Electrical power and energy transfers

Fluorescent tube lamps can be used in schools and office buildings for lighting.

The photograph shows an engineer installing a fluorescent tube lamp.

Diagram 1 shows a simplified view of the components of a fluorescent tube lamp.

When the lamp is on, a large voltage is applied between the positive electrode and the negative electrode.

State what is meant by the term voltage.

The large voltage causes electrons to accelerate from the negative electrode towards the positive electrode.

An electron gains 1.0 × 10^–16J of energy when it accelerates between the electrodes.

Show that the voltage between the electrodes is about 600 V.

[magnitude of electron charge = 1.6 × 10^–19C]

The electron gains 1.0 × 10^–16J of energy in its kinetic store when it accelerates from the negative electrode to the positive electrode.

Calculate the speed of an electron when it reaches the positive electrode.

Assume the electron is initially at rest.

[electron mass = 9.1 × 10^–31kg]

When the electrons accelerate between the electrodes, they collide with mercury atoms.

Energy is transferred to the mercury atoms during the collisions. This causes the mercury atoms to emit ultraviolet light.

Atoms in the fluorescent coating absorb this ultraviolet light, which is then re‑emitted as light from a different part of the electromagnetic spectrum.

Diagram 2 shows this process.

Suggest why the tube must have a fluorescent coating for the lamp to operate effectively.

Explain why the fluorescent tube lamp is dangerous if the fluorescent coating becomes damaged.

Photograph 1 shows a power supply for some electrical equipment. The electrical equipment is connected to the mains supply.

Explain why the power supply is completely covered in plastic.

The electrical equipment is connected to the mains power supply of 230 V.

State the formula linking power, current and voltage.

The electrical equipment has a power of 350 W.

Calculate the current in the equipment when it is being used.

Photograph 2 shows a circuit breaker.

Explain the advantages of using a circuit breaker rather than a fuse.

The photograph shows an electric heater connected to the mains electricity supply.

(Source: © Yevhen Prozhyrko / Shutterstock)

The circuit the heater is connected to is fitted with a circuit breaker, which breaks the circuit if the current gets too high.

Give an advantage of using a circuit breaker instead of using a fuse.

The voltage of the mains electricity supply is 230 V.

State the formula linking power, current and voltage.

The normal operating current of the heater is 11 A.

Calculate the input power to the heater for this current.

Give your answer in kW.

The circuit breaker has a rating of 16 A.

Suggest a reason why the heater may switch off before it reaches its normal operating current.

The diagram shows a domestic lighting circuit.

Explain an advantage of using this circuit for domestic lighting.

When switch 1 is closed, the current in lamp 1 is 22 mA.

Give the name of the charged particle that moves in an electric current.

Show that lamp 1 has a power of about 5 W.

Calculate the energy transferred by lamp 1 when it is on for 30 seconds.

The circuit is connected to the mains supply. Mains voltage is 230 V.

State what is meant by the term voltage.

Switches 1 and 3 are closed, which turn on lamps 1 and 3.

Switch 2 is open.

Calculate the current in the mains supply.

Diagram 1 shows a generator inside a small wind turbine. The generator is connected to a lamp and the windmill blades.

When the coil rotates in the direction of the arrow, the ammeter displays a small current.

Explain how the generator produces a current.

The generator in the wind turbine acts as an alternating current (a.c.) power supply.

Diagram 2 shows an electric circuit containing the generator being used to charge a mobile phone battery.

Direct current (d.c.) is needed to charge the battery.

Explain why there is a diode in the circuit.

Explain how the current in the battery will change if the wind speed increases.

The mean voltage across the battery is 7.2 V.

The battery gains 14 kJ of energy in 8400 seconds.

Calculate the mean current in the battery.

The photograph shows a water bath that a technician uses to heat some water.

The water bath is filled with water at an initial temperature of 15C.

Calculate the initial temperature of the water in kelvin.

The technician heats the water to a final temperature of 60C.

Describe how the energy of the water molecules changes as the temperature of the water increases.

The table shows some information about the heating element in the water bath and the heating process.

