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1.5 ELECTRICAL PROPERTIES OF MATERIALS

1.5 ELECTRICAL PROPERTIES OF MATERIALS. 1.5.1 Electric charge 1.5.2 Electric current 1.5.3 Voltage 1.5.4 Resistance 1.5.5 Power loss. Electrostatic Force. If a charged object comes near another charged object a force acts between them. The force F is given by the equation:

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1.5 ELECTRICAL PROPERTIES OF MATERIALS

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  1. 1.5 ELECTRICAL PROPERTIES OF MATERIALS • 1.5.1 Electric charge • 1.5.2 Electric current • 1.5.3 Voltage • 1.5.4 Resistance • 1.5.5 Power loss

  2. Electrostatic Force If a charged object comes near another charged object a force acts between them. The force F is given by the equation: where Q1 and Q2 are the charges, r is the distance between them and  is the "permittivity" which is 8.85 10-12 C2N-1m-2 for air (or free space).

  3. 1.5 ELECTRICAL PROPERTIES OF MATERIALS • 1.5.1 Electric charge • 1.5.2 Electric current • 1.5.3 Voltage • 1.5.4 Resistance • 1.5.5 Power loss

  4. Electromagnetic force The force between two parallel wires carrying steady currents is: where I1 and I2 are the currents in the wires, l is the length of the wires, a is the separation of the wires  is called the magnetic permeability and has a value of 4 10-7 N/A2 for air.

  5. Alternating Current This is the easiest way to generate electricity and the current is called alternating current (ac) as opposed to direct current (dc) which is a continuous flow in one direction. In this country the current in the main supply reverses direction 50 times per second, i.e. the frequency is 50 cycles per second or 50 Hertz.

  6. Safety If you are working with high voltage direct current systems you should be aware that they are far more dangerous than alternating current. 50V dc can kill but 110V ac is normally safe.

  7. Effects of electromagnetism • If a wire carrying direct current is flowing close to a conductor opposite charges will build up on each side of the conductor. This may, for example, cause corrosion. • If a wire carrying alternating current is flowing close to a conductor it will induce a current in it. Fortunately at mains frequency this effect is not normally great but it increases with frequency. High frequency cables must therefore be "screened" with a conductor.

  8. 1.5 ELECTRICAL PROPERTIES OF MATERIALS • 1.5.1 Electric charge • 1.5.2 Electric current • 1.5.3 Voltage • 1.5.4 Resistance • 1.5.5 Power loss

  9. Definition of Voltage If the energy required to work against the electrostatic force and move one Coulomb of charge along a wire from one point to another is one Joule then the voltage between the points is one Volt. Thus:  W = QV where W is the work in Joules, Q is the charge in Coulombs and V is the voltage. Note that the voltage is also known as the potential difference.

  10. Electric Power Power is the rate of doing work and is measured in Joules/second or Watts. Current is the rate of flow of charge in Coulombs/second or Amps. Thus: P = VI where P is the power in Watts. and I is the current in Amps. This gives the rate of delivery of power down a wire.

  11. 1.5 ELECTRICAL PROPERTIES OF MATERIALS • 1.5.1 Electric charge • 1.5.2 Electric current • 1.5.3 Voltage • 1.5.4 Resistance • 1.5.5 Power loss

  12. Resistance The resistance is defined as the ratio between the voltage drop along the wire and the current in it. Thus: Where R is the resistance in Ohms ().

  13. Voltage V Resistance R Current = V/R Current = 2V/R Resistance R Effect of doubling area Current = V/R

  14. Resistivity where R is the resistance of the wire, L is its length, A is its cross sectional area and  is the resistivity of the material from which the wire is made. The resistivity does not depend on the dimensions of the wire.

  15. Capacitance When a voltage is applied across a capacitor charge is stored in it. The capacitance is defined as: capacitance = stored charge applied voltage and is measured in Coulomb/volt or Farad (named after the inventor called Faraday) A one Farad capacitor would be very large. Real capacitors are rated in MicroFarad

  16. 1.5 ELECTRICAL PROPERTIES OF MATERIALS • 1.5.1 Electric charge • 1.5.2 Electric current • 1.5.3 Voltage • 1.5.4 Resistance • 1.5.5 Power loss

  17. Equations for power Combining the equations for power loss and resistance gives: where I is the current flow along the wire and V is the voltage between the ends of the wire.

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