Cambridge IGCSE0625

Electromagnetic effects

Physics 0625 Chapter Notes

What this chapter covers

Electromagnetic effects - Electromagnetic inductionElectromagnetic effects - The a.c. generatorElectromagnetic effects - Magnetic effect of a currentElectromagnetic effects - Force on a current-carrying conductorElectromagnetic effects - The d.c. motorElectromagnetic effects - The transformer
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In this chapter

  1. 1Magnetic Fields from Electric Currents
  2. 2The Motor Effect and Fleming's Left-Hand Rule
  3. 3How DC Electric Motors Work
  4. 4Electromagnetic Induction
  5. 5AC Generators and Fleming's Right-Hand Rule
  6. 6Transformers and How They Work
  7. 7High-Voltage Power Transmission

1. Magnetic Fields from Electric Currents

When an electric current flows through a conductor, it creates a magnetic field around it. The shape and direction of this field depend on the shape of the conductor. For a long, straight wire, the magnetic field consists of concentric circles centred on the wire. The direction of the field can be determined using the Right-Hand Grip Rule: if you point your right thumb in the direction of the conventional current, your fingers curl in the direction of the magnetic field lines. For a coil of wire, called a solenoid, the magnetic field inside is strong, uniform, and directed along the axis of the coil, much like the field of a bar magnet. The strength of the field can be increased by increasing the current, increasing the number of turns in the coil, or placing a soft iron core inside the solenoid (creating an electromagnet).

Key term

Right-Hand Grip Rule: A rule for finding the direction of the magnetic field around a current-carrying conductor: point the thumb of your right hand in the direction of the current, and your fingers will curl in the direction of the magnetic field.

Examiner insight

Examiners look for clearly drawn diagrams. For magnetic fields, ensure your field lines are continuous, do not cross, and have clear arrows indicating direction.

Common pitfall

Confusing the magnetic field of a straight wire (circles) with the field of a solenoid (similar to a bar magnet).

Worked example 13 marks

The diagram shows a long, straight wire carrying a current into the page, represented by the symbol ⊗. Sketch the magnetic field produced by this current, and indicate its direction.

  1. 1

    Step 1: Identify the rule to use. The Right-Hand Grip Rule determines the direction of the magnetic field around a current-carrying wire.

  2. 2

    Step 2: Apply the rule. Point your right thumb into the page, in the direction of the current.

  3. 3

    Step 3: Observe the direction of your fingers. Your fingers curl in a clockwise direction.

  4. 4

    Step 4: Draw the field lines. The magnetic field lines are concentric circles around the wire. Draw at least two circles.

  5. 5

    Step 5: Add direction arrows. Draw arrows on the circles pointing in the clockwise direction.

Worked example 22 marks

A student creates a solenoid by wrapping wire around a plastic tube. State two ways they could increase the strength of the magnetic field inside the solenoid.

  1. 1

    Method 1: Increase the current flowing through the wire. A larger current produces a stronger magnetic field.

  2. 2

    Method 2: Increase the number of turns (coils) per unit length. A more tightly wound coil concentrates the magnetic field, making it stronger.

  3. 3

    Method 3 (Bonus): Insert a soft iron core inside the solenoid. This makes it an electromagnet and significantly strengthens the magnetic field.

Recap

  • An electric current produces a magnetic field.
  • The Right-Hand Grip Rule is used to find the direction of the magnetic field around a wire.
  • A solenoid is a coil of wire that produces a strong, uniform magnetic field inside it.
  • The strength of a solenoid's field increases with current and the number of turns.
  • Placing a soft iron core inside a solenoid creates a powerful electromagnet.

Quick check

  1. What is a solenoid?1 mark
  2. If a current in a vertical wire flows upwards, what is the direction of the magnetic field to the east of the wire?1 mark

2. The Motor Effect and Fleming's Left-Hand Rule

When a wire carrying a current is placed in a magnetic field, it can experience a force. This is known as the motor effect. This happens because the magnetic field from the current interacts with the external magnetic field. The force is greatest when the wire is perpendicular to the magnetic field lines. To predict the direction of the force (or motion), we use Fleming's Left-Hand Rule. Hold your left hand so your thumb, first finger, and second finger are all at right angles to each other. Your First finger points in the direction of the Field (from North to South), your seCond finger points in the direction of the Current (conventional, + to -), and your ThuMb points in the direction of the resulting force or Motion. The size of the force can be increased by using a stronger magnet, a larger current, or a greater length of wire in the field.

Key term

Motor Effect: The phenomenon where a force is exerted on a current-carrying conductor when it is placed in a magnetic field.

