Electric current

1. Electric current Physics 122 Lecture VII

2. Concepts • Primary concepts: • Electric current • Resistor and resistivity • Electric circuit Lecture VII

3. Laws • Ohm’s law • Power in electric circuits Lecture VII

4. Electric current - - - - - - - + + + + + + + • A flow of charge is called an electric current Note: net charge =0 • It is measured in ampere (A=C/s) • Need free charge to have electric current. Use conductors. Lecture VII

5. Skiing  electric circuit High PE High PE Low PE Skiers Charges go from points with high PE to low PE To complete the circuit need a device that brings you back to high PE: Ski lift Battery Low PE Lecture VII

6. Electric circuit • Need free charge  electric circuit must consist of conductive material (wires). • Electric circuit must be closed. • Battery supplies constant potential difference – voltage. e - • Battery converts • chemical energy into • electric energy. Symbol for battery Lecture VII

7. Electric circuit b). Will not work, Circuit is at the same potential (+), no potential difference - voltage. c). Will work. a). Will not work, Circuit is not closed Lecture VII

8. Ohm’s law • Electric current is proportional to voltage. • Coefficient in this dependence is called resistance R • Resistance is measured in Ohm (W = V/A) I R V Lecture VII

9. Resistors • First digit • Second digit • Multiplier • Tolerance • 2.5 x103W +- 5%. Lecture VII

10. Resistivity • traffic Electric current • Long narrow street  high resistance • Condition of the road  material property called resistivityr. r is measured in W m L – length of the conductor A – its area. Lecture VII

11. Resistance and Temperature • When electrons move through the conductor they collide with atoms: • Resistivity grows with temperature ( more collisions) r0 – resistivity measured at some reference temperature T0 a – temperature coefficient of resistivity Lecture VII

12. Resistance and Temperature • When electrons move through the conductor they collide with atoms: • Temperature of the conductor increases because of the current (through collisions) • Electrical energy is transformed into thermal energy • Resistors dissipate energy • Power – energy per unit of time- (in W=J/s) dissipated by a resistor Lecture VII

13. Electric power • Electric energy can be converted into other kinds of energy: • Thermal ( toaster) • Light (bulbs) • Mechanical (washer) • Chemical • Electric power (energy per unit of time): Lecture VII

14. Test problem • You have an open working refrigerator in your room. It makes your room • A hotter • B colder Lecture VII

15. Test problem • A light bulb is connected to a battery. It is then cooled and its resistance decreased. Brightness is proportional to consumed power. The light bulb burns • A Brighter • B dimmer P=IV P=I2R P=V2/R Lecture VII

16. Alternating current (AC) • Voltage changes sign  current changes the direction I Req ~ Lecture VII

17. Electric circuits: resistors • Current in=current out I1=I2 • No electrons are lost inside • Resistors dissipate power (energy/time) • P=I2R • Drop of voltage over a resistor DV=-IR: • V2=V1-IR R I2,V2 I1,V1 Lecture VII

18. Electric circuits: wires I2,V2 I1,V1 • We assume that wire have very small resistance (R=0) • Current in=current out I1=I2 • Power dissipated in wires • P=I2R=0 • Drop of voltage over a resistor DV=-IR=0 • V2=V1 • From the point of electric circuit wires can be • stretched, • Bended • Straightened • Collapsed to a point without changing the electrical properties of the circuit I2,V2 I1,V1 I2,V2 I1,V1 Lecture VII

19. Electric circuit: battery R1 R2 R3 Energy conservation • Drop of voltage in electric circuit is always equal to voltage supplied by an external source (e.g. battery). • Current (the effective flow of positive charge) goes from + to – • Electrons (negative charge!) go from – to + I V Lecture VII

20. Electric circuits: branches • Charge is conserved • Current – what goes in, goes out I1 I I2 I I3 V Lecture VII