DCcircuitheeeeeeeeeeeeeeeeeeeeeeeeesry.pptx

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1. DC circuit theory

2. A-level: A battery maintains an electric field through the circuit. This enables it to do work on charges wherever there is a potential difference e.g. in a filament. Electromotive force is the energy supplied per unit charge. (work done on each coulomb of charge) Potential difference (p.d.) is the energy transferred per unit charge between 2 points in a circuit. (work done by each coulomb of charge) Unit (for both) is the volt = joule/coulomb EMF and potential difference

3. Resistors in series V = V 1 + V 2 [conservation of energy] IR = IR 1 + IR 2 R = R 1 + R 2 R is always larger than any of R 1 , R 2 etc Resistors in parallel I = I 1 + I 2 [conservation of charge] V / R = V / R 1 + V / R 2 1/ R = 1/ R 1 + 1/ R 2 R is always smaller than any of R 1 , R 2 etc Resistor networks

4. Useful for constructing sensors In pairs, sketch a dark sensor a heat sensor a cold sensor Potential dividers

5. Real power supplies Demonstrations: 12V DC supply lighting more and more lamps in parallel EHT with a 1.5V lamp What’s happening?

6. Graphical representation

7. Resistance in a wire microscopic picture: free electrons drifting through a metal (polycrystalline, each crystal having an ionic lattice) constant of proportionality is resistivity, unit  m a material property Compare with rules for R networks. VPL simulation.

8. Current and charge Current is rate of flow of charge e.g. 10 16 electrons pass a point every second Demonstration: Conduction by ‘coloured’ ions

9. Drift velocity where n is the number of free electrons per unit volume A is the cross sectional area Δ x is a small length along the wire e is the charge of an electron

10. Comparing copper with tungsten The difference in drift velocities explains why incandescent lamps glow white hot while their connecting wires stay safely at room temperature. metal electrons per m 3 electron drift velocity in mm s -1 copper 8.5 × 10 28 ~0.02 tungsten 3.4 × 10 28 ~250

11. Conductivity Metal wires conduct extremely well. Conductance G = I / V , unit siemens (symbol S) depends on the number of carriers available ratio I / V is 'effect per unit of cause‘ Note: conductance is the reciprocal of resistance conductivity,   [unit S m -1 ] is the reciprocal of resistivity

12. Capacitance a measure of how much charge a capacitor can separate at a given p.d. unit of capacitance: farad (symbol F) demonstration super-capacitor Note: There are rules for adding capacitors in networks.