AQA Physics Magnetic Fields Section 7 Transformers

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

current Magnetic Field produced by an AC Current If an AC current is passed through a coil it produces an alternating magnetic field. time

Transformers A transformer consists of two coils: A primary coil and a secondary coil. A soft iron core exists through the primary coil to maximise the magnetic field it produces. An alternating current is passed through the primary coil.

Transformers A transformer consists of two coils: A primary coil and a secondary coil. A soft iron core exists through the primary coil to maximise the magnetic field it produces. An alternating current is passed through the primary coil.

Transformers A transformer consists of two coils: A primary coil and a secondary coil. A soft iron core exists through the primary coil to maximise the magnetic field it produces. An alternating current is passed through the primary coil.

Transformers A transformer consists of two coils: A primary coil and a secondary coil. A soft iron core exists through the primary coil to maximise the magnetic field it produces. An alternating current is passed through the primary coil.

Transformers A transformer consists of two coils: A primary coil and a secondary coil. A soft iron core exists through the primary coil to maximise the magnetic field it produces. An alternating current is passed through the primary coil.

Transformers A transformer consists of two coils: A primary coil and a secondary coil. A soft iron core exists through the primary coil to maximise the magnetic field it produces. An alternating current is passed through the primary coil.

Transformers A soft iron core exists above through the secondary coil and above the two coils to maximise the flux linkage through the secondary coil. The alternating magnetic flux produced by the primary coil passes through the secondary coil (the magnetic field lines cut through he wires in the secondary coil). An EMF/current is induced in the secondary coil.

Primary Coil Number of Turns = N P Secondary Coil Number of Turns = N S Transformers VPVP VSVS (for a perfectly efficient transformer) If V S >V P – a “step-up” transformer If V S < V P – a “step-down” transformer

Transformers (for a perfectly efficient transformer) To conserve energy: When voltage is stepped up, current is stepped down by the same amount. When voltage is stepped down, current is stepped up by the same amount. Energy must always be conserved. The rate at which energy is input into the primary coil must be equal to the rate at which energy is output from the secondary coil.

Inefficiencies in Transformers The efficiency of a transformer can be expressed as follows: All transformers are inefficient for a number of reasons…. Heat Loss in Wires Electrical energy in converted into the internal energy in the wires of the coils due to their own resistance.

Eddy Currents Electrons are free to move in the soft iron core that exists between and links the two coils. The changing magnetic field around the core causes these electrons to circulate (induced current). These currents are known as Eddy currents. Electrical energy in converted into the internal energy in the iron core due to the resistance to these Eddy currents. Inefficiencies in Transformers The efficiency of a transformer can be expressed as follows: All transformers are inefficient for a number of reasons…. changing magnetic field circulating Eddy current

Eddy Currents Eddy currents are reduced by laminating the core. The thin strips only accommodate small Eddy currents therefore keeping the amount of heating and loss of energy to a minimum. Inefficiencies in Transformers The efficiency of a transformer can be expressed as follows: All transformers are inefficient for a number of reasons…. Laminated Transformer Core

Flux Losses Not all of the magnetic flux from the primary coil links with the secondary coil. Inefficiencies in Transformers The efficiency of a transformer can be expressed as follows: All transformers are inefficient for a number of reasons…. Coils are usually wound on top of each other for this reason. Primary Coil Secondary Coil

Hysteresis Losses Soft magnetic materials such as iron (the material in the core of the coils of a transformer) gains and looses magnetism easily – which is why it is used as the core material (it is nit desirable for the core to remain magnetic when the coil is turned off). Iron is not perfectly soft. Once magnetised in one direction it resists being magnetised in the reverse direction. This reversal requires energy which heats up the coil. Inefficiencies in Transformers The efficiency of a transformer can be expressed as follows: All transformers are inefficient for a number of reasons….

At the consumer end the voltage is stepped down to more useable voltages. Transportation of Electricity Power Station Consumer Step-down Transformer High Voltage Low Current Step-up Transformer The voltage is stepped up using a transformer at the power station. This steps down the current. Electricity is transported at lower current to reduce loss of energy due to heating of wires.