Topic questions Magnetism and electromagnetism The motor effect
22 questions · 199 marks · about 3.5 hours
The diagram shows a simple direct current (d.c.) electric motor.
Explain why the coil turns continuously when the motor is connected to a direct current supply.
You should refer to the brushes and the split‑ring commutator in your answer.
Also involves Electrical power and energy transfers · Gravity and weight
A student investigates the magnetic force on a current‑carrying metal rod.
The student places a U‑shaped magnet on a sensitive balance.
They connect a rigid metal rod to a low-voltage power supply, and then fix the rod in place so that it passes between the poles of the magnet.
The diagram shows part of the student’s apparatus.
When there is a current in the metal rod, the rod experiences a force due to the magnetic field of the magnet.
The reading on the balance changes because the magnet experiences a force that is the same magnitude as the force on the rod, but in the opposite direction.
The student can increase the current in the metal rod up to a maximum of 5.0 A.
Suggest why the student should only have the power supply switched on for short periods of time.
Before the power supply is switched on, the reading of the balance is 194.95 g.
After the power supply is switched on, the reading of the balance is 193.80 g.
Calculate the force exerted on the current‑carrying rod by the magnet.
The student investigates how the balance reading varies as they change the current in the metal rod.
The table shows the student’s results.
| Current in A | Balance reading in g |
| 0.00 | 194.95 |
| 0.50 | 194.80 |
| 1.00 | 194.65 |
| 1.50 | 194.50 |
| 2.00 | 194.35 |
| 2.50 | 194.20 |
| 3.00 | 194.05 |
Plot the student’s results on the grid.
Draw the line of best fit.
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Current in A
The student changes the connections between the metal rod and the power supply to reverse the direction of the current.
Draw another line on the graph to show how the balance reading will vary with current after this change has been made.
Diagram 1 shows two magnetic poles.
Draw three field lines on Diagram 1 to show the uniform magnetic field in the shaded region between the two poles.
Diagram 2 shows a current‑carrying wire in the uniform magnetic field between the poles of a magnet.
The arrow on the wire shows the direction of current.
Draw another arrow on Diagram 2 to show the direction of the force on the wire.
Diagram 3 shows a rectangular loop of wire carrying a current in a uniform magnetic field.
Explain why the loop starts to turn but stops when the loop is vertical.
Also involves Circular motion
This question is about electromagnets.
Describe the construction of a simple electromagnet that is producing a magnetic field.
You may draw a diagram to help your answer.
A proton moves through a uniform magnetic field produced by a strong electromagnet.
The shaded area in the diagram represents the magnetic field.
The initial velocity, v, of the proton is also shown.
Use the left-hand rule to determine the direction of the force acting on the proton.
Explain how the force on the proton changes as the proton moves through the magnetic field.
You may add to the diagram to help your answer.
Suggest why the velocity of the proton changes.
Also involves Circular motion · Charged particles in magnetic fields
The diagram shows a machine that can be used to measure the speed of fast‑moving protons.
At the start, the proton is attracted towards a negatively charged plate.
Give a reason why the proton is attracted to the negatively charged plate.
The proton accelerates at 1.90 × 1011m / s2 from rest to a speed of 1.38 × 105m / s.
Show that the time taken for this acceleration is about 7 × 10–7s.
The proton passes through a hole in the negatively charged plate and enters an area where there is a magnetic field.
The magnetic field exerts a force on the proton, as shown in the diagram.
This force causes the proton to follow a circular path without changing speed.
Give the direction of the magnetic field.
Suggest how increasing the strength of the magnetic field will affect the proton when it is moving in the magnetic field.
This question is about magnets.
Which of these substances is not attracted to a bar magnet?
Diagram 1 shows a bar magnet.
k% S
Diagram 1
Draw magnetic field lines on diagram 1 to show the shape and direction of the magnetic field around the bar magnet.
Some bar magnets are made of steel.
Explain why steel is a good material for making bar magnets.
Draw an arrow on diagram 2 to show the direction of the force on the wire due to the magnetic field.
Assume that the magnetic field is uniform.
State two changes that could be made that would decrease the magnitude of the force on the wire in diagram 2.
A device called a metal detector can be used to find metal buried underground.
The metal detector has two circuits, each containing a coil of copper wire.
Diagram 1 shows the circuit for the transmitter coil.
Suggest why there is a magnetic field around the transmitter coil.
The cell supplies direct current (d.c.). The electronics in diagram 1 change the direct current into alternating current (a.c.) in the coil.
Describe the difference between direct current (d.c.) and alternating current (a.c.).
Diagram 3 shows the circuit for the receiver coil.
As a result of the alternating current in the gold ring, there is an alternating current in the receiving coil.
Explain how an alternating current in the receiving coil causes a sound to be emitted from the loudspeaker.
