How does a step-up transformer differ from a step-down transformer in terms of coil construction?

A step-up transformer has more turns on the secondary coil than the primary coil to increase voltage. Conversely, a step-down transformer has more turns on the primary coil than the secondary coil to decrease voltage.

What is the relationship between voltage and current in a transformer, assuming it is 100% efficient?

They are inversely proportional. If the transformer steps up the voltage, the current must decrease by the same factor to ensure the total power ($P = V \times I$) remains constant.

Why are step-up transformers used at power stations instead of sending electricity directly at low voltage?

Stepping up the voltage reduces the current flowing through the transmission cables. Since heating in wires is proportional to the square of the current ($P = I^2R$), a lower current results in significantly less energy being wasted as heat.

Why will a transformer not work if connected to a 12V DC battery?

Transformers require a changing magnetic field to induce a potential difference in the secondary coil. A DC battery provides a constant current, which creates a static magnetic field that cannot induce voltage.

What is a common misconception regarding the iron core's role in a transformer?

Many believe the iron core conducts electricity from the primary to the secondary coil. In reality, the core only conducts the magnetic field; the two coils are electrically insulated from each other.

What happens to the efficiency of the National Grid if step-up transformers are removed?

Efficiency would drop drastically because electricity would be transmitted at higher currents. This would cause the transmission wires to heat up significantly, wasting a large portion of the generated energy.

Define the 'Primary Coil' in the context of a transformer.

The primary coil is the input coil of the transformer that is connected to the initial alternating current power source. It generates the initial changing magnetic field in the core.

What is the function of the 'Secondary Coil'?

The secondary coil is the output coil where a potential difference is induced by the changing magnetic field of the core. It delivers the modified voltage to the rest of the circuit.

What is an 'Alternating Current' (AC)?

AC is an electric current that periodically reverses direction and changes its magnitude continuously with time. This constant change is what allows transformers to operate via induction.

State the conservation of power equation for an ideal transformer.

The equation is $V_p I_p = V_s I_s$, where $V$ is potential difference, $I$ is current, and the subscripts $p$ and $s$ refer to the primary and secondary coils respectively.

Use of Transformers | AQA GCSE Science

1. Definition & Core Concepts

A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It is primarily used to change the potential difference (voltage) of an alternating current (AC) supply.

The device consists of two distinct coils of wire: the primary coil (input) and the secondary coil (output). These coils are wrapped around a common magnetic iron core, which facilitates the transfer of magnetic flux between the two circuits.

Transformers only function with alternating current (AC) because a changing magnetic field is required to induce a voltage in the secondary coil; a steady direct current (DC) would produce a static magnetic field and no induction.

Diagram of a transformer showing primary and secondary coils wound around a rectangular iron core.

2. Underlying Principles

The operation of a transformer is based on the principle that an alternating current in the primary coil creates a continuously changing magnetic field within the iron core. This changing field passes through the secondary coil, inducing an alternating potential difference across its ends.

The ratio of the potential difference across the primary and secondary coils is directly proportional to the ratio of the number of turns of wire on each coil. This relationship allows for precise control over the output voltage.

The iron core is used because it is a soft magnetic material that is easily magnetized and demagnetized, ensuring that the magnetic field is efficiently guided from the primary to the secondary coil with minimal loss.

3. Methods & Techniques

Step-up Transformers

Function: These increase the potential difference from the input to the output.
Construction: They feature more turns of wire on the secondary coil than on the primary coil ( $N_s > N_p$ ).
Application: Used at power stations to boost voltage to hundreds of thousands of volts before transmission through the National Grid.

Step-down Transformers

Function: These decrease the potential difference to safer, usable levels.
Construction: They feature fewer turns on the secondary coil than on the primary coil ( $N_s < N_p$ ).
Application: Used in local substations and near homes to reduce voltage to standard domestic levels (e.g., 230V in the UK).

4. Key Distinctions

Feature	Step-up Transformer	Step-down Transformer
Voltage Change	Increases ( $V_{out} > V_{in}$ )	Decreases ( $V_{out} < V_{in}$ )
Current Change	Decreases ( $I_{out} < I_{in}$ )	Increases ( $I_{out} > I_{in}$ )
Coil Ratio	More turns on Secondary	More turns on Primary
Grid Location	Between Power Station and Cables	Between Cables and Consumers

The inverse relationship between voltage and current is critical: as voltage is stepped up, the current is stepped down. This is governed by the principle of conservation of energy, where (assuming 100% efficiency) $P = V_p I_p = V_s I_s$ .

5. Exam Strategy & Tips

Efficiency Logic: Always link 'high voltage' to 'low current' and 'low current' to 'reduced thermal energy loss'. This is the most common multi-mark explanation required in physics exams.
Coil Identification: When given a diagram, count the loops. If the output side has more loops, it is a step-up transformer. If it has fewer, it is step-down.
The AC Requirement: If a question asks why a transformer won't work with a battery (DC), the answer must state that DC provides a constant magnetic field, whereas induction requires a changing magnetic field.
Sanity Check: Ensure your calculated output voltage matches the transformer type. A step-up transformer should never result in a lower output voltage than the input.

6. Common Pitfalls & Misconceptions

Energy Creation: A common mistake is thinking transformers 'create' power. In reality, they only change the ratio of voltage to current; the total power output can never exceed the power input.
Core Material: Students often confuse the iron core with the wires. The core is NOT there to conduct electricity between the coils (the coils are insulated); it is there to conduct the magnetic field.
Current vs. Voltage: Many learners incorrectly assume that increasing voltage also increases current in the transmission lines. In the context of a transformer, $V$ and $I$ are inversely proportional to keep Power ( $P=VI$ ) constant.

Use of Transformers

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Use of Transformers

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Methods & Techniques

Step-up Transformers

Step-down Transformers

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Step-up Transformers

Step-down Transformers