Electrical Power (SQA National 5 Physics): Exam Questions

A student investigates the relationship between the power developed in a resistor and the resistance of the resistor. The voltage across the resistor and the temperature of the resistor are kept constant during the investigation.

The graph shows the results.

The voltage across the resistor is

A dehumidifier is an appliance that extracts water from the air around it.

One particular dehumidifier operates at 230 V a.c. and has a power rating of 0.35 kW.

State the fuse rating that should be used for this dehumidifier.

Calculate the resistance of the dehumidifier.

An electric iron operates at 230 V a.c.

The power rating of the iron is 1750 W.

Calculate the current in the iron when it is operating.

The table gives information about the wavelength and output power of various lasers.

Light from a CO₂ laser is used in dental treatment.

During the dental treatment the CO₂ laser emits short pulses of light.

The average energy per pulse of light is 42.5 mJ.

Calculate the average time for each pulse.

A student is investigating connecting different combinations of resistors in circuits.

The student sets up a circuit as shown.

(i) Calculate the current in the circuit.

[4]

(ii) Calculate the power dissipated in the 120 Ω resistor.

[3]

The student then sets up a different circuit as shown.

(i) Determine the total resistance of this circuit.

[4]

(ii) State how the power dissipated in the 120 Ω resistor in this circuit compares to the power dissipated in the 120 Ω resistor in the circuit in part (a) (ii).

Justify your answer.

[2]

A hot water dispenser is used to heat enough water for one cup at a time.

The rating plate for the hot water dispenser is shown.

The hot water dispenser takes 26 s to heat enough water for one cup.

Show that the energy supplied to the hot water dispenser during this time is 91 000 J.

A motor raises a mass of 2400 kg vertically through a height of 5.0 m in 2 minutes.

The minimum power rating of the motor to raise the mass is

A toaster is connected to a 230 V mains supply and switched on.

The heating element in the toaster has a power rating of 1.2 kW.

A student makes the following statements about the toaster:

I The heating element transfers 12 000 J of electrical energy each second.

II The plug of the toaster should be fitted with a fuse rated at 13 A.

III The charge passing through the heating element each second is 230 C.

Which of these statements is/are correct?

A length of wire has a resistance of 8.0 Ω.

The current in the wire is 1.5 A.

The power developed in the wire is

The microwave oven has a power rating of 750 W.

The microwave oven is switched on for 180 s in order to sterilise the bottle.

Calculate the total energy used by the microwave oven during this time.

The filament of a lamp has a resistance of 4·0 Ω.

The lamp is connected to a 12 V supply.

The power developed by the lamp is

A filament lamp consists of a thin coil of resistance wire surrounded by a low pressure gas, enclosed in a glass bulb.

Using your knowledge of physics, comment on the suitability of this design as a light source.

A student carries out an experiment, using the apparatus shown, to determine a value for the specific heat capacity of water.

The student switches on the power supply and the immersion heater heats the water.

The joulemeter measures the energy supplied to the immersion heater.

The student records the following measurements.

energy supplied to immersion heater = 21 600 J

mass of water = 0·50 kg

initial temperature of the water = 16 °C

final temperature of the water = 24 °C

reading on voltmeter = 12 V

reading on ammeter = 4∙0 A

Calculate the time for which the immersion heater is switched on in this experiment.