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The transformer equation says primary voltage ÷ secondary voltage = turns on the primary coil ÷ turns on the secondary coil. Free GCSE Physics notes.
Voltage and Turns Ratio in Transformers is part of Induced potential, transformers and the National Grid (physics only) (HT only) in GCSE AQA Higher Physics.
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The transformer equation says primary voltage ÷ secondary voltage = turns on the primary coil ÷ turns on the secondary coil. Free GCSE Physics notes.
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Use this lesson to stay inside Induced potential, transformers and the National Grid (physics only) (HT only) while connecting the detail back to the wider Magnetism and electromagnetism topic.
Read the core explanations from Voltage and Turns Ratio in Transformers before testing yourself from memory.
Your phone charger steps down 230V mains electricity to just 5V. Without this voltage transformation, your phone would be destroyed instantly. In exam answers, always state whether the transformer is step-up or step-down and link that directly to the turns ratio.
Transformers are everywhere in our electrical world. They boost voltage for efficient power transmission across the National Grid, then step it back down for safe use in homes. The key to how they work lies in a simple mathematical relationship between voltage and the number of wire turns in their coils.
Understanding this relationship isn't just academic - it's essential for calculating real-world electrical systems. You'll see this formula in every GCSE physics exam, and it's the foundation for understanding how electricity reaches your home safely.
Picture a transformer as two separate coils of wire wrapped around the same iron core. The primary coil receives the input voltage, whilst the secondary coil delivers the output voltage. The process occurs through electromagnetic induction - changing current in the primary creates a changing magnetic field that induces voltage in the secondary.
The crucial insight is this: the voltage ratio depends entirely on how many turns of wire each coil has. More turns means higher voltage, fewer turns means lower voltage. This gives us the fundamental transformer equation that governs all voltage calculations.
This equation can be rearranged to find any unknown value. If you know three of the four variables, you can calculate the fourth. The key is setting up the ratio correctly - primary values go together, secondary values go together.
The terms 'step-up' and 'step-down' describe what happens to the voltage, not the current. A step-up transformer increases voltage from primary to secondary, whilst a step-down transformer decreases it. The key is comparing the number of turns on each coil.
Here's the pattern: more turns on the secondary means step-up, fewer turns on the secondary means step-down. Your phone charger is a step-down transformer - it has many turns on the primary (connected to 230V mains) and few turns on the secondary (producing 5V output).
**Trap**: Many students think step-up means the primary voltage is higher. Actually, step-up means the secondary voltage is higher than the primary voltage. The 'step' refers to the direction of change from input to output.
Transformer calculations follow a standard pattern. You'll always use the same equation, but you need to rearrange it depending on what you're asked to find. The key is identifying which values are primary and which are secondary.
Worked example: Finding Secondary Voltage
A step-up transformer has 150 turns on the primary coil and 600 turns on the secondary coil. If the primary voltage is 25V, calculate the secondary voltage. [3 marks]
Answer: $V_s = 100V$
Notice this is a step-up transformer because the secondary voltage (100V) is greater than the primary voltage (25V). This matches our expectation since the secondary coil has more turns than the primary coil.
Worked example: Finding Number of Turns
A step-down transformer reduces 240V to 12V. If the primary coil has 2000 turns, how many turns are on the secondary coil? [3 marks]
Answer: $n_s = 100 \text{ turns}$
Transformer questions are mark-losers if you're not careful with the setup. The most common mistake is mixing up primary and secondary values when substituting into the equation. Always label your values clearly before substituting.
Another frequent error is forgetting that the equation gives you ratios, not absolute values. If you're asked whether a transformer is step-up or step-down, you must compare the voltages or turns - don't just state the type without justification.
**Trap**: Students often confuse the direction of step-up and step-down. Remember: step-up means the output (secondary) voltage is higher than the input (primary) voltage. The 'step' describes what happens to the voltage as it passes through the transformer.
Units are crucial in transformer calculations. Voltages must be in volts (V), and turns are just numbers (no units). Always include units in your final answer, and check that your answer makes physical sense - a step-up transformer cannot produce lower voltage than its input.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The transformer equation says primary voltage ÷ secondary voltage = turns on the primary coil ÷ turns on the secondary coil. Primary values go together and secondary values go together, and turns are just numbers with no units.
Compare the turns: if the secondary coil has more turns than the primary, it is a step-up transformer and the output voltage is higher; if it has fewer turns, it is a step-down transformer. Step-up means the secondary voltage is higher than the primary voltage.
A phone charger is a step-down transformer: it steps 230V mains electricity down to just 5V. It has many turns on the primary coil, connected to the mains, and few turns on the secondary coil.
Substitute into the transformer equation and solve. A transformer with 150 turns on the primary and 600 on the secondary and a primary voltage of 25V gives 150/600 = 1/4, so the secondary voltage is 25 × 4 = 100V.
Rearrange the transformer equation for the unknown number of turns. A step-down transformer that reduces 240V to 12V with 2000 turns on the primary gives 240/12 = 20 = 2000/ns, so the secondary has 100 turns.
A transformer with 150 primary turns, 600 secondary turns and 25 V on the primary gives 100 V on the secondary. Vp/Vs = np/ns gives 25/Vs = 150/600 = 1/4, so Vs = 25 × 4 = 100 V, a step-up transformer.
A step-up transformer cannot produce a lower voltage than its input; its secondary coil has more turns than its primary, so the secondary voltage is higher. A calculated answer that is lower means an error has been made.
The number of turns on a coil has no units; it is just a number. Voltages must be in volts (V), and the final answer should always include units.
The terms step-up and step-down describe what happens to the voltage, not the current. A step-up transformer increases voltage from the primary to the secondary coil.
The turns ratio compares the number of turns on the primary and secondary coils, and it equals the voltage ratio: primary voltage ÷ secondary voltage = primary turns ÷ secondary turns. More turns on the secondary means a higher output voltage.
The most common mistake in transformer calculations is mixing up primary and secondary values when substituting. Each value should be marked as primary or secondary first, with primary values together and secondary values together.
The primary coil is the input coil connected to the power source, receiving the original alternating voltage. The secondary coil is the output coil, connected to the device being powered.
Whether a transformer is step-up or step-down must be justified by comparing the voltages or turns: more turns on the secondary, so a higher secondary voltage, means step-up. Simply stating the type without comparison is not enough.
A phone charger steps down 230 V mains electricity to just 5 V; without this, the phone would be destroyed instantly. It has many turns on the primary coil and few turns on the secondary coil.