Electromagnetic Induction, Alternators, Dynamos & Transformer Equations
In Triple Science GCSE Physics, the generator effect (electromagnetic induction) is the exact inverse of the motor effect: moving an electrical conductor relative to a magnetic field so that it cuts through magnetic field lines induces a potential difference across the ends of the conductor (and an induced current if the circuit is complete). This principle powers electrical generators, where rotating a coil in a magnetic field produces either alternating current in an alternator (using slip rings) or direct current in a dynamo (using a split-ring commutator), as well as moving-coil microphones. Finally, transformers combine electromagnetic induction in two coils wound around a laminated soft iron core to step alternating voltages up or down according to V_p / V_s = n_p / n_s.
- The Generator Effect (Electromagnetic Induction)
- If an electrical conductor moves relative to a magnetic field (or if the magnetic field around a stationary conductor changes) so that the conductor cuts across magnetic field lines, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, an induced current flows. Magnitude of induced p.d. is increased by: moving the wire/magnet faster, using a stronger magnetic field, or using a coil with more turns. Reversing the direction of motion or the polarity of the magnet reverses the direction of the induced p.d./current.
- Opposing Magnetic Field (Conservation of Energy)
- An induced current always generates a magnetic field that opposes the original change (either the movement of the conductor or the change in magnetic field). For example, pulling a North pole out of a solenoid induces a South pole at that end of the solenoid to attract the magnet back and resist the pull.
- Alternators (AC) vs Dynamos (DC)
- Alternators generate alternating current (AC) using two continuous slip rings and brushes. On a voltage-time oscilloscope trace, an alternator produces a sine wave that alternates between +V and -V (peak p.d. occurs when the coil is horizontal and cutting field lines at 90°; zero p.d. occurs when the coil is vertical and moving parallel to field lines). Dynamos generate direct current (DC) using a split-ring commutator, which swaps contacts every half-turn so the voltage-time graph stays entirely positive (bumps above the x-axis).
Mitosis (2n → 2n) vs Meiosis (2n → n) & Ribosome Protein Synthesis
1. Interphase (DNA Replication)
Diploid parent cell (2n = 46 in humans). DNA replicates so each chromosome consists of two identical sister chromatids; subcellular organelles (mitochondria, ribosomes) also double.
Diploid germ cell (2n = 46). DNA replicates once to form homologous pairs of doubled chromosomes ready for two rounds of division.
Key Concepts & Exam Definitions (6)
- The Generator Effect (Electromagnetic Induction)
- If an electrical conductor moves relative to a magnetic field (or if the magnetic field around a stationary conductor changes) so that the conductor cuts across magnetic field lines, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, an induced current flows. Magnitude of induced p.d. is increased by: moving the wire/magnet faster, using a stronger magnetic field, or using a coil with more turns. Reversing the direction of motion or the polarity of the magnet reverses the direction of the induced p.d./current.
- Opposing Magnetic Field (Conservation of Energy)
- An induced current always generates a magnetic field that opposes the original change (either the movement of the conductor or the change in magnetic field). For example, pulling a North pole out of a solenoid induces a South pole at that end of the solenoid to attract the magnet back and resist the pull.
- Alternators (AC) vs Dynamos (DC)
- Alternators generate alternating current (AC) using two continuous slip rings and brushes. On a voltage-time oscilloscope trace, an alternator produces a sine wave that alternates between +V and -V (peak p.d. occurs when the coil is horizontal and cutting field lines at 90°; zero p.d. occurs when the coil is vertical and moving parallel to field lines). Dynamos generate direct current (DC) using a split-ring commutator, which swaps contacts every half-turn so the voltage-time graph stays entirely positive (bumps above the x-axis).
- Moving-Coil Microphones
- Use the generator effect to convert pressure variations in sound waves into variations in electrical current (the exact reverse of a loudspeaker). Sound waves hit a flexible diaphragm, causing it and an attached coil of wire to vibrate back and forth inside the magnetic field of a permanent magnet. As the coil cuts the magnetic field lines, it induces an alternating potential difference and current at the exact same frequency as the sound wave.
- How a Transformer Works (AC Only)
- A basic transformer consists of a primary coil and a secondary coil wound on a soft iron core. 1. An alternating potential difference (AC) applied across the primary coil causes an alternating current to flow. 2. This produces an alternating (constantly changing) magnetic field in theprimary coil and throughout the soft iron core. 3. The changing magnetic field in the iron core cuts through the secondary coil, inducing an alternating potential difference across the secondary coil (at the same frequency).
- Transformer Equations (Turns Ratio & 100% Efficiency)
- 1. Turns ratio equation: V_p / V_s = n_p / n_s (where V_p and V_s are the potential differences across the primary and secondary coils in V, and n_p and n_s are the number of turns on the primary and secondary coils). In a step-up transformer, n_s > n_p so V_s > V_p; in a step-down transformer, n_s < n_p so V_s < V_p. 2. Power equation (assuming 100% efficiency, so electrical power output = electrical power input): V_s x I_s = V_p x I_p.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Why is zero potential difference induced when a wire is moved parallel to magnetic field lines?
What happens to the peak voltage and the frequency on an alternator's oscilloscope trace if the coil is rotated twice as fast?
Compare the rings used in an AC alternator versus a DC dynamo.
Compare a loudspeaker and a microphone in terms of energy transfer and physics effect.
State the two transformer equations on the GCSE Physics equation sheet.
