Resistors and I-V characteristics

The battery inside an electric toothbrush is recharged when the toothbrush is placed on its charger.

Figure 9 shows part of the toothbrush and the charger.

Explain how the current is produced in the secondary coil of the transformer in Figure 9.

The potential difference across the primary coil is 230 V.

The potential difference across the secondary coil is 1.5 V.

The primary coil has 1840 turns of wire.

Calculate the number of turns of wire in the secondary coil.

number of turns =_........

In Figure 9, a diode is connected in series with the rechargeable battery.

Explain why a diode is needed in the toothbrush in this part of the circuit.

Figure 10 shows a simplified diagram of the split ring commutator in a d.c. electric motor.

Explain the function of the commutator in a d.c. electric motor.

Figure 4 is a circuit diagram.

The current at P is

Some students investigate resistors in parallel.

The students set up a circuit containing four identical resistors.

The circuit used is shown in Figure 5.

Figure 5

The students measure the current from the power supply and the voltage (p.d.) across the resistors.

On Figure 5, draw a voltmeter connected to measure the voltage (p.d.) across the resistors.

T he students remove one resistor and measure the current and voltage again with only 3 resistors in the circuit.

T hey repeat the measurements of current and voltage with only 2 resistors in the circuit and then with only 1 resistor in the circuit.

Figure 6 is a table of their results.

Using data from the table in Figure 6, predict the current from the power supply when there are 4 resistors in the circuit.

current =_........

Using data from the table in Figure 6, calculate the resistance of only 1 resistor.

resistance =_........

Using data from the table in Figure 6, explain what happens to the total resistance of the circuit as the number of resistors in parallel decreases.

A student investigates how current varies with potential difference across a filament lamp.

The student uses a power supply, a variable resistor, the filament lamp and two meters.

Part of the circuit diagram is shown in Figure 19.

Complete the circuit diagram needed for this investigation.

Figure 19

Another student repeats the investigation in part (a) using a data logger.

The data logger records observations using sensors instead of meters. The sensors are connected to a computer to collect and display the observations.

The data logger collects 555 pairs of data in 2 minutes.

The results are shown in Figure 20.

Suggest one advantage of using a data logger instead of meters in this investigation.

Describe how current varies with potential difference in the graph in Figure 20.

Use data from the graph in Figure 20 to show how the resistance changes with potential difference for the filament lamp.

Which of these equations is correct?

The unit of work is the joule.

Starting with the meaning of work, we may obtain an equivalent unit of work as Nm.

Using work F d

unit of work unit of force unit of distance Nm

The unit of potential difference is the volt.

Explain how, starting with the meaning of potential difference, we may obtain an equivalent unit of potential difference.

Figure 3 shows some of the apparatus that students use to determine the resistance of a piece of iron wire.

Add connecting wires, a voltmeter and an ammeter to complete the circuit in Figure 3 so that the students can determine the resistance of the piece of iron wire.

The students extend the investigation to determine how the resistance of the iron wire changes with its length.

Give the name of one additional piece of apparatus the students would need.

Figure 4 shows a graph of the results.

Draw a straight line of best fit on Figure 4.

Use Figure 4 to estimate the resistance of a 100 cm length of the iron wire.

The variable resistor shown in Figure 3 is used to prevent the iron wire from becoming too hot.

Explain how the variable resistor is used to prevent the iron wire from becoming too hot.

The potential difference (voltage) across another piece of wire is 1.56 V.

The current in the wire is 0.45 A.

Calculate the resistance of this piece of wire.

Use the equation

Which of these shows the correct circuit symbol for a thermistor?

A student investigates how the resistance of a thermistor varies with temperature.

Figure 16 shows a graph of the results of this investigation.

Describe how the resistance of this thermistor varies with temperature.

Draw the tangent to the curve at a temperature of 30 °C, to find the rate of change of resistance with temperature at 30 °C.

State the unit.

rate of change of resistance with temperature at 30 °C =_........ unit_........

Figure 17 shows the apparatus used for this investigation.

Explain one improvement in measurement that the student could make in the investigation.