Calculate the energy transferred by the heating element in the water bath during the heating process.

Calculate the mass of water being heated.

Assume that all the energy is transferred to the thermal store of the water. [for water, specific heat capacity = 4200 J / kgC]

QUESTION CONTINUES ON THE NEXT PAGE

Some water evaporates as a gas from the water bath.

Describe the arrangement of particles in a gas.

Describe two differences between evaporation and boiling.

This question is about a filament lamp.

Which of these is the correct circuit symbol for a filament lamp?

The filament lamp emits visible light.

The table gives some statements about visible light.

Place ticks () in the boxes to show which statements are correct for visible light.

The diagram shows a ray of light from the filament lamp incident on the reflective side of a curved mirror.

Complete the diagram by drawing

the normal line where the ray is incident on the mirror.

the reflected ray of light.

The filament lamp is connected in a circuit with a switch and a battery of three cells.

When the switch is on, the filament lamp transfers 120 J of energy in a time of 3.0 minutes.

Each cell has a voltage of 1.5 V.

Calculate the current in the filament lamp.

A small plotting compass is placed near the wires in the circuit.

When the switch is turned on, the compass needle moves to a new position.

Give a reason why the compass needle moves.

The diagram shows the screen of an oscilloscope when a sound wave is detected, and the oscilloscope settings.

Give the name of the piece of equipment that is connected to the oscilloscope to detect the sound wave.

Use the trace on the oscilloscope to determine the time period of the detected sound wave.

Calculate the frequency of the detected sound wave.

State the formula linking energy transferred, charge and voltage.

The effective voltage of the oscilloscope trace can be calculated using the formula

Use the effective voltage to calculate the energy transferred when 6.3 × 10⁻⁵C of charge passes through the oscilloscope.

A family has a television set.

The television set has a low power mode called standby.

When on standby, the power rating of the television set is 0.27 W.

Calculate the energy transferred to the television set on standby in 12 hours.

In normal use, the current in the television set is 0.31 A.

Explain how a fuse works to protect the television set if there is a fault.

Explain why a 13 A fuse is not an appropriate choice of fuse to use in the plug of this television set.

The circuit diagram shows a motor, power supply, switch and variable resistor connected in series.

The switch is open and the motor is not moving.

State how much energy is in the kinetic energy store of the motor.

The switch is closed.

The motor takes 0.42 seconds to reach its maximum speed of rotation.

The mean current in the motor is 3.9 A.

The voltage across the motor is 7.1 V.

Show that the work done on the motor during the 0.42 seconds is about 12 J.

Complete the Sankey diagram for the electric motor during the 0.42 seconds by giving the missing information.

store of surroundings

2 J

Use the Sankey diagram to calculate the efficiency of the process of bringing the motor to its maximum speed of rotation.

A cleaning product is applied to a car using a sponge pad.

The sponge pad is rubbed against the car to apply the cleaning product.

Some parts of the car are made of metal and other parts are made of plastic.

The metal parts of the car are earthed.

Explain why the pad becomes charged when rubbing the plastic parts, but not when rubbing the metal parts.

The sponge pad is held near a metal post that is connected to the ground.

The sponge pad discharges with a small spark through the air to the metal post.

The sponge pad stores 5.0 mJ of energy in its electrostatic store.

The voltage between the sponge pad and the metal post is 6000 V.

Calculate the charge transferred by the spark.

The small spark between the sponge pad and the metal post demonstrates that the sponge pad is charged.

Describe a different experiment that could demonstrate that the sponge pad is charged.

You may draw a diagram to support your answer.

The diagram shows a small computer in two difference cases, X and Y.

The electronic chip in the computer is hot when in use. Each case is designed to cool the computer chip.

Case X is made of white plastic and has a fan.

Case Y is made of black-painted aluminium metal and has no fan. There is a metal block that is in contact with the case and the chip.

Explain the main method of heat transfer from the chip to the surroundings for case X.

Explain the main method of heat transfer from the chip to the surroundings for case Y.

State the formula linking power, current and voltage.