Examiner insight

When asked to use Fleming's Left-Hand Rule, explicitly state the direction of the field and current before concluding the direction of the force. This shows clear, logical working.

Common pitfall

Mixing up Fleming's Left-Hand Rule (for motors) with his Right-Hand Rule (for generators). Remember: 'Left for Force, Right for Current'.

Fun fact

High-speed maglev (magnetic levitation) trains use powerful electromagnets and the motor effect to both levitate above the track and propel forward at speeds over 600 km/h.

Worked example 12 marks

The diagram shows a wire placed between the poles of a U-shaped magnet. The current flows in the direction shown. Determine the direction of the force on the wire.

  1. 1

    Step 1: Identify the rule. We need to find the direction of a force on a current-carrying wire in a magnetic field, so we use Fleming's Left-Hand Rule.

  2. 2

    Step 2: Identify the Field direction. The magnetic field (B) goes from the North pole to the South pole, so it points downwards. Point your First finger downwards.

  3. 3

    Step 3: Identify the Current direction. The current (I) is flowing into the page. Point your seCond finger into the page.

  4. 4

    Step 4: Determine the Force direction. With your fingers correctly aligned, your ThuMb points to the right. Therefore, the force on the wire is to the right.

Recap

  • A current-carrying wire in a magnetic field experiences a force (the motor effect).
  • Fleming's Left-Hand Rule is used to find the direction of the force.
  • Remember: Thumb = Motion/Force, First finger = Field, Second finger = Current.
  • The force is zero if the current is parallel to the magnetic field.
  • The force is increased by a stronger field, larger current, or longer wire.

Quick check

  1. What does each finger and the thumb represent in Fleming's Left-Hand Rule?3 marks
  2. What happens to the force on a wire if the direction of the current is reversed?1 mark

3. How DC Electric Motors Work

A simple direct current (DC) motor uses the motor effect to create continuous rotation from electrical energy. It consists of a coil of wire free to rotate between the poles of a magnet. When current flows, one side of the coil experiences an upward force while the other side experiences a downward force (according to Fleming's Left-Hand Rule). These two forces create a turning effect, or torque, causing the coil to spin. The key component for continuous rotation is the split-ring commutator. This is a metal ring split into two halves. As the coil rotates, the brushes that supply the current lose contact with one half of the commutator and make contact with the other. This cleverly reverses the direction of the current in the coil every half turn. This reversal ensures that the force on each side of the coil always pushes it in the same direction of rotation, preventing it from just rocking back and forth.

Key term

Split-ring commutator: A device in a DC motor that reverses the direction of the current in the coil every half turn to ensure continuous rotation in one direction.

Examiner insight

Examiners award high marks for explanations that clearly link the commutator's action to the reversal of current, which in turn reverses the direction of the force on a specific side of the coil, thus maintaining a constant direction of torque.

Common pitfall

Describing the motor's action without mentioning the crucial role of the split-ring commutator in reversing the current.

Worked example 14 marks

A simple DC motor is rotating. Explain the role of the split-ring commutator.

  1. 1

    Step 1: State the basic function. The purpose of the split-ring commutator is to reverse the direction of the current flowing into the coil.

  2. 2

    Step 2: Explain when this happens. This reversal occurs every half turn (every 180 degrees of rotation).

  3. 3

    Step 3: Explain why this is necessary. After half a turn, the side of the coil that was moving up is now on the side that needs to move down. By reversing the current, the direction of the force on that side is reversed (as per Fleming's Left-Hand Rule).

  4. 4

    Step 4: Conclude the overall effect. This ensures the turning force (torque) on the coil is always in the same direction (e.g., always clockwise), causing continuous rotation rather than the coil just oscillating.

Recap

  • A DC motor converts electrical energy into kinetic energy.
  • It uses the motor effect on a coil of wire in a magnetic field to produce rotation.
  • The split-ring commutator is essential for continuous rotation.
  • The commutator reverses the current in the coil every half turn.
  • The speed of the motor can be increased by increasing the current, using a stronger magnet, or having more turns on the coil.

Quick check

  1. Without a split-ring commutator, what would a DC motor's coil do when connected to a power supply?1 mark
  2. Name two parts of a simple DC motor besides the coil.2 marks

4. Electromagnetic Induction

Electromagnetic induction is the process of creating a voltage (or electromotive force, e.m.f.) in a conductor by changing the magnetic field around it. This is the principle behind generators. There are two main ways to do this: move a conductor (like a wire) through a stationary magnetic field, or move a magnet near a stationary conductor. In either case, the conductor is 'cutting' through magnetic field lines, which induces an e.m.f. If the conductor is part of a complete circuit, an induced current will flow. The size of the induced e.m.f. (and therefore the current) can be increased by: 1. Moving the wire or magnet faster. 2. Using a stronger magnet. 3. Using a coil of wire with more turns (so more wire cuts the field). The direction of the induced current will oppose the change that created it (this is known as Lenz's Law). For example, if you move the north pole of a magnet towards a coil, the induced current will create a north pole at that end of the coil to repel it.

