2 now in parallel shown in figure 3

7. Close the switch and record the readings on the voltmeter and ammeter

8. Use the equation for resistance to calculate it.

9. The results should show that in series:

The resistance of the combined resistors is equal to the sum of the two individual resistances. This is because the electrons flow through just one path through both resistors, so the current does too

The results should show that in parallel: The resistance of the combined resistors is less than the sum of the two individual resistances. This is because the electrons are split between the different paths (or 'loops') but the resistors still have the same potential difference across them

Evaluating the resistance experiments for safety/errors :

-The first crocodile clip (connected to the circuit, not the wire) must start at 0 on the ruler

Otherwise, this could cause a zero error in your measurements of the length

-Both the ammeter and voltmeter should be checked to start from 0

-Only allow small currents to flow through the wire

This keeps the temperature of the wire constant, so it doesn't change its resistance

Required Practical 4: Investigating I–V Characteristics

The aim of the experiment is to use circuit diagrams to construct appropriate circuits to investigate the I–V characteristics of a variety of circuit elements (These include a fixed resistor at a constant temperature, a lamp and diode)

Variables:

Independent variable = Potential difference, V. Dependent variable = Current, I. Control variables: Potential difference of the power supply, Use of the same equipment eg. wires, diodes

Method:

1. Set up the circuit with the fixed resistor

2. Vary the voltage across the component by changing the resistance of the variable resistor, using a wide range of voltages (between 8-10 readings). Check the appropriate voltage reading on the voltmeter

3. For each voltage, record the value of the current from the ammeter 3 times and calculate the average current

4. Increase the voltage further in steps of 0.5 V and repeat steps 2 and 3

5. Make sure to switch off the circuit in between readings to prevent heating of the component and wires

6. Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current)

7. Replace the fixed resistor with the filament lamp, then the diode, repeating the experiment for each.

8. Plot a graph of average current against voltage (an I–V graph) for each component

If the I–V graph is a straight line, it is an ohmic conductor. This is expected from the fixed resistor - This means it obeys Ohm's Law: V = IR

If the I–V graph is a curve, it is a non-ohmic conductor. This is expected from the filament lamp and diode.

Compare the results from the graphs obtained to the known I–V graphs for the resistor, filament lamp and diode. These should look like: (see gallery folder)