The small computer operates at a voltage of 5.1 V with a current of 2.9 A.

Calculate the power of the small computer.

Give the unit.

A student uses the circuit shown in diagram 1 to investigate how the current changes with voltage for a filament lamp.

Give the name of component Y.

Give a reason why component Y is included in the circuit.

The graph shows some of the student’s results.

State the formula linking charge, current and time.

Determine the current in the lamp when the voltage across the lamp is 10 V.

Calculate the charge transferred through the lamp in 30 s when the voltage across the lamp is 10 V.

Give the unit.

Calculate the time for the lamp to transfer 250 J of energy when the voltage across the lamp is 10 V.

The student disconnects the cell and reconnects it with its terminals reversed.

Complete the graph to show how the current in the lamp varies with voltage across the lamp when the cell is connected with its terminals reversed.

The student replaces the filament lamp with a light emitting diode (LED) and replaces the cell with an alternating current (a.c.) power supply, as shown in diagram 2.

The student also removes the ammeter and voltmeter from the circuit.

Explain why the LED flashes on and off in this circuit.

Diagram 1 shows a simple loudspeaker.

The coil is connected to an alternating current (a.c.) supply.

Describe how the loudspeaker produces sound.

Diagram 2 shows two loudspeakers connected in series with a variable resistor.

The variable resistor is set to 5.0 Ω.

The total voltage across the two loudspeakers is 0.75 V and the current in the circuit is 0.15 A.

Show that the total power of the two loudspeakers is about 0.1 W.

[power = current × voltage]

A student varies the resistance of the variable resistor.

The table shows the power of the loudspeakers for different resistance values of the variable resistor.

Plot the student’s results on the grid.

Draw a curve of best fit.

Diagram 3 shows the loudspeakers connected in series to a cell.

Diagram 4 shows the loudspeakers connected in parallel to the same cell.

Comment on how the total resistance of the loudspeakers in diagram 3 compares with the total resistance of the loudspeakers in diagram 4.

Diagram 1 shows a trolley seen from above.

A copper rod is attached to the front of the trolley.

The rod is connected to a voltmeter fixed to the trolley.

Diagram 2 shows the path of the trolley, backwards and forwards through a very strong magnetic field directed into the page.

The shaded area shows the magnetic field.

A voltage is induced in the copper rod as the trolley moves through the magnetic field.

Explain why the sign of the voltmeter reading changes as the trolley moves backwards and forwards.

Give a reason why the magnitude of the induced voltage might change.

The voltmeter is replaced by a resistor.

A charge of 1.410^–4C flows in the resistor during a time of 0.78 s.

Calculate the mean current in the resistor.

The thermal energy store of the resistor increases by 2.310^–6J as energy is transferred to it electrically.

Calculate the mean voltage across the resistor when a charge of 1.410^–4C transfers this energy.

The diagram shows a circuit containing a cell, two switches and two resistors, R₁ and R₂.

A light emitting diode (LED) is represented using the symbol

Draw a LED on the circuit diagram that will indicate when there is a current in resistor R₁.

Both switches in the circuit are now closed.

Draw an ammeter on the circuit diagram that will measure the current in the cell.

The voltage across the cell is 4.5 V.

Explain how the voltage across R₂ compares with the voltage across the cell.

Switch 1 is now opened whilst switch 2 remains closed.

State the formula linking energy transferred, charge and voltage.

The voltage across the cell stays constant at 4.5 V.

Calculate the charge transferred through the cell when the cell transfers 4.1 J of energy.

A student investigates how the current in resistor R₂ varies with the voltage across it.

Describe how the circuit could be modified for the student to do this investigation.

This question is about an electric fan.

A battery supplies a voltage of 12 V and a current of 0.25 A to the fan.

The fan is switched on for 12 seconds and the fan gains 25 J in its kinetic energy store.

Calculate the efficiency of this energy transfer.

The diagram shows part of the electric motor inside the fan.

Explain why the coil starts to rotate when there is a current in the coil.

Which side of the coil moves vertically upwards?

