Currents Of Electricity JPJC Notes

1 JURONG PIONEER JUNIOR COLLEGE 9749 H2 PHYSICS CURRENT OF ELECTRICITY Content 1 Electric current 2 Potential difference 3 Resistance and resistivity 4 Electromotive force Learning Outcomes Students should be able to: (a) show an understanding that electric current is the rate of flow of charge. (b) derive and use the equation I = nAvq for a current -carrying conductor, where n is the number density of charge carriers and v is the drift velocity. (c) recall and solve problems using the equation Q = It. (d) recall and solve problems using the equation V = W Q . (e) recall and solve problems using the equations P = VI, P = I2R and P = 2V R . (f) define the resistance of a circuit component as the ratio of the potential difference across the component to the current passing through it and solve problems using the equation V = IR. (g) sketch and explain the I-V characteristics of various electric al components such as an ohmic resistor, a semiconductor diode, a filament lamp and a negative temperature coefficient (NTC) thermistor. (h) sketch the resistance-temperature characteristic of an NTC thermistor. (i) recall and solve problems using the equation R = A l . (j) distinguish between electromotive force (e.m.f.) and potential difference (p.d.) using energy considerations. (k) show an understanding of the effects of the internal resistanc e of a source of e.m.f . on the terminal potential difference and output power.

2 Introduction • In everyday life, we are familiar with electric currents flowing in wires and other conductors. Electric current is said to be set up along a conductor when there is a flow of charges in a particular direction. • We first examine electric current from a macroscopic point of view: i.e. current measured using laboratory apparatus. Then, we examine it from a microscopic point as flows of electrons and their drift velocity. • Fig. 1.1 shows a circuit diagram where a direct current is flowing, driven by a cell. It also shows electrons in the wire moving away from the negative terminal (since electrons are negatively-charged and we know that like charges repel) and move towards the positive terminal (since unlike charges attract) of the cell. Fig. 1.1 Electron movement in an electric circuit Note: ❖ Scientists first thought that positive charges flow from the positive terminal of a cell to the negative terminal. This is the conventional current direction. The d irection of conventional current is thus the direction of the flow of positive charges in a circuit. ❖ However, it was later found that a current in a metal wire is in fact a flow of negatively - charged electrons in the opposite direction. That is, electrons flow from the negative (−) to the positive (+) terminal outside the battery. ❖ Nevertheless, the conventional current is still being used. ❖ When solving proble