Key term

Electromagnetic Induction: The process of generating an electromotive force (e.m.f.) across a conductor by placing it in a changing magnetic field.

Examiner insight

Clear answers will always refer to a 'change in magnetic field' or 'cutting of magnetic flux' as the cause of induction. Simply saying 'the magnet moves' is less precise.

Common pitfall

Thinking that a current is induced simply by a wire being in a magnetic field. The key is *change* – the wire must be moving, or the field must be changing.

Worked example 14 marks

A student pushes the North pole of a bar magnet into a coil of wire connected to a sensitive ammeter. The ammeter needle deflects to the right.a) What is observed when the magnet is held stationary inside the coil?b) What is observed when the magnet is pulled out of the coil?c) What could the student do to get a larger deflection?

  1. 1

    a) When the magnet is stationary, the magnetic field through the coil is not changing. Therefore, no e.m.f. is induced, and the ammeter reads zero.

  2. 2

    b) When the magnet is pulled out, the direction of the change in magnetic field is reversed. This induces an e.m.f. and current in the opposite direction. The ammeter needle will deflect to the left.

  3. 3

    c) To get a larger deflection (a larger induced e.m.f.), the student could: move the magnet faster, use a stronger magnet, or use a coil with more turns.

Recap

  • A voltage (e.m.f.) is induced in a conductor when it cuts magnetic field lines.
  • This can be done by moving the conductor or changing the magnetic field.
  • An induced current flows only if there is a complete circuit.
  • The induced e.m.f. is larger for faster movement, a stronger magnet, or more turns in a coil.
  • No change in magnetic field means no induced e.m.f.

Quick check

  1. State the three factors that affect the size of an induced e.m.f.3 marks
  2. Does a stationary wire placed in a strong magnetic field have a current induced in it?1 mark

5. AC Generators and Fleming's Right-Hand Rule

An AC generator (or alternator) converts mechanical energy into electrical energy using electromagnetic induction. It consists of a coil rotated within a magnetic field. As the coil spins, the wires on each side cut through the magnetic field lines, inducing an e.m.f. During one half of the rotation, one side of the coil moves up, and the other moves down, inducing a current in one direction. In the next half of the rotation, the sides move in the opposite directions, so the induced current reverses. This creates an alternating current (AC). The direction of the induced current can be found using Fleming's Right-Hand Rule (different from the Left-Hand Rule for motors). Hold your right hand with the thumb, first, and second fingers at right angles. Your First finger points in the direction of the Field, your ThuMb points in the direction of Motion of the wire, and your seCond finger shows the direction of the induced Current. The output voltage varies sinusoidally, being maximum when the coil is moving fastest across the field lines (horizontal) and zero when it is moving parallel to them (vertical).

Key term

Fleming's Right-Hand Rule: A rule for finding the direction of an induced current: Thumb represents Motion, First finger represents Field, and Second finger represents Current.

Worked example 13 marks

The diagram shows a simple AC generator coil rotating clockwise. For the position shown, determine the direction of the current in side AB of the coil.

  1. 1

    Step 1: Identify the rule. We need to find the direction of an induced current, so we use Fleming's Right-Hand Rule.

  2. 2

    Step 2: Identify the Motion. Side AB is moving upwards. Point your ThuMb upwards.

  3. 3

    Step 3: Identify the Field. The magnetic field goes from North to South, so from left to right. Point your First finger to the right.

  4. 4

    Step 4: Determine the Current. Your seCond finger now points out of the page. So, the current in side AB flows from B to A.

Worked example 23 marks

Sketch a graph of the output voltage of an AC generator against time for one full rotation, starting with the coil horizontal.

  1. 1

    Step 1: Set up the axes. Label the y-axis 'Voltage / V' and the x-axis 'Time / s' or 'Rotation'.

  2. 2

    Step 2: Starting point. When the coil is horizontal, it cuts field lines at the fastest rate, so the induced voltage is at a maximum. Start the graph at a peak.

  3. 3

    Step 3: Quarter turn. After a quarter turn (90 degrees), the coil is vertical and moving parallel to the field. It cuts no field lines, so the voltage is zero. The graph crosses the x-axis.

