Chapter 22 Current and Resistance

Contents Electric Current Batteries and EMF Resistance and Resistivity Ohm’s Law and simple circuits Energy and Power

Electric Current

Current (I): Rate of flow of electric charge. Unit: Coulomb/second = Ampere (A) Example: Charge flows thru a wire. 3 Coulombs of charge flows past some point every 6 seconds. How much current is there? River current: Flow of water Electric current: Flow of charge Andre Ampere ( )

Plasma Globe: Electric Current thru Ionized Gas Plasma The globe is filled with neon and argon gas. Electricity is used to create a high electric field which ionizes the gas: Electrons are stripped from the neon atoms, leaving positively-charged neon atoms. This mixture of positive and negative charges is called a plasma: The 4 th state of matter. The plasma charges can conduct electricity along pathways which excite the neon atoms and cause them to glow.

Conductors Conductors: Materials with lots of “loose” charges available to flow. Examples: METALS

Insulators Insulators: Materials with no loose charges, so no current can flow. Examples: Plastic, glass, rubber, …

Positive or Negative Current? A frequent source of confusion when studying electricity is that inside metal wires, it is electrons thus (-) charges that carry current, however we will always define current as the flow of (+) charge! Thus positive current flows in the opposite direction as the electrons in the wire. Current flows from higher potential (+) to lower potential (-) I

Conservation of Current Electric current, like water flowing thru pipes, is conserved: Current can’t be created or destroyed inside the wires. The current going into a junction equals the current going out.

Current Continuity (a)Which way does current flow in this circuit? (b)A light bulb’s brightness depends on how much current flows thru it. Which light bulb will be brighter? (a)Which way does current flow in this circuit? (b)Rank the bulbs from brightest to dimmest.

Batteries and EMF

Electric Circuit (1) Electric circuits are similar to moving water thru pipes using a pump: The same amount of current must flow thru each part of the loop.

Electric Circuit (2) Electric circuit: Electric charge (current) flows thru wires. A battery or power supply causes the current to flow.

Voltage Voltage: “Electrical Pressure” that causes current to flow. Unit: Volt (V) More accurate definition: Voltage is how much energy (Joules) each charge has, or how much work each charge can do: Volt = Joules/Coulomb From plumbing analogy: It is the pressure difference that causes water to flow. It is the voltage difference (potential difference ΔV) that causes current to flow:

“Battery Man” A battery uses chemical potential energy to move the charges. Chemical energy  Electrical energy Circuit diagram:

Battery Voltage Terminology 1.5 Volts is called the: Battery voltage Terminal voltage Potential difference ΔV between the terminals EMF E “Electro Motive Force”

Batteries in Series: Increasing the Voltage 4.5 volts Each battery raises the potential by another 1.5 volts.

Resistance and Resistivity

Resistance Resistance (R) : Opposition to current flow. Unit: Ohm Symbol: What causes resistance? Collisions between electrons and atoms inside the wires. KE from electrons transferred to wire metal atoms. Demo: Use solid spring model with marbles. This is how light bulbs, toasters, and hair dryers work!

Resistance: Converting Electric Energy into Heat and Light

Demo Demo heating of paper clip using battery charger (Use 10A setting, use 2 stands, 2 clamps, 2 posts with screws to hold paper clip, pie tin to catch residue) After demo: Is it safe to touch electrodes with hands? Explain: Body’s resistance is very high compared to paper clip (High R  Low I)

Resistivity and Resistance Resistivity (ρ): A material property that measures how resistive the material is to electric current. Good conductors: Low resistivity Poor conductors: High resistivity Resistance (R) depends on the resistivity and the dimensions of the material:

Resistivity, Resistance, Resistors Resistivity (ρ): A material property that measures how resistive the type of material is to electric current. Units: Ω·m Resistance (R): A measure of how hard it is to pass current thru an object. Depends on resistivity and dimensions of object. Units: ohms (Ω) Resistor: A circuit element that intentionally adds a controlled amount of resistance to a circuit.

Ohm’s Law and Simple Circuits

The Relationship Between I, V, and R: Ohm’s Law So far we’ve learned: Voltage increases Current: Resistance decreases Current: Combine results: Ohm’s Law ΔV = I R Voltage across resistor (volts) Current thru resistor (amps) Resistance (ohms) Δ

Simple Circuits and Ohm’s Law How much current flows in this circuit? What is the battery voltage in this circuit? How much resistance does the circuit have?

Electrical Energy and Power

Electrical Power Recall from Physics 101 chapter 10: Power = Work done (or energy transferred) per unit time Electrical Power P = V I Unit: Joules/second = Watts (W)

Power Delivered Power Dissipated Power is delivered to the circuit by the battery: Power is dissipated from the circuit by the resistance: Electrical Power Delivered: P = V I V = battery voltage I = current thru battery Electrical Power Dissipated: P = V I = I² R = V² /R V = voltage across resistor = IR I = current thru resistor = V/R

Examples of Electrical Power A flashlight with a 9V battery draws 0.2 amps of current. How much power does the battery supply? How much power does the light bulb give off? Battery supplies power: P = V I = (9 V)(0.2 A) = 1.8 W Bulb dissipates power: P = V I = (9V)(0.2A) = 1.8 W Conservation of energy!!

Another example: Electrical power Your electric hairdryer is rated at 1500 W. How much energy do you use when you dry your hair in 3 minutes? Electrical energy  Heat (Thermal energy)