Diagram 1 shows a machine used for demonstrating electrostatics.

When the handle is turned, the dome becomes positively charged.

In terms of electron movement, give a reason why the metal dome becomes positively charged.

When the handle is turned for 15 s the dome gains 0.50 J of energy in its electrostatic store as it becomes charged.

The voltage between the dome and the earth is 120 kV.

Calculate the mean charging current during this time.

The metal dome is discharged.

A thin metal case is then placed on top of the metal dome, as shown in Diagram 2.

When the handle is turned, the thin metal case moves upwards away from the dome.

Explain why the thin metal case starts to move upwards.

Explain why the metal case reaches a maximum height above the metal dome.

This question is about electric current.

State what is meant by the term electric current.

Give the name of the particles that flow if there is an electric current in a wire.

An electric heater contains wires that heat up when there is a current in the wires.

The electric heater has a power of 1.4 kW when connected to a mains voltage of 230 V.

State the formula linking power, current and voltage.

Calculate the current in the wires.

Explain why the current in the wires causes the temperature of the wires to increase.

The photograph shows a mains-operated, decorative lamp, X.

Lamp X has seven identical bulbs that are connected in series.

Give a disadvantage of connecting the bulbs in series.

Suggest an advantage of connecting the bulbs in series.

Each bulb has a working resistance of 390 Ω.

The voltage across each bulb is 33 V.

State the formula linking voltage, current and resistance.

Calculate the current in each bulb.

Calculate the energy transferred to lamp X when it is used for 2.5 hours.

Another decorative lamp, Y, uses five of the same 390 Ω bulbs connected in series.

Both lamps are mains operated.

Explain how the brightness of a single bulb in lamp Y compares to the brightness of a single bulb in lamp X.

The photograph shows a hair dryer plugged into the mains supply.

The hair dryer contains a fuse.

State which wire in the hair dryer should be in series with the fuse.

The fuse is an electrical safety feature used in mains-operated domestic appliances.

State two other electrical safety features that can be used in mains-operated domestic appliances.

The hair dryer has a current of 9.6 A.

The mains supply voltage is 230 V.

State the formula linking power, current and voltage.

Calculate the power of the hair dryer.

Give the unit.

The hair dryer contains a coil of wire which is used to heat air passing through the hair dryer.

Explain why the coil of wire heats up when there is a current in it.

Explain how a fuse protects a domestic appliance.

The diagram shows part of a device used to demonstrate electrostatic charge.

Negatively charged water droplets fall into bucket A.

Describe how bucket A becomes negatively charged.

Explain why the negatively charged droplets spread out as they fall.

A droplet hits bucket A with a speed of 3.8 m/s.

Calculate the kinetic energy of the droplet when it hits bucket A.

The total charge stored in bucket A is –1.110^–10 C.

This charge passes through the air between the buckets in 9.2 × 10^–3 s, causing a spark between bucket A and bucket B.

Calculate the mean current between the buckets.

The spark transfers a charge of –1.110^–10 C.

The mean voltage between the buckets is 1.7 kV.

Calculate the energy transferred by the spark.

A toaster is an electrical device used for toasting bread.

The toaster contains thin metal wires.

These wires get hot when there is a current in them.

The wires transfer energy to the bread by heating and by radiation.

Give a reason why the wires in the toaster are connected in parallel.

State the formula linking power, current and voltage.

The power rating of the toaster is 2.8 kW.

Calculate the total current in the toaster.

[mains voltage = 230 V]

The toaster contains 48 thin metal wires.

Calculate the current in each of the thin metal wires.

Welding is a process where pieces of metal are melted and then joined together.

The diagram shows apparatus used for welding.

The welding apparatus uses the transformer to decrease the voltage from 230 V.

State the name of this type of transformer.

State the formula linking the turns ratio, the input voltage and the output voltage for a transformer.

The table gives some information about the transformer.

Use information from the table to calculate the output current of the transformer.

Assume that the transformer is 100% efficient.

A fuse is installed in the plug which supplies the input current.

Which fuse should be used in this plug?

Give your answer to 2 significant figures.