  4. 4

    Step 4: Half turn. After a half turn (180 degrees), the coil is horizontal again but moving in the opposite direction. The voltage is at a maximum but in the opposite direction (a negative peak).

  5. 5

    Step 5: Three-quarter turn. After 270 degrees, the coil is vertical again, and the voltage is zero.

  6. 6

    Step 6: Full turn. After 360 degrees, the coil is back at the start, with maximum positive voltage. Connect the points with a smooth, sinusoidal curve.

Recap

  • An AC generator uses electromagnetic induction to produce alternating current.
  • It works by rotating a coil in a magnetic field.
  • Fleming's Right-Hand Rule determines the direction of the induced current.
  • The output voltage and current are sinusoidal, changing direction periodically.
  • Maximum voltage is induced when the coil moves perpendicular to the field lines.

Quick check

  1. What is the key energy conversion that takes place in a generator?1 mark
  2. What is the difference between Fleming's Left-Hand and Right-Hand rules?2 marks

6. Transformers and How They Work

A transformer is a device used to increase (step-up) or decrease (step-down) the voltage of an alternating current. It consists of two separate coils of wire, the primary coil and the secondary coil, wrapped around a continuous soft iron core. When an alternating voltage is applied to the primary coil, it drives an alternating current, which creates a continuously changing magnetic field in the iron core. The soft iron core concentrates this changing magnetic field and guides it through the secondary coil. This changing magnetic field induces an alternating e.m.f. (voltage) in the secondary coil. This process is called mutual induction. Crucially, transformers only work with AC, because a changing magnetic field is required for induction. A DC supply would create a steady magnetic field, inducing no voltage in the secondary coil (except for a brief moment when it's switched on or off).

Vp / Vs = Np / Ns

Vp * Ip = Vs * Is (for a 100% efficient transformer)

Key term

Transformer: A static device that transfers electrical energy from one AC circuit to another by electromagnetic induction, changing the voltage level in the process.

Examiner insight

When explaining how a transformer works, you must mention 'alternating current', 'changing magnetic field', and 'induced e.m.f.'. Missing any of these key steps will lose marks.

Common pitfall

Applying the transformer equations to a DC supply. Remember, induction requires a *changing* magnetic field, which DC does not provide.

Worked example 13 marks

A transformer has 200 turns on its primary coil and is connected to a 230 V mains supply. If it is designed to produce an output voltage of 11.5 V, how many turns should the secondary coil have?

  1. 1

    Step 1: Write down the transformer equation: Vp / Vs = Np / Ns.

  2. 2

    Step 2: Identify the known values: Vp = 230 V, Vs = 11.5 V, Np = 200.

  3. 3

    Step 3: Rearrange the equation to find Ns: Ns = Np * (Vs / Vp).

  4. 4

    Step 4: Substitute the values: Ns = 200 * (11.5 / 230).

  5. 5

    Step 5: Calculate the result: Ns = 200 * 0.05 = 10. The secondary coil should have 10 turns.

Worked example 23 marks

A step-up transformer is 100% efficient. It converts a 25 V AC supply to 400 V. If the current in the primary coil is 8.0 A, calculate the current in the secondary coil.

  1. 1

    Step 1: Write down the power equation for a 100% efficient transformer: Primary Power = Secondary Power, so Vp * Ip = Vs * Is.

  2. 2

    Step 2: Identify the known values: Vp = 25 V, Vs = 400 V, Ip = 8.0 A.

  3. 3

    Step 3: Rearrange the equation to find Is: Is = (Vp * Ip) / Vs.

  4. 4

    Step 4: Substitute the values: Is = (25 * 8.0) / 400.

  5. 5

    Step 5: Calculate the result: Is = 200 / 400 = 0.5 A. The current in the secondary coil is 0.5 A.

Recap

  • Transformers change AC voltages using electromagnetic induction.
  • They have a primary coil, a secondary coil, and a soft iron core.
  • A step-up transformer has more secondary turns than primary turns (Ns > Np) and increases voltage.
  • A step-down transformer has fewer secondary turns than primary turns (Ns < Np) and decreases voltage.
  • Transformers do not work with DC.
  • For an ideal transformer, input power equals output power (Vp * Ip = Vs * Is).

