Electromagnetic Induction and Transformers
Electromagnetic induction, the generator effect, is the process of inducing a potential difference, and a current in a complete circuit, across a conductor by changing the magnetic field around it, for example by moving a wire through a magnetic field or rotating a coil inside one. It is the reverse process to the motor effect and is the working principle of generators, which produce electricity from movement, and transformers, which change the size of an alternating voltage.
Method
- Recognise the three basic ways to induce a potential difference: move a straight conductor through a magnetic field, move a magnet into or out of a stationary coil, or rotate a coil inside a magnetic field; in every case, what matters is that the magnetic field experienced by the conductor is changing.
- Learn the four factors that increase the size of the induced potential difference (and current): a stronger magnetic field, a greater number of turns on the coil, a larger area of the coil, and a faster rate of movement.
- Remember that reversing the direction of movement, or the direction of rotation, reverses the direction (polarity) of the induced potential difference and current.
- For a rotating-coil generator, know that as the coil turns, the rate at which it cuts through field lines constantly changes, so the size and direction of the induced potential difference vary throughout each rotation. A generator that uses slip rings produces alternating current (a.c.), which regularly reverses direction, giving an alternator, while a generator that uses a split-ring commutator produces direct current that always flows the same way, giving a dynamo.
- For transformer questions, learn the structure: a primary coil and a secondary coil, both wound around the same soft iron core, which are not electrically connected to each other. An alternating current in the primary coil produces a constantly changing magnetic field in the core, which induces an alternating potential difference in the secondary coil.
- Use the turns-ratio equation, primary voltage / secondary voltage = number of primary turns / number of secondary turns (Vp / Vs = Np / Ns), to calculate an unknown voltage or number of turns. A step-up transformer has more turns on the secondary coil than the primary (Vs greater than Vp), and a step-down transformer has fewer turns on the secondary than the primary (Vs less than Vp).
- Use the transformer power equation, primary voltage x primary current = secondary voltage x secondary current (Vp Ip = Vs Is), which assumes the transformer is 100 percent efficient. Remember a transformer that steps voltage up must step current down by the same factor, and vice versa.
- For National Grid questions, explain why electricity is transmitted at a very high voltage and correspondingly low current: a lower current through the transmission cables means less energy is dissipated as heat due to the resistance of the cables, which makes transmission more efficient, so step-up transformers raise the voltage for transmission and step-down transformers lower it again to a safe, usable voltage before it reaches homes.
Worked example
A step-down transformer has 2000 turns on its primary coil and 100 turns on its secondary coil. The primary coil is connected to an alternating supply of 230 V. Calculate the secondary (output) voltage.
- Write the turns-ratio equation: primary voltage / secondary voltage = primary turns / secondary turns (Vp / Vs = Np / Ns).
- Rearrange to make secondary voltage the subject: Vs = Vp x Ns / Np.
- Substitute the values: Vs = 230 x 100 / 2000.
- Calculate: 230 x 100 = 23000, then 23000 / 2000 = 11.5.
- Final answer: the secondary voltage is 11.5 V, and since Ns is less than Np this confirms the transformer is a step-down transformer, as stated.
Practice questions
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Q1State two ways of inducing a potential difference across a conductor using a magnetic field.Show answer
Answer: Any two of: move the conductor through a magnetic field; move a magnet into or out of a coil; rotate a coil inside a magnetic field. The key requirement each time is a changing magnetic field around the conductor.
Q2State two factors that increase the size of the potential difference induced in a coil.Show answer
Answer: Any two of: a stronger magnetic field; a greater number of turns on the coil; a larger coil area; a faster rate of movement or rotation.
Q3State the difference between an alternator and a dynamo.Show answer
Answer: An alternator uses slip rings and produces alternating current (a.c.), which regularly reverses direction; a dynamo uses a split-ring commutator and produces direct current (d.c.), which always flows in the same direction.
Q4A transformer has 500 turns on its primary coil and 2500 turns on its secondary coil. State, with a reason, whether this is a step-up or a step-down transformer.Show answer
Answer: Step-up, because the secondary coil has more turns than the primary coil (Ns greater than Np), so the secondary voltage will be greater than the primary voltage.
Q5A transformer's primary coil is connected to a 12 V supply and has 60 turns. Calculate the number of turns needed on the secondary coil to produce an output of 3.0 V.Show answer
Answer: 15 turns (Ns = Np x Vs/Vp = 60 x 3.0/12 = 60 x 0.25 = 15)
Q6A transformer has a primary current of 2.0 A and a primary voltage of 230 V. The secondary voltage is 460 V. Assuming the transformer is 100 percent efficient, calculate the secondary current.Show answer
Answer: 1.0 A (Vp Ip = Vs Is, so Is = Vp Ip / Vs = 230 x 2.0 / 460 = 460/460 = 1.0 A)
Q7Explain why electricity is transmitted through the National Grid at a very high voltage rather than at the voltage used in homes.Show answer
Answer: A high transmission voltage means a lower current is needed to transmit the same power; a lower current means less energy is wasted as heat, dissipated due to the resistance of the transmission cables, so transmission is more efficient.
Q8State the name of the component at the centre of a transformer's primary and secondary coils, and state its purpose.Show answer
Answer: A soft iron core; it carries and concentrates the changing magnetic field from the primary coil to the secondary coil so that a potential difference is induced efficiently in the secondary coil.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A step-up transformer is used to increase the voltage from a 400 V generator to 11000 V for transmission along a power line. The primary coil has 200 turns. Calculate the number of turns needed on the secondary coil.
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A transformer has a primary voltage of 230 V and a primary current of 4.0 A. The secondary voltage is 46 V. Calculate the secondary current, assuming the transformer is 100 percent efficient.
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Explain how a step-up transformer and a step-down transformer are used together to transmit electrical power efficiently through the National Grid from a power station to homes, referring to voltage, current and energy losses.
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