Chapter 21 Electric Current & DC Circuits

Transcription

1 Chapter 21 Electric Current & DC Circuits Optional: Ammeters and Voltmeters (21-8) Optional: RC Circuits (21-9) Water

2

3

4

5 U

6 Reservoirs store potential energy in water at high elevation. This energy comes from the Sun.

7 Figure 21-3

8 Electric Current Electric current is the flow of electric charge from one place to another. If a charge ΔQ flows past a point in time Δt, then there is a current: In an electric circuit, charge always returns to its starting point (eg., a battery).

9 Current is defined as the amount of positive charge that flows in a chosen direction. (regardless of the type of particle that s actually moving) Examples: -In electronic devices, electrons (-) flow -In human nerve cells, ions (eg. Ca+, Na+, K+) flow If the chosen direction is up, then: -Positive charge flowing up or -Negative charge flowing down have the same effect.

10 Figure 21-2

11 Parts of a Circuit: 1. Battery: Provides charges with potential energy 2. Wire: made of a conductor 3. Load: component that does something, eg a light bulb 4. Switch: creates an open circuit (off) battery has + and - poles or terminals A circuit with no load is called a short circuit

12 Short Circuit! In a short circuit, a high current flows from one terminal of the battery to another....so high that the wire can heat up and start a fire. To prevent this, a fuse can be placed in the circuit. The fuse will blow if the current exceeds a certain level (eg 12 Amps)

13 A blown fuse results in an open circuit: no current flows. It must be replaced. A circuit breaker is a switch that prevents short circuits.

14 Batteries A battery uses chemicals to make a voltage difference (ΔV) between its (+ and -) terminals. This ΔV pushes charges around, creating a current. A battery s Voltage (Vbat) is also represented as: battery symbol NOTE: the term Electromotive Force is just a Voltage (units: V)... not a force.

15 Its OK to assume all currents are + charges in motion. They have high potential energy when near the + pole. They are attracted to the - pole. Battery

16 Insulators resist the flow of electrons... They have higher resistivity But, it s more useful to describe how a single object (eg light bulb) resists the flow of Current when a Voltage difference is placed on it. This quantity is called: RESISTANCE (R)

17 The flow of current (I) depends on the voltage difference (V) across an object: Solving for the resistance, we find The units of resistance, volts per ampere, are also called ohms (Ω)

18 A space heater Electrons in the wire bump around & heat up

19 Simple circuit: Battery & Light Which direction does current flow?

20 Circuit model: light is represented as a resistor 600 Question: What Current flows through this circuit?

21 Power I just switched off a small desk lamp and a large searchlight. Which used more energy? The searchlight consumes more energy each second. It has a larger Power. Power = Energy per time. Energy = Power x time. Units: Watts = Joules per second. (W=J/s)

22 Power in Electric Circuits The purpose of circuits is to do work from us: to convert electrical potential energy into some other useful type of energy. When charge (Q) moves through a voltage (V), its potential energy (U) changes: The change in energy per second is the power: =J/s

23 Power in Electric Circuits Power can also be written in terms of V, I, R *. The power used by a resistor R is: The energy supplied to a circuit by a battery becomes: light, sound, mechanical work, or heat, etc. Question How many Joules of energy does a 100 Watt lightbulb use in 1 hour?

24 Energy and Power Units Power is expressed in Watts (W). 1 Watt= 1Joule per second (W=J/s) REPEAT: Watt = 1 Joule Per Second (also kilowatts, megawatts, gigawatts... etc) Anything that consumes or produces energy over time has a power associated with it. appliances, computers require certain power a light bulb requires, eg. 40 Watts. a power plant provides 1000 MW = 1GW of power. Um, is that 1 GW per day?

25 Energy and Power Units Energy is power x time. Pay electric bills? You are buying energy. You must pay for every Joule of energy you use. But...silly electricity companies measure in energy a different unit: the kilowatt-hour (kwh). A kilowatt is unit of power A kilowatt-hour (kwh) is unit of energy. Each kilowatt-hour of electricity costs about $0.10

26 Resistors in Series Resistors connected end-to-end are said to be in series. The same current flows through each. They can be replaced by a single resistor with an equivalent resistance (Req). This simple circuit can now be solved with Ohm s Law: V= E = I R

27 Resistors in Series The current through each resistor is the same. So the total voltage difference is the sum of the voltage differences across each resistor. Hence: