Current and electric circuits Chapter 4 Chapter 1

resistors in series and in parallel For resistors in series, the current through each resistor is the same. The equivalent resistance equals the sum of each single resistance. For resistors in parallel, the current is split into different minor currents. The inverse of the equivalent resistance equals the sum of the reciprocals of the individual resistances.

electric power Electric power, as in dynamics, is the energy transformed by a device per unit time: The unit of power is the watt, W. For example, the resistance of a 40 W automobile headlight designed for 12 V is For ohmic devices, we can make the following substitutions: In an electricity bill, what is reported, is not the cost of power, but of energy: the power consumption multiplied by time. Energy is measured in joules, but the electricity supplier measures it in kilowatt-hours, kWh: 1 kWh = (1000 W) × (3600 s) = 3.60 x 106 J.

power in household circuits The wires used in homes to carry electricity have very low resistances. However, if the current is high enough, the power will increase and the wires can become hot enough to start a fire. To avoid this, fuses or circuit breakers are used, which disconnect the supply when the current drawn goes above a predetermined value. When the current exceeds certain value, the metallic ribbon melts and the circuit breaks. The fuse must then be replaced.

superconductivity Some materials have resistivity that suddenly drops to zero when the temperature is very low. This temperature is called the critical temperature Tc. At T < Tc, a superconducting material has zero resistivity. At Tc, the resistivity becomes non-zero and increases with temperature like other materials. C Experiments have shown that currents, once started, can flow through these materials for years without decreasing, even in the absence of a potential difference. In the Large Hadron Collider (LHC) at CERN (European Organisation for Nuclear Research) more then 1200 superconducting magnets are used to bend the path of accelerating particle beams and to keep them on course.

learning the basics j j j j j 1. The current is defined as the amount of energy per unit of time. T F 2. An electric company asks us to pay for the electric power multiplied by the time it is used. T F 3. A superconducting material has infinite resistivity when the temperature drops below a “critical temperature” Tc. T F j j j j j

applying the concepts 2. Draw: ▶ three resistors in parallel ▶ a series of three resistors ▶ a series made of two sets of two resistors in parallel ▶ two parallel series of three resistors