Electric Current and Resistance

Chapter 17

Batteries

Batteries create a difference in potential [J/C] between two leads called the anode and the cathode.

Anode and cathode are different types of metal which react with the electrolyte (solution) inside the battery.

Anode is the positive side and cathode is the negative side.

Chemical energy transforms to electrical energy.

Batteries

The battery is capable of maintaining a difference in potential energy.

Any device that can maintain a potential difference is called a power supply.

Batteries create DC (direct current) because charge flows in one direction.

Terminal Voltage vs. Emf

Emf stands for electromotive force but it is NOT a force but a voltage!

Emf gives the potential difference across a battery when nothing is connected (no current flows) – this is a maximum voltage

When current flows through the battery, the battery provides some internal resistance that slightly reduces this Emf

Terminal voltage is the ‘operating voltage’ of a battery.

Normally Emf and terminal voltage are essentially the same.

V = Emf - IR

Emf

A ‘non-ideal’ battery has a large internal resistance.

Circuit Symbols Learn the basic

symbols for creating electric circuits!

A circuit is a complete loop through which current can flow.

Practice

Use the appropriate symbols to sketch a complete circuit containing two 6 V batteries in series wired to two identical capacitors in parallel, followed by two resisters in series.

Current

Static electricity (chapters 15, 16) refers to charges that are not moving.

Electric current refers to charges that flow.

Electric current tells how much charge flows per second

I = q/t [Coulomb/sec] = [Ampere] = [A]

Current Electric current give

the charge flowing past a particular area per second.

Though it is electrons that actually flow, current is defined as the flow of positive charge.

Example

Suppose there is a steady current of 0.50 A in a flashlight bulb lasting for 2.0 minutes. How much charge passes through the bulb in this time? How many electrons does this represent?

Drift Velocity

Electrons in a wire don’t ‘flow’ in the same manner as water in a pipe.

In the absence of a potential difference, V, the electrons in a conductor move randomly at high speeds, making many collisions with atoms.

When a potential difference is applied, this random motion changes: electrons begin to drift in the direction of the voltage.

Drift Velocity Electrons move

opposite the direction of the electric field.

When voltage is applied, their random motion becomes slightly less random…

Homework

# 10 - 13, 21 - 24 page 586

Also # 1 – 7, 15 - 19 if not already done

Resistance and Ohm’s Law

Current flows less easily through thinner wires than through thicker wires.

Materials that resist the flow of electric current are caller resistors.

Resistance is the opposition to the flow of electricity.

For a given voltage difference, current will be smaller if the resistance of a material is higher.

R = V/I

Resistance and Ohm’s Law

R = V/I [Volt/Amp] = [Ohm] = [Ω]

V = IR is Ohm’s Law

If you know the total resistance in a circuit powered by a particular voltage, you can find the current.

Example

Any room in the house that is exposed to water and electrical voltage can present hazards. For example, suppose a person steps out of a shower and inadvertently touches an exposed 120 V wire (frayed end of the hairdryer) with a wet finger. When wet, the human body has a resistance of only 300Ω. Find the current in the person’s body.

Factors Influencing Resistance

Resistance is inversely proportional to the cross sectional area of a wire and directly proportional to length:

R = ρ L/A where ρ is the materials resistivity

Resistivity

The resistivity, ρ, of a material may increase with temperature.

ρ=ρ0(1+αΔT) where α = temperature coefficient of resistivity

R = R0(1+αΔT)

Resistivities

Example

A platinum wire has a resistance of 0.5 Ω at zero degrees Celsius. It is placed in a water bath where its resistance rises to 0.6 Ω. Find the temperature of the water bath.

Superconductivity Resistance increases as temperature

increases. Therefore resistance decreases as

temperature decreases… Superconductivity occurs when the

resistance is exactly zero. Temperatures near 100K produce

superconductivity (very difficult to achieve outside of a lab environment)

Electric Power

Power = Work/time = V·q/t = V·I = [J/C][C/s]

[Watt] P = VI The power provided by a battery as it

pushes charge through a potential difference P = VI.

This formula is valid as long as voltage and current are constant over time.

Power

Power is also used (dissipated) by each resistor in the circuit (resistors turn energy into heat)

P = VI = (IR)I = I2R P = VI = V(V/R) = V2/R

Example

Consider two appliances that operate at the same voltage. Appliance A has a higher power rating than Appliance B. a) How does the resistance of A compare with the resistance of B?

Example

A computer system includes a monitor with a power requirement of 200 W, whereas a countertop broiler/ toaster oven is rated at 1500 W. Calculate the resistance of each if they are designed to run at 120 V?

Summary

V = IR Ohm’s Law P = VI Power P = I2R = V2/R

If power rating is higher, resistance is lower for appliances operating at the same voltage.

Homework

Read Examples 17.7 and 17.8 on pages 582 – 582

Do # 27 – 29, 36, 38, 42, 44, 48, 52, 62, 63, 66, 68, 72, 73, 78, 79 Chapter 17.