What is a switch-mode transformer and how does it differ from a traditional transformer?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Explain how a transformer with more turns on its secondary coil than on its primary coil steps up an alternating potential difference, and explain why a transformer will not work with a 12 V DC car battery. (Triple Higher Tier) [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. An alternating potential difference applied across the primary coil causes an alternating current to flow in the primary coil [1 mark]. 2. This alternating current produces a constantly changing (alternating) magnetic field in the primary coil, which magnetises the soft iron core [1 mark]. 3. The soft iron core channels this changing magnetic field through the secondary coil; as the changing magnetic field cuts the secondary coil, it induces an alternating potential difference across the secondary coil (by the generator effect) [1 mark]. 4. Because there are more turns on the secondary coil than on the primary coil (n_s > n_p), more field lines are cut, so the induced secondary potential difference is greater than the primary potential difference (V_s > V_p) [1 mark]. 5. A 12 V DC battery supplies a constant direct current, which creates an unchanging (static) magnetic field in the iron core; since the magnetic field is not changing, zero potential difference is induced in the secondary coil [1 mark].
- Q2
A laptop charger contains a step-down transformer connected to the 230 V UK mains supply. The primary coil has 1,150 turns and the secondary coil has 100 turns. Assuming the transformer is 100% efficient and the laptop draws a current of 3.45 A from the secondary coil, calculate: (a) the output potential difference across the secondary coil, and (b) the current in the primary coil. (Triple Higher Tier) [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. (a) State the transformer turns ratio equation: V_p / V_s = n_p / n_s [1 mark]. 2. Substitute values: 230 / V_s = 1,150 / 100 = 11.5 [1 mark]. 3. Solve for secondary p.d.: V_s = 230 / 11.5 = 20 V [1 mark]. 4. (b) State the 100% efficiency transformer power equation: V_p x I_p = V_s x I_s [1 mark]. 5. Substitute values and solve for I_p: 230 x I_p = 20 x 3.45 = 69 W -> I_p = 69 / 230 = 0.30 A [1 mark].
- Q3
Explain how a moving-coil microphone converts the sound waves of a singer's voice into an electrical signal that can be sent to an amplifier. (Triple Higher Tier) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Longitudinal sound waves (pressure variations in the air) hit the flexible diaphragm of the microphone, causing the diaphragm to vibrate back and forth at the same frequency as the sound wave [1 mark]. 2. A coil of wire attached to the diaphragm vibrates back and forth inside the magnetic field of a cylindrical permanent magnet [1 mark]. 3. As the moving coil cuts across the magnetic field lines, a potential difference is induced across the ends of the coil by the generator effect [1 mark]. 4. Because the coil continuously changes direction as it vibrates, it induces an alternating current (AC) in the microphone circuit with the exact same frequency and amplitude pattern as the original sound wave [1 mark].
- Q4
A student drops a bar magnet North-pole first through a vertical coil of wire connected to a sensitive centre-zero ammeter. Describe and explain what happens to the ammeter reading as the magnet (a) enters the top of the coil, (b) is momentarily at the middle, and (c) falls fast out of the bottom of the coil. (Triple Higher Tier) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. (a) As the North pole enters the top of the coil, the coil cuts the changing magnetic field lines, inducing a current that deflects the ammeter needle in one direction (e.g., to the positive right) [1 mark]. 2. By Lenz's Law, the top of the coil becomes an induced North pole to repel the approaching magnet and oppose the motion [1 mark]. 3. (b) In the middle, the reading drops through zero and reverses direction [1 mark]. 4. (c) As the South pole leaves the bottom of the coil, the magnetic field is changing in the opposite sense and the magnet has accelerated under gravity (moving faster), so the ammeter needle deflects in the opposite direction (negative left) with a larger peak reading [1 mark].
Common Exam Questions (4)
What is the key difference between an alternator and a dynamo?
Both rotate a coil inside a magnetic field to induce electricity via the generator effect, but they use different connections to the external circuit. 1. An alternator uses slip rings and brushes, so each side of the coil stays connected to the same brush; as the coil rotates, the induced potential difference and current reverse every half-turn, producing Alternating Current (AC). 2. A dynamo uses a split-ring commutator and brushes, which swaps the connections every half-turn so the current in the external circuit always flows in the same direction, producing Direct Current (DC).
Why does an induced current always oppose the change that made it (Lenz's Law)?
An induced current generates its own magnetic field around the coil, and this magnetic field always acts to oppose the original change (movement or magnetic field change) that caused it. For example, if you push the North pole of a bar magnet into a coil, the near end of the coil becomes a North pole to repel your magnet, meaning you have to do mechanical work pushing the magnet in (conserving energy).
Why do transformers only work with Alternating Current (AC) and not with Direct Current (DC)?
This is a guaranteed 3-mark exam question. To induce a potential difference in the secondary coil via the generator effect, the magnetic field in the iron core must be constantly changing. A direct current (DC) in the primary coil produces a constant, unchanging magnetic field, so zero potential difference is induced in the secondary coil. An alternating current (AC) constantly changes direction and magnitude, producing a constantly changing magnetic field in the iron core that cuts the secondary coil and induces an alternating potential difference.
Why is the core of a transformer made of soft iron, while the coils are made of insulated copper wire?
Soft iron is a magnetic material that is easily magnetised and demagnetised as the AC changes direction 50 times a second, channeling and strengthening the changing magnetic field from the primary coil to the secondary coil. The coils are made of low-resistance copper wire coated in plastic insulation so that the current flows around all the turns of the coil rather than short-circuiting through the metal iron core.
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