Potential and Capacitance

Potential Difference (V) Potential difference between two points is the work done to transfer a one coulomb of charge between the points. i.e V = W Q Unit Volt V or J C-1 Volt The p.d. between two points 1 volt if one joule of work is done bringing one coulomb of charge from one point to the other Potential at a point is the p.d. between a point and the Earth, where the Earth is at zero potential

Measuring Potential Electroscope Voltmeter The spread of the leaves is a measure of the potential between the leaves and the case. Voltmeter

Two oppositely charged plates are 5cm apart Two oppositely charged plates are 5cm apart. The electric field strength between them is 2000N/C. (i) What is the force on 1C placed between the plates? (ii) Find the work done in bringing the charge from one plate to another. (iii) What is the potential difference between the plates.

The potential difference between two plates is 2000V The potential difference between two plates is 2000V. An electron is released from one plate and moves to the other. What is its speed when is reaches the second plate. (e=1.6x10-19 C and m=9.1 x 10-31Kg)

Capacitance

Potential is proportional to charge V α Q C V = Q C is capacitance

Capacitance (C) Capacitance is the ratio of the charge on a conductor to its p.d. i.e. C = Q V Unit Farad F or C V-1 Capacitor stores charge Symbol

The presence of a negative plate reduces the potential of the positive plate and then more charge can be stored on this plate. This effectively increases capacitance C. Q=charge V=potential

Energy Stored in a Charged Capacitor W = ½ CV 2 Experiment to show that capacitors store energy. Connect an electrolytic capacitor to a d.c. supply. This charges the capacitor. Disconnect the capacitor from the power supply and connect to a bulb. The bulb lights showing that there is energy stored in a capacitor.

Conduction in Capacitors Capacitors conduct AC. Capacitors do not conduct DC. AC The bulb lights for a short while as charge builds up on the capacitor, then it goes out as capacitors do not conduct DC. The bulb stays lit.

A Capacitor

Factors affecting capacitance Capacitance is proportional to area of overlap. Capacitance is inversely proportional to distance between plates. Capacitance is proportional to the permittivity of the dielectric Parallel Plate Capacitor C =  A d A = area of overlap of plates d = distance between plates  = permittivity of dielectric (insulator between plates)

Experiment investigating the factors affecting capacitance. Apparatus: multimeter set to read capacitance, 2 metal plates Method: Connect the plates to the multimeter. Reduce the overlap area and note that the capacitance drops. Increase the distance between the plates and note that the capacitance decreases. Conclusion: Capacitance is proportional to area of overlap and inversely proportional to the distance between the plates.

Capacitors Uses of Capacitors Tune radio stations (variable capacitor) Smooth out variations in d.c. (Allow a.c. to flow but block d.c.) Camera flash

Potential Difference (V) This can also be said to be the energy lost by 1 coulomb as it moves between 2 points in a circuit Note: W = VQ Divide both sides by t (time) W = VQ t t P = VI (P = W and I = Q ) t t

Electromotive Force (e.m.f.) e.m.f. (E) is a voltage applied to a cct. Unit Volt Sources of potential/voltage/emf Simple Cell Primary Cell Secondary Cell Thermocouple Mains Electric cell is a device that converts chemical energy into electrical energy and is a source of e.m.f.

Simple Cell Copper and zinc plates are electrodes (negative cathode and positive anode) Copper sulphate with dilute sulphuric acid is the electrolyte This simple cell can’t be recharged as the chemicals are used up as a current flows e- Anode Cathode Cu + Zn - Zn Cu2+ Cu Zn2+

Primary Cell This type of cell can’t be recharged. Also known as dry cells as the electrolyte tends to be a chemical paste.

Secondary Cell This is a cell that can be recharged Also known as an accumulator Car battery is a lead-acid accumulator