Resistance and Ohm's lawIB MYP Physics: Subtopic test

1. 1
   A technician in a workshop tests a resistor from a heating unit. She connects it to a power supply with a voltmeter and an ammeter. The voltmeter reads 12 V and the ammeter reads 0.60 A. The resistor stays at a constant temperature throughout the test.

   (a)

   What is the resistance of the resistor?

   [1 mark]

   • A7.2 Ω
   • B0.050 Ω
   • C12.6 Ω
   • D20 Ω

   (b)

   Which statement is Ohm's law?

   [1 mark]

   • AThe resistance of any component is always constant.
   • BThe current through an ohmic conductor is directly proportional to the potential difference across it, at constant temperature.
   • CThe potential difference across a conductor is always equal to its resistance.
   • DThe current in a conductor decreases as the potential difference increases.

   (c)

   The technician doubles the potential difference to 24 V. Calculate the new current, assuming the resistor is ohmic at constant temperature.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student plots current against potential difference for three components in turn: a fixed resistor, a filament lamp and a diode. She increases the potential difference from zero in small steps and then reverses the connections to test the diode in the opposite direction.

   (a)

   The graph for the filament lamp curves so that the line becomes less steep as the p.d. increases. What is the reason?

   [1 mark]

   • AThe filament gets hotter, so its resistance increases
   • BThe filament gets hotter, so its resistance decreases
   • CThe potential difference stops increasing
   • DThe lamp behaves like a diode

   (b)

   What happens to the current in the diode when the connections are reversed?

   [1 mark]

   • AIt doubles
   • BIt flows the opposite way by the same amount
   • CAlmost no current flows
   • DIt stays exactly the same

   (c)

   Describe the shape of the current–potential difference graph for the fixed resistor and state what this shows about its resistance.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A group of students investigates how the length of a piece of nichrome wire affects its resistance. They connect the wire to a cell, with an ammeter and a voltmeter, and use a metre rule to change the length of wire in the circuit.

   (a)

   Describe how the students can find the resistance of the wire at each length, and state one safety precaution.

   [3 marks]

   (b)

   The results were: 0.20 m gave 1.0 Ω, 0.40 m gave 2.1 Ω, 0.60 m gave 2.9 Ω and 0.80 m gave 4.1 Ω. Describe the trend, explain it in terms of electrons and atoms, and evaluate the effect of the wire heating up on the results.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A hotel in Dubai plans to install an automatic system for its outdoor garden lights and a temperature-controlled cooling system for its server room. The garden lights will contain a light-dependent resistor (LDR) and the cooling system will contain a thermistor.

   (a)

   Explain how an LDR and a thermistor can be used to make these two systems work automatically.

   [6 marks]

   (b)

   Evaluate whether the hotel should install the automatic system.

   [6 marks]

   Total for question 4: 12 marks

End of questions