The diagrams show some equipment that the physicist Ørsted used in an investigation in 1820.
Diagram 1 shows the position of eight compass needles around a wire with no current in the wire.
The compass needles line up with and show the direction of the Earth’s magnetic field lines.
Diagram 2 shows the position of the same compass needles when a current is in the wire.
Diagram 2
Explain why the compass needles turn when the current is switched on.
Using evidence from the compass needles in diagram 2, draw on diagram 2 the shape and direction of a magnetic field line produced by the current in the wire.
Suggest what happens to the magnetic field when the current in the wire is reversed.
The current in the wire is turned off.
Diagram 3 shows the wire placed in a uniform magnetic field.
Diagram 3
The current is now switched on.
Draw an arrow to show the direction of the force on the wire.
The power supply in diagram 3 is replaced by an ammeter.
When the wire is moved, a current is detected.
Explain which direction the wire is moved in the magnetic field to produce a current in the wire.
Explain why a current is produced in the wire when the wire is moved in the magnetic field.
This question is about magnetic fields.
A student positions a thick wire vertically through the centre of a horizontal card.
The student then passes a constant current through the wire in the downward direction, as shown in diagram 1.
On diagram 1, draw the shape and direction of the magnetic field produced by the current in the wire.
Describe a method the student could use to show the shape of the magnetic field produced by the current in the wire.
The student then removes the card and sets up a second wire next to the first wire, as shown in diagram 2.
The current in both wires is in the downward direction.
The student observes that the wires move towards each other.
Explain why the wires move towards each other.
Also involves Resistors and I-V characteristics
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.
| Resistance of variable resistor in Ω | Power of loudspeakers in W |
| 0.0 | 0.000 |
| 2.5 | 0.100 |
| 5.0 | 0.113 |
| 7.5 | 0.108 |
| 10.0 | 0.100 |
| 12.5 | 0.092 |
| 15.0 | 0.084 |
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.
Also involves Errors and uncertainties
Photograph 1 shows part of an ammeter that can measure very small currents.
A student moves a wire between the poles of a strong magnet and uses the ammeter to measure the induced current.
Determine the current reading shown by the ammeter in photograph 1.
Predict a value for the current if the movement of the wire was repeated with the poles of the magnet reversed.
Predict a value for the current if the wire is moved faster between the poles of the magnet.
Photograph 2 shows the structure of the ammeter. The ammeter has a needle attached to a coil of wire which can move freely around a magnet. When there is a current in the coil, the needle moves. The larger the current, the more the coil turns.
Suggest how the design of the ammeter could be modified to increase its sensitivity.
Photograph 3 shows part of the ammeter after a student has adjusted the ammeter in error.
The needle on the ammeter does not point to zero when there is no current.
Which of these factors has been decreased by the student’s adjustment?
Suggest how the student could correct the readings taken from this ammeter due to this error without adjusting the ammeter.
These equations may be required for both International GCSE Physics (4PH1) and International GCSE Combined Science (4SD0) papers.
4. Energy resources and energy transfers
The equations on the following page will only be required for International GCSE Physics.
These additional equations may be required in International GCSE Physics papers 2P
Also involves Charged particles in magnetic fields
The SPS is a particle accelerator in Geneva.
The SPS can accelerate sulfur particles to speeds almost as fast as the speed of light.
Neutral sulfur particles can become positively charged sulfur particles.
Describe the difference between a neutral sulfur particle and a positively charged sulfur particle.
Diagram 1 shows a section of the SPS.
Positively charged sulfur particles are introduced at point P and accelerate to the right.
Explain why the positively charged sulfur particles accelerate.
The sulfur particles enter a large tube and complete a circular orbit.
Diagram 2 shows a sulfur particle, travelling with velocity v, in the tube.
A magnetic force acts on the positively charged sulfur particle.
This force makes the particle travel in a circle.
The force is labelled F.
Use the left-hand rule to determine the direction of the applied magnetic field.
The tube has a radius of 1.1 km and the sulfur particle has an orbital speed of 2.9 × 108m/s.
Calculate the orbital period of the sulfur particle.
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?
A racing car uses an engine and an electric motor to power its wheels.
The electric motor can only operate at maximum power for part of each lap of the race.
The electric motor has a maximum power output of 1.2
105W.
The electric motor transfers 4.0
106J of energy when it operates at maximum power.
Calculate the amount of time the electric motor can operate at maximum power.
The diagram shows a simplified view of a direct current (d.c.) electric motor.
State what is meant by the term direct current.
Explain the purpose of the brushes and the commutator in the d.c. motor.
The electric motor in the racing car can also be used as a generator to charge a battery pack. It is known as a motor generator unit (MGU).
The rotating wheels of the car cause the magnets in the MGU to spin near a coil of wire.
This induces a current in the coil when the coil is connected to the battery pack.
Name the energy store of the battery pack that increases when the battery pack is being charged.
