Physics - Electromagnets

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  • Created by: Chesca
  • Created on: 16-05-13 16:26

Electromagnets

Like poles repel ; unlike poles attract

Made of insulated wire wrapped around an iron bar (core)

  • current passed along wire, magnetic field created
  • magnetic field magnatises iron bar strongly

Current switched off = iron bar loses magnetisum

Iron ideal over steel, as Iron can lose magnetisum when current switched off....Steel can't!

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Uses of Electromagnets

  • Scrapyard crane: Steel frame of vehicle sticks to electromagnet, when current passed through coil.
  • When current switched off...steel frame is dropped
  • The Circuit Breaker: switch in series with an electromagnet
  • switch held in place by 'iron catch'
  • too much current passes through, magnetic field stronger, switch pulled open
  • Electric Bell: Bell connected to battery, iron armature pulled onto electromagnet.
  • Switch opens, electromagnet switched off
  • As result...armature springs back, switch closes
  • The Relay: used to switch electrical machine(motor) on or off.
  • Current passes through electromagnet, armature pulled onto magnet.
  • Armature turns about pivot and closes switch gap.
  • Small current switches on a larger current
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The Motor Effect

When a current is passed along a wire in a magnetic field and the wire is not parallel to the lines of the magnetic field, a force is exerted on the wire by the magnetic field.

Relies on:

  • Magnetic field
  • Current
  • Force

Force increased by: increasing current/using stronger magnet/greatest when wire is perpendicular to magnetic field

Direction of force at right angles to wire and field.

  • Direction of force is reversed, direction of current & magnetic field also reversed.
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Electric Motor

Uses motor effect (control speed using current) (reverse direction by reversing current)

  • Rectangular coil of insulated wire (armature coil) forced to rotate
  • Coil connected to two 'brushes' to the battery
  • brushes press on split ring commutator(reverses the current round the coil every half turn)

When current passed through coil, it spins because:

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

 

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The Loudspeaker

Designed to make diaphram attached to coil vibrate when alternating current passes through the coil

When current passes through coil, force due to motor effect makes coil move.

Each time current changes direction, force reverses it's direction. So coil is repeatedly forced backwards and forwards.

  • Motion makes diaphram vibrate so sound waves created :)
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Electromagnetic Induction

The process of inducing (creating) a potential difference (voltage) in a wire by moving the wire so it cuts across the lines of force of a magnetic field.

  • If part of a complete circuit, an electric current passes around the circuit
  • Connect insulated wire to ammeter.
  • Move wire between the poles of a U-shaped magnet and watch the ammeter.
  • Ammeter pointer should deflect as a current is generated when wire cuts across magnetic field.

Ammeter deflects = movement of wire causes induced pd

  • creating current which shows on the ammeter
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Transformers

Electricity enters your house from the National Grid, via cables.

  • Alternating pd (voltage) is used
  • Transformer = changes the size of the alternating pd

Has two coils of insulated wire, both wound around the same iron core

  • Alternating current in primary coil
  • Creates a changing magnetic field in the iron core
  • This induces (creates) an alternating pd in the secondary coil as it cuts through the magnetic field

If bulb connected across secondary coil, induced pd creates alternating current in sc = bulb lights up. Electrical energy has been transferred from primary to secondary

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Types of transformers

Step -up transformer = makes pd across secondary coil greater than pd in primary coil. Secondary coil has more turns of coil!

Step-down transformer = makes pd across secondary coil less than pd in primary coil. Secondary coil has fewer turns of coil!

  • used in low voltage supply to step mains pd down to 230V

 

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Transformers in action

Transformers ONLY work with alternating current

  • direct current = no changing magnetic field - secondary pd is 0!

Primary and secondary coils in 'practical transformer' are wound around same part of coil

  • Core is layered 'laminated' to cut out induced currents in iron layers.
  • If not laminated, effiency would decrease!

 

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Switch mode transformers

Different to other transformers:

  • operates at frequencies between 50,000 Hz & 20,000 Hz.
  • Has a 'ferrite' core, much lighter than iron core.

Used in mobile phone chargers:

  • light and smaller than traditional transformer
  • uses little power when no device connected across its output terminals

 

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Transformer equation and stuff

The higher the grid pd, the greater the efficiency of transferring electrical power through the grid.

  • Lowering pd means less heat and energy is wasted!
  • pd across primary (Vp)        =     number of turns on primary (Np)
  • pd across secondary (Vs)    =    number of turns on secondary (Ns)

Step up transformer = number of secondary turns is greater than primary turns

Step down transformer = number of secondary turns is less than primary turns

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Efficiency

Transformers are almost 100% efficient

Power supplied to transformer= primary current X primary pd

Power delievered by transformer = secondary current X secondary pd

For 100% efficiency....

  • power supplied to transformer = power delievered by transformer

 

 

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Physics case study

We use physics in hospitals whenever:

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

Measure blood pressure, ECG potential differences and exposure to ionising radiation in hospitals.

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

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