Quick check

  1. Why is the core of a transformer made of soft iron?1 mark
  2. If Ns > Np, is the transformer step-up or step-down?1 mark

7. High-Voltage Power Transmission

Electrical power is generated in power stations and transmitted across the country to homes and industries via a network of cables called the National Grid. A significant challenge is minimising energy lost as heat in these long cables. The power lost in a cable is given by the formula P_loss = I²R, where 'I' is the current and 'R' is the resistance of the cable. This equation shows that the power lost is proportional to the square of the current. Therefore, to minimise energy loss, it is crucial to transmit the electricity using the lowest possible current. We also know that electrical power is calculated by P = VI. This means for a fixed amount of power being transmitted, if we use a very high voltage (V), the current (I) will be very low. This is the strategy used: step-up transformers at power stations increase the voltage to very high levels (e.g., 400,000 V), which reduces the current. The electricity is then transmitted at this high voltage. Near towns and homes, a series of step-down transformers reduce the voltage to a safer, usable level (e.g., 230 V in the UK).

P = VI

P_loss = I²R

Key term

National Grid: The high-voltage electric power transmission network that connects power stations and consumers across a country.

Examiner insight

Top-tier answers will use both P=VI and P=I²R to construct a logical argument. Explicitly stating that power loss is proportional to the *square* of the current demonstrates a deeper understanding.

Common pitfall

Stating that high voltage reduces energy loss without explaining *why*. The key is that high voltage allows for low current, and it is the low current that directly reduces the I²R heat loss.

Fun fact

The buzzing sound you hear under high-voltage power lines is caused by the strong electric field ionising the air around the cables, a phenomenon called corona discharge.

Worked example 15 marks

A power station generates 200 MW of power. It is transmitted along cables with a total resistance of 5 Ω. Calculate the power lost in the cables if the power is transmitted ata) 50,000 V andb) 400,000 V.

  1. 1

    Parta) Transmitting at 50,000 V:

  2. 2

    Step 1: Calculate the current (I) using P = VI. I = P / V = 200,000,000 W / 50,000 V = 4000 A.

  3. 3

    Step 2: Calculate the power loss using P_loss = I²R. P_loss = (4000)² * 5 = 16,000,000 * 5 = 80,000,000 W or 80 MW.

  4. 4

    Partb) Transmitting at 400,000 V:

  5. 5

    Step 3: Calculate the new current. I = P / V = 200,000,000 W / 400,000 V = 500 A.

  6. 6

    Step 4: Calculate the new power loss. P_loss = I²R = (500)² * 5 = 250,000 * 5 = 1,250,000 W or 1.25 MW.

  7. 7

    Step 5: Compare the results. Transmitting at the higher voltage reduces the power loss from 80 MW to 1.25 MW, which is a huge saving.

Recap

  • Power is transmitted at very high voltages to minimise energy loss.
  • High voltage allows for low current for the same amount of power (P = VI).
  • Power lost as heat in cables is given by P = I²R.
  • Low current dramatically reduces the power lost in the cables.
  • Step-up transformers increase voltage at power stations.
  • Step-down transformers decrease voltage for consumers.

Quick check

  1. What is the main reason for transmitting electricity at high voltages?1 mark
  2. What two types of transformers are essential for the National Grid?2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Explain, in detail, how a simple DC motor works. Your explanation should mention the motor effect, Fleming's Left-Hand Rule, and the role of the split-ring commutator.6 marks
  2. A transformer is used to power a 12 V, 24 W lamp from a 230 V mains supply. Assuming the transformer is 100% efficient, calculate: a) the current drawn by the lamp, b) the current drawn from the mains supply.4 marks
  3. State three ways to increase the magnitude of the e.m.f. induced in a coil by a moving magnet.3 marks
  4. Draw a diagram of a simple AC generator. Use arrows to show the direction of the magnetic field, the direction of rotation of the coil, and the direction of the induced current in each side of the coil at the instant the coil is horizontal.4 marks
  5. Explain why transformers will not work with a direct current (DC) supply.3 marks
  6. A long, straight, vertical wire carries a current downwards. Describe the magnetic field produced by this current and state how you would determine its direction.3 marks
  7. A power station needs to transmit 440 MW of power. The transmission cables have a total resistance of 4.0 Ω. The power is stepped up to a voltage of 400 kV for transmission. Calculate the power lost as heat in the cables.4 marks
  8. Distinguish between a step-up transformer and a step-down transformer in terms of their construction and their effect on voltage and current.4 marks
  9. The diagram shows a wire suspended in a magnetic field. When the switch is closed, the wire moves. State the direction of motion of the wire and explain what would happen to the motion if: a) the current was reversed, b) the magnetic poles were swapped.3 marks
  10. Lenz's Law is a consequence of the conservation of energy. It states that the direction of an induced current is always such as to oppose the change that caused it. Explain how this applies when you push the north pole of a magnet towards a coil.3 marks

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