1. Effects of an ElectricCurrent Chap 24
2. Types of effects• Heating effect• Chemical effect• Magnetic effect
3. Heating effect• When current flows in a wire, the wire heats up (e.g. Kettle)• Exp. To show the heating effect of an electric current p 273• Joule studied this effect and found experimentally that the amount of heat energy given out by a wire depends on several factors-
4. W (heat energy ) ∝ I2 ∝R ∝ t (time)THUS W ∝ R I2t W = k R I2 tFor SI units the constant =1 W = R I2 t
5. Joule’s Law• Power = rate at which work is done = rate at which heat is produced = W /t• P = R I2 Joule’s Law• The rate at which heat is produced in a conductor is directly proportional to the square of the current provided the resistance is constant P ∝ I2• Questions p274
6. Experiment• To verify Joule’s Law (i.e. to verify that the rise in temperature (which is a measure of the power generated) of a wire in a given time is proportional to the square of the current• p275
7. High Voltage energytransmission.• Electricity is generated at power stations and transmitted to consumers via wires.• As the electricity passes along the wires, the wires will heat• According to Joule’s law, the rate at which they heat (power) is proportional to the (current)2
8. • The electrical energy which heats the wires is useless, and so should be minimised• Thus, before transmission the electricity is transformed to a very low current (and consequently a very high voltage).
9. • When this travels through the wires a small amount is lost as heat in the wires (since heat (power) generated is proportional to the current squared).• This high voltage signal is too dangerous for domestic use, so it is transformed back to lower voltages, higher currents before it enters houses.
10. Chemical effect of anelectric current• When an electric current passes through a liquid it may cause a chemical reaction to occur in the liquid-electrolysis• The liquid is called the electrolyte• The metal plates placed in the liquid are called the electrodes (+ plate = anode, -plate = cathode)
11. • The entire system is called a voltameter• See exp. P277• If the electrodes take part in the chemical reaction they are called active electrodes, if they don’t, they are called inert or inactive electrodes
12. Electrolysis of water• Hydrogen and oxygen form at the two electrodes in the ratio 2:1 (see diagram p278)
13. Uses of electrolysis• Used for electroplating-coating one metal with a thin layer of another metal in order to prevent rusting and also to improve the appearance• Used to purify metals and to extract metals from their ores• Used to coat a thin layer of dielectric on to the plate of an electrolytic capacitor
14. Experiment• To demonstrate the chemical effect of an electric current p277• To investigate the variation of current with pd for copper sulphate solution with copper electrodes p278
15. I and V for differentconductors• 1. a metallic conductor-if the temperature remains fairly constant, the resistance is constant and I is directly proportional to V I V
16. • 2. A Filament bulbAs current flows through the filament it heats and so the resistance increases, so increasing V does not result in as big an increase in I I V
17. • 3. A semiconductor (e.g. a thermistor)• As the semiconductor heats up its resistance drops so that a given change in voltage will produce a bigger change in current I V
18. • 4. An electrolyte-in this case the resistance stays constant, so there is a straight line relationship. For active electrodes the graph obeys Ohm’s Law I Electrolyte with active electrodes V
19. • 5. If the electrodes are inert then the system behaves as a cell, with an emf. The applied voltage must be bigger than this emf before current will start to flow I Electrolyte with inert electrodes V
20. • 6. A Gas - Discharge tube+ - Gas at low pressureDue to cosmic rays and backgroundradiation there are always some ions beingformed in the tube, which recombine overtime. If a p.d. is applied across the tube,these ions move towards the electrodes,and so a current flows. As the p.d.increases the no. of ions and thus thecurrent increases (region O→A in graph)