Physics Tuition Singapore by The Physics Coach » O-level Units http://www.thephysicscoach.com Top physics lecturer and UK PSC scholar Mon, 15 Apr 2013 03:09:40 +0000 en-US hourly 1 Monopolar Motor http://www.thephysicscoach.com/monopolar-motor/ http://www.thephysicscoach.com/monopolar-motor/#comments Tue, 28 Feb 2012 13:15:46 +0000 admin http://thephysicscoach.com/?p=326 This is an interesting application of electromagnetism that a physics teacher can show his student: a monopolar motor. The simplest type of DC motor there is.

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]]> http://www.thephysicscoach.com/monopolar-motor/feed/ 0 Fuse Blowing http://www.thephysicscoach.com/fuse-blowing/ http://www.thephysicscoach.com/fuse-blowing/#comments Sun, 05 Feb 2012 07:31:17 +0000 admin http://thephysicscoach.com/?p=196 2011 GCE O Level Physics Paper 1 Question 33

An electrical cable contains three wires: live, neutral and earth. The cable is correctly wired to a plug which contains a 3A fuse. The insulation becomes damaged and bare metal wires show. Five possible events can occur.

  • A person touches the earth wire
  • A person touches the neutral wire
  • A person touches the live wire
  • The live wire touches the neutral wire
  • The live wire touches the earth wire

How many of these five events cause the fuse in the plug to blow?

A. 1
B. 2
C. 3
D. 4

The answer is B. Only in the last two cases will there be a surge in current big enough to cause the fuse to blow as these are what constitute a short circuit. A person touching the earth or neutral wire will not experience any current.

Current is dependent on resistance and the voltage available to push it through that resistance. As the body resistance of a person is quite high, touching the live wire does not bring about a current high enough to blow the fuse, although the current might be high enough to kill you. The fuse exists to prevent fire due to overheating in the event of a short circuit, not to prevent electrocution.

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