Chapter 20 Electric Current and Ohm’s law

Chapter 20 Electric Current and circuit Resistance and Resistivity Ohm’s Law Energy and Power in Electric Circuits Ammeters and Voltmeters

Learning Objective Focus on Conservation law of charge Conservation law of energy 3

3 What would happen? More electric potential difference - more charges move 3 Video - electric current animation

4 Electric current The flow of charge particle from one place to another. The electric current is defined the rate of flow of electric charge. 4

Example 6 q=1.00 C I=0.300 mA

How fast current runs? In a common DC circuit, electrons move at speeds of A.a fraction of a centimeter per second B.many centimeters per second C.the speed of light D.the speed of a sound wave 7

Lights come on as soon as a switch is flicked. Most electrical signals carried by currents travel at a significant fraction of the speed of light. Electrical shock wave moves through the system at nearly the speed of light. 8

5 What do you need to light a bulb? 5

Electric circuits - A closed path through which charge can flow, returning to its starting point. A simple circuit and the standard schematic representation of a battery, conducting wire, and load (a resistor). Schematics are very useful in visualizing the main features of a circuit. 3

Electric Current The direction of current flow – from the positive terminal to the negative one. Conventional current – the flow of + charge

Electric Current A battery uses chemical reactions to produce a potential difference between its terminals. It causes current to flow through the flashlight bulb similar to the way the person lifting the water causes the water to flow through the paddle wheel.

Relationship 13

Resistance and Ohm’s Law The current that flows through most substances is directly proportional to the voltage V applied to it. This important relationship is called Ohm's law. It can be viewed as a cause-and-effect relationship, with voltage the cause and current the effect. I ∝ V.

Resistance and Ohm’s Law If voltage drives current, what blocks current movement it? The electric property that blocks current (crudely similar to friction and air resistance) is called resistance R. Resistance is defined as inversely proportional to current, or

Conduction of Electricity and Heat Good electrical conductors are often good heat conductors, too. This is because large numbers of free electrons can carry electrical current and can transport thermal energy. Moving charges collide with atom and molecules in substance and transfers energy to substance. 16

Resistance and Ohm’s Law Resistance – the property of matter determining how much current will flow. Solving for the resistance; The units of resistance, volts per ampere, are called ohms:

18 Check Your Understanding Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply. A.decrease the voltage B.decrease the resistance C.increase the voltage D.increase the resistance 18

19 Check Your Understanding The current through a 10 ohm resistor connected to a 120 V power supply is A.1 A B.10 A C.12 A D.120 A 19

20 Check Your Understanding When a 10 V battery is connected to a resistor, the current in the resistor is 2 A. What is the resistor's value? A.2 Ω B.5 Ω C.10 Ω D.20 Ω 20

21 Check Your Understanding A 10 Ω resistor has a 5 A current in it. What is the voltage across the resistor? A.0.5 V B.2 V C.15 V D.50 V 21

22 Use the Ohm's law equation to determine the missing values in the following circuits. 22

23 Check Your Understanding If the resistance of a circuit were tripled, then the current through the circuit would be ____. A.one-third as much B.three times as much C.unchanged D.... nonsense! There would be no way to make such a prediction. 23

Resistance and Ohm’s Law Ohm's law really defines resistance for certain materials. Ohm's law is not universally valid. The many substances for which Ohm's law holds are called ohmic. Ohmic materials have a resistance R that is independent of voltage V and current I. These include good conductors like copper and aluminum.

Resistance and Ohm’s Law Simple circuits with resistors that have different resistances and thus different slopes.

Resistance and Ohm’s Law The materials which follow Ohm's law by having a linear relationship between voltage and current are known as ohmic materials. Some materials exhibit a nonlinear voltage-current relationship and hence are known as nonohmic materials.

Resistance and Ohm’s Law The lengths and diameters of four wires are given below: Are they all have same or different resistances if they connected in electric circuit which has same current and voltage?

Resistance and Ohm’s Law Two wires of the same length and diameter will have different resistances if they are made of different materials. The resistance of an object depends on its shape and the material of which it is composed. This property of a material is called the resistivity.

Check understanding The lengths and diameters of four wires are given below: If the four wires are made from the same material, which of the following is true? Select two answers. A.Resistance of Wire 3 > Resistance of Wire 2 B.Resistance of Wire 1 > Resistance of Wire 2 C.Resistance of Wire 1 < Resistance of Wire 4 D.Resistance of Wire 4 < Resistance of Wire 3

Resistance and Ohm’s Law The difference between insulators, semiconductors, and conductors can be clearly seen in their resistivities:

Check understanding Two lamps, one with a thick filament and one with a thin filament of the same material, are connected in series to a battery. The voltage is greater across the lamp with the A.thick filament B.thin filament C.current is the same in both

Electric Power and Energy Power – the rate of energy use Electric energy depends on both the voltage involved and the charge moved. PE = qV 4

The electric power 6

Example A 120 V motor operates at 13 A. Determine the power and the energy used in one hour of operation. 10

The cost of Electricity The more electric appliances you use and the longer they are left on, the higher your electric bill. kilowatt-hours (kWh) are a measure of energy. E = Pt

Example If the cost of electricity in your area is 12 cents per kWh, what is the total cost of using a 60-W incandescent bulb for 1000 hours if the bulb cost 25 cents? If we replace this bulb with a compact fluorescent light that provides the same light output, but at one-quarter the wattage, and which costs $1.50 but lasts 10 times longer (10,000 hours), what will that total cost be? 36

Alternating Current versus Direct Current Once the current is established, it is thus also a constant. Direct current (DC) is the flow of electric charge in only one direction. It has a constant-voltage circuit. Alternating current (AC) is the flow of electric charge that periodically reverses direction. If the source varies periodically (sinusoidal), the circuit is known as an alternating current circuit. 37

Alternating Current versus Direct Current 38 DC voltage and current are constant in time, once the current is established. A graph of voltage and current versus time for 60-Hz AC power. The voltage and current are sinusoidal.

39 Some circuit symbols used in schematic diagrams are shown below. 39

Ammeters and Voltmeters An ammeter is a device for measuring current. The current in the circuit must flow through the ammeter; therefore the ammeter should have as low a resistance as possible, for the least disturbance.

Ammeters and Voltmeters A voltmeter measures the potential drop between two points in a circuit. It therefore is connected in parallel; in order to minimize the effect on the circuit, it should have as large a resistance as possible.