P3 Using magnetic fields to keep things moving

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  • Created by: Phoebe
  • Created on: 06-05-13 12:04

3.1 Electromagnets

The force between two magnets: like poles repel, unlike poles attract.

A magnetic field line, or line of force is the line along which a plotting compass points.

An electromagnet consists of a coil of insulated wire wrapped round an iron core.

When a current flows through a wire, a magnetic field is produced around the wire. 

When the current is switched off the iron loses its magnetism. This temporary magnetism makes electromagnets very useful.

When a current flows through the wire the iron becomes strongly magnetised.

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3.2 The motor effect

When we place a wire carrying an electric current in a magnetic field, it may experience a force. This is called the motor effect.

In the motor effect, the force:

  • is increased if the current or the strength of the magnetic field is increased
  • is at right angles to the direction of the magnetic field and to the wire
  • is reversed if the direction of the current or the magnetic field is reversed

An electric motor has a coil which turns when a current is passed through it.

Fleming's left hand rule is used to determine the direction of the force.


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More motor effect

  • the speed of a motor is increased by increasing the size of the current
  • the direction of the motor can be reversed by reversing the direction of the current

When a current passes through the coil, the coil spins because:

  • a force acts on each side of the coil due to the motor effect
  • the force on one side of the coil is in the opposite direction to the force on the other side

The force is a maximum if a wire is at an angle of 90 degrees to the magnetic field, and at zero is the wire is parallel to the magnetic field.

The "split-ring" commutator reverses the direction of the current around the coil every half-turn.

Because the sides always swap over each half-term, the coil is always pushed.

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3.3 Electromagnetic induction

If an electrical conductor "cuts" through magnetic field lines, a potential difference is induced across the ends of the conductor.

Electromagnetic induction is the process of creating a potential difference using a magnetic field.

When a conductor cuts the lines of a magnetic field, a potential difference is induced across the ends of the conductor.

When an electromagnet is used, it needs to be switched on or off to induce a potential difference.

A potential difference is induced only when the wire or coil and the magnetic field move relative to each other.

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More electromagnetic induction

The size of the induced potential difference is increased by increasing:

  • the speed of movement
  • the strength of the magnetic field
  • the number of turns on the coil

Potential difference is induced only when the wire or coil and the magnetic field move relative to each other.

If the direction of movement of the wire or coil is reversed, or the polarity of the magnet is reversed, the direction of the reduced potential difference is also reversed.

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3.4 Transformers

A transformer consists of two coils of insulated wire, called the primary and secondary coils.

These coils are wound onto the same iron core.

When an alternating current passes through the primary coil, it produces an alternating magnetic field in the cores, which continually expands and collapses.

A transformer only works on a.c. because a changing magnetic field is necessary to induce a.c. in the second coil.

A step up transformer makes the p.d. across the secondary coil greater than the pd across the primary coil. Its secondary coil has more turns than its primary coil.

Vice versa for a step down transformer

A switchmode transformer is lighter and smaller than an ordinary transformer, and operates at a high frequency.

There is no current in the iron core, just a magnetic field.

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More about transformers

A switch mode transformer:

  • operates at a much higher frequency 
  • is lighter and smaller
  • uses very little power when there is no device connected across its output terminals
  • has a ferrite core

Iron can be magnetised.

A cell only supplies d.c. current, and cannot be used for a transformer.

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3.5 Transformers in action

The higher the potential difference at which electrical energy is transmitted across the Grid, the smaller the energy wasted in the cables.

Transformers are used to step potential differences up or down.

The transformer equation is:

primary pd, Vp/ secondary coil, Vs = Np/Ns

Where: Np = no. primary turns     Ns = no.secondary turns

For a step-down transformer, Ns is less than Np.

For a step-up transformer, Ns is greater than Np.

For a 100% efficient transformer: Vp x Ip = Vs x Is

Where: Ip = primary current   1s = secondary coil

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3.6 A physics case study

We use physics in hospital whenever:

  • blood pressure (or temperature) is measured
  • an ECG recording is made
  • an endoscope is used
  • a scanner is used

We measure blood pressure, ECG potential differences and exposure to ionising radiation is hospitals.

A CT scanner uses X-rays, which are ionising radiation and can therefore damage living cells.

  • CT scanner: used to give a picture of organs in the body
  • An endoscope: used to observe cavities in the body
  • An ECG machine: used to measure the PD generated by the heart
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