Explain why the kinetic energy store of the car must decrease when the MGU charges the battery pack.
Also involves Estimation and orders of magnitude · Charged particles in magnetic fields
This question is about nuclear fusion.
The incomplete nuclear equation shows the fusion of two isotopes of hydrogen.
Identify particle X in the nuclear equation.
Each fusion reaction releases approximately 3
10–12J of energy.
Estimate how many reactions are needed to produce 1 J of energy.
Give your answer to 1 significant figure.
Explain why this fusion reaction will only happen at high temperature and high pressure.
A tokamak is an experimental nuclear fusion reactor.
Strong magnetic fields are used to contain the reactants inside the tokamak.
The diagram shows a positively charged hydrogen nucleus in a uniform magnetic field directed out of the page.
magnetic field out of page
positively charged hydrogen nucleus
The nucleus experiences a force acting to the left because of the magnetic field.
Deduce how the nucleus must behave to experience this force.
Suggest how engineers working on the tokamak could increase the force on the nucleus.
The diagram shows the construction of a simple loudspeaker.
A coil of wire is wrapped around a paper tube attached to the loudspeaker cone.
When there is an alternating current (a.c.) in the coil, the cone moves.
Explain how the loudspeaker produces a sound wave.
This question is about a radio powered by a person turning a handle.
The radio has a battery which stores energy when the handle is turned.
The diagram shows the part of the radio called a generator.
The generator produces a voltage which does electrical work on the battery.
Explain how the generator produces a voltage.
The radio receives a radio wave of frequency 93 MHz.
State the formula linking speed, frequency and wavelength of a wave.
Calculate the wavelength of the radio wave.
[speed of radio waves 
3.0 × 108 m/s]
The signal received by the radio is converted into an alternating current (a.c.) signal.
Describe how the loudspeaker in the radio converts this a.c. signal into a sound wave.
State a modification that would increase the force on the loudspeaker coil.
Diagram 1 shows a loop of wire connected to a power supply.
Describe how to find the shape of the magnetic field produced by the loop of wire.
Diagram 2 shows the view from above the card with two cross-sections of the loop of wire.
Key
Diagram 2
On diagram 2, draw the magnetic field produced by the two cross-sections of the loop of wire.
These diagrams show the magnetic field acting on a cross-section of wire when the current in the wire is out of the page.
Which diagram shows the correct direction of the force acting on the cross-section of wire due to the magnetic field?
The magnetic field acting on the cross-section of wire is produced by the current in the other side of the loop of wire.
Explain why there is no effect on the direction of the force on the cross-section of wire when the direction of the current in the loop is reversed.
A machine called the synchrocyclotron (SC) was designed to cause protons to move in a circular path using strong magnetic fields.
The SC used electromagnets to produce the strong magnetic fields.
Describe the construction of a simple electromagnet.
You may draw a diagram to support your answer.
The diagram shows a proton moving at velocity v in the magnetic field produced by the SC.
The direction of the magnetic field is into the page.
Using the left hand rule, determine the direction of the force acting on the proton due to the magnetic field.
The SC is surrounded by very thick concrete walls.
The radiation produced by the SC created atoms of radioactive barium-133 in the concrete walls.
Barium-133 has a half-life of 10.5 years.
State what is meant by the term half-life.
In 1990, the SC stopped being used at CERN, a research centre.
In 1990, the largest number of barium-133 atoms were found 40 cm deep in the concrete walls.
Since 2014, members of the public have been allowed to visit the SC at CERN.
Explain why the radioactive barium-133 in the concrete walls is not a risk to people visiting the SC.
Also involves Circular motion
This question is about a proton moving in a uniform magnetic field.
Describe how to use two permanent magnets to produce a uniform magnetic field.
You may draw a diagram to support your answer.
The diagram shows a proton moving in a circular path at constant speed in a uniform magnetic field.
This motion is similar to the movement of a planet in the Solar System.
Draw an arrow to show the direction of the force on the proton.
Accurately measure the diameter of the proton’s path.
The proton completes one orbit of its circular path in a time period of 8.7 × 10–6 seconds.
Calculate the orbital speed of the proton.
This question is about magnetic fields.
Diagram 1 shows a positively charged proton moving downwards in a uniform magnetic field.
magnetic field
Diagram 1
The proton experiences a force to the right.
What is the direction of the magnetic field?
When a current passes through a flat circular coil, a magnetic field is produced.
Complete diagram 2 by drawing the magnetic field of the flat circular coil.
Diagram 2
A wireless charging base uses a magnetic field to charge the battery of a mobile phone.
There is an alternating current in a coil of wire in the charging base.
There is another coil of wire connected to the battery in the mobile phone.
Explain how the wireless charging base charges the battery of the mobile phone.
Discuss the advantages and disadvantages of using a high current in the wireless charging base.
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