In this investigation, the resistance can be measured in two ways.

Method 1 – use an ohmmeter.

Method 2 – use an ammeter and a voltmeter.

Method 1 Method 2

using an ohmmeter using an ammeter and voltmeter

Figure 18

Explain why method 2 gives more precise results than method 1.

Figure 17 shows two different types of the same circuit component.

What is the name of this circuit component?

Which row in the table describes the way that ammeters and voltmeters should be connected with a component in a circuit?

Figure 18 shows the results from an experiment where the potential difference (voltage) across a filament lamp was varied.

The current and voltage were measured.

Describe the relationship between the current and the voltage as shown in the graph in Figure 18.

Use the values of the voltage and current at point P and at point Q on the graph in Figure 18 to complete the table in Figure 19.

Calculate the resistance of the filament lamp when the voltage is 4.5 V and the current is 51 mA.

Use the equation

Explain why the resistance of the filament lamp changes as the voltage across it increases.

Figure 4 shows the results from an experiment where the potential difference (voltage) across a filament lamp was varied.

The current and voltage were measured.

Describe the relationship between the current and the voltage as shown in the graph in Figure 4.

Use the values of the voltage and current at point P and at point Q on the graph in Figure 4 to complete the table in Figure 5.

Calculate the resistance of the filament lamp when the voltage is 4.5 V and the current is 51 mA.

Use the equation

Explain why the resistance of the filament lamp changes as the voltage across it increases.

A student investigates resistors connected in parallel using a number of resistors. Each resistor has the same resistance.

Figure 19 shows a circuit diagram with one resistor, R.

Add to Figure 19:

a voltmeter to find the potential difference across resistor R

another resistor in parallel with resistor R.

State the measurements that the student must take to find the overall resistance of the resistors in parallel.

The student investigates how the overall resistance of the circuit changes when additional resistors are added in parallel to R.

Each resistor has the same resistance.

Figure 20 shows the results of the student’s investigation.

State the resistance of a single resistor.

Comment on the relationship between the overall resistance of the circuit and the number of resistors in parallel.

Use information from Figure 20 to support your answer.

A different circuit is then set up with two resistors as shown in Figure 21.

Calculate the potential difference across the 15 Ω resistor.

Calculate the total power dissipated when there is a current of 0.20 A in the two resistors.

Use the equation

power produced_........

Which of these symbols is used to represent a thermistor in an electrical circuit?

A student investigates how the current in a lamp changes with the potential difference across the lamp.

The student uses the results to calculate the resistance of the lamp.

The results are shown in the table in Figure 17.

One value of resistance is missing from the table in Figure 17.

Calculate the value of resistance that is missing from the table.

missing resistance =_........ Ω

The student writes this conclusion:

‘The resistance of the lamp is directly proportional to the potential difference.’

Comment on the student’s conclusion. Use information from Figure 17 in your answer.

*(c) Figure 18 shows a battery connected to a filament lamp.

Explain, in terms of the movement of charged particles, how energy is transferred from the battery, through the lamp, to the surroundings.

Which of these symbols is used to represent a thermistor in an electrical circuit?

A student investigates how the current in a lamp changes with the potential difference across the lamp.

The student uses the results to calculate the resistance of the lamp.

The results are shown in the table in Figure 5.

One value of resistance is missing from the table in Figure 5.

Calculate the value of resistance that is missing from the table.

missing resistance =_........ Ω

The student writes this conclusion:

‘The resistance of the lamp is directly proportional to the potential difference.’

Comment on the student’s conclusion. Use information from Figure 5 in your answer.

The student used a power supply that had fixed output voltage settings. Each of these outputs was a whole number of volts.

Describe how the student could add a component to the circuit that would provide a continuously variable voltage across the lamp.

Figure 18 shows identical filament lamps connected together to a 12 V power supply.

Calculate the potential difference across each lamp.

The power output of each lamp is 0.75 W

Calculate the resistance of each lamp.

*(b) Explain, with the aid of a circuit diagram, the method a student could use to investigate how the resistance of a single lamp changes with the potential difference across the lamp.

Equations