Electric Circuits

Symbol, units, alternative units for charge

Q, coulomb (C), 1 e.c \= 1.6x10^-19 C

Symbol and units for electric potential (voltage)

V, volt (V), J/C

Equations not in packet for electric potential- relating Ep/E and q

Ep = qv

E = qV

Equations not in packet for electric potential relating W and q

W\= qv

equation not in packet for electric potential relating E and d

v \= Ed

Units/symbols for energy

E, Joule (J). 1 eV \= 1.6x10^-16 J

Units/symbols for current

I, ampere (A), C/s

equation for current not in packet

I \= change in q /change in t

drift speed equation

rearrange I \=nAvq (in packet)

symbol for charge density

n

define charge density

number of charge carriers per unit volume (m^3)

units and symbols for resistance

R, ohm, V/A

units and symbols for resistivity

p, ohm x meter

Units and symbols for power

P, watt (W) 1 W \= 1 J/s, \= 1 VA

units and symbols for energy, work

E, W, Q, joule

How does a battery cause a light bulb to light up?

one battery terminal is at a high electric potential (+), the other is at a low (-) one. this difference in potential makes an electric field in both the wire and the bulb's filament, causing free electrons in the circuit to start moving toward the positive terminal. Along the way they hit the positive stationary lattice ions, transferring kinetic energy. The increase in Ek in lattice ions in the filament becomes thermal energy, and the filament gets hot enough to glow.

Wire R is cut into thirds of equal length, arranged in parallel. What is the equivalent resistance?

R/q

How many MJ in one Kw/Hr?

3.6 x 10^6

A copper wire of diameter .65 mm carries a current of .25 A. There are 8.5x10^28 charge carriers in each cubic meter of copper. Calculate the drift speed of the charge carriers

1. find A using pi r ^2
2. v = I/nqA
3. I = .25, n = 8.5x10^28, q = 1.6x10^-19, A = pir^2
4. =5.5x10^-5 m/s

What is the cross-sectional area equation?

A \= pi x (r ^2)

What is the difference between a source of emf and a potential difference?

EMF \= voltage rise, potential increase. Potential difference \= voltage drop, potential decrease

What does it mean when something is a source of EMF

it converts from some other form of energy into electrical energy

What does it mean when something is a source of potential difference?

it converts from electrical energy into some other form of energy

What is a cell?

a container in which a chemical reaction occurs to convert chemical potential energy into electrical energy. A source of EMF

What is a battery?

Two or more cells connected together

What is a primary cell?

Non-rechargeable cell

What is a secondary cell?

A rechargable cell

A cell-phone battery is marked as 90 Ma h 12 V 1.08 Wh. What quantity is being measured as 90 mAh?

The charge. 90 x 10^-3 c/s x 3600 s \= 324 c

Define (charge) capacity

A quantity used to measure the ability of a cell to release charge

A battery whose capacity is 90 MA h means that before it dies you can run it

a. At 90 mA for x hours

b. at 45 mA for x hours

c. at 9 mA for x hours

a. 1 hour \n b. 2 hours \n c. 10 hours

Ideal cell vs real cell

Ideal has no internal resistance and voltage across the terminals (terminal potential difference) is constant over time

Real has small internal resistance that increases over time as chemicals are used up. Voltage across terminals (terminal potential difference) decreases over time

Describe the terminal potential difference of a real cell

decreasing over time

Define EMF

electromotive force: the total energy per unit charge supplied around a circuit by the battery

Energy that is used by both the exterior circuit and the interior chemical processes of the cell

Remains constant as the battery charges

Define terminal voltage (Vterm)

The potential difference across the terminals of the battery.

The energy that is available for use by the exterior circuit.

Decreases as the battery charges

In an ideal cell, how are emf and Vterm related

emf \= Vterm

In a real cell, how are EMF and Vterm related?

emf \> Vterm

How does terminal potential difference vary with time in a cell? Draw the graph on page 4.

Vterm loses its initial value quickly, then has a stable, constant value for most of its life, then rapidly decreases to zero as the cell discharges completely

In which direction should current flow to recharge a secondary cell, and why?

Backwards through the cell, from positive to negative to reverse the chemical reaction within the cell.

describe the current in a series circuit

the same for all resistors. It \= I1 \= I2

describe the voltage in a series circuit

split in proportion to resistance. Vt \= V1 + V2

describe the resistance in a series circuit

The total adds up. Rt \= R1 + R2

describe the power (measure of brightness) in a series circuit

Total adds up. Pt \= P1 + P2

write the Ratio Relationship for series circuits

R1/R2 \= V1/V2

What is a potential divider?

Resistors in series split the potential of the source between them

describe the current in a parallel circuit

split in inverse proportion to resistance. It \= I1+I2

describe the voltage in a parallel circuit

same for all resistors. Vt \= V1 \= V2

describe the resistance in a parallel circuit

total adds down. Rt \= (1/R1 + 1/R2)^-1

describe the power (measure of brightness) in a parallel circuit

total adds up. Pt \= P1 + P2

write the Ratio Relationship for parallel circuits

R1/R2 \= I2/I1 (bigger R gets less current)

What is an ammeter? What is the resistance of an ideal ammeter? Where should an ammeter be placed relative to the battery in a circuit?

Ammeter measures current, ideally 0 R, placement is in series with battery

What is a voltmeter? what is the resistance of an ideal voltmeter? where should a voltmeter be placed relative to the battery in a circuit?

Measures potential difference, ideally infinite R, placement is in parallel with battery

When there are two emfs in series in the same direction, how do you find voltage?

add the voltages

When there are two emfs in series in opposite directions, how do you find the voltage?

subtract the voltages

When there are two cells in parallel how do you find the emf?

same emf, add internal resistances

What are kirchoff's circuit laws?

1. Total voltage rises = total voltage drops
2. total current into a junction = total current out

difference between potentiometer and variable resistor

sliding connector on potentiometer can be a variable resistor. Adding multiple connections onto a potentiometer can create a potential divider (like a light with a dimmer switch)

Define r

Internal resistance- the resistance supplied by materials within the cell

Define V

change in energy over unit charge (joules per coulomb)

define I

change in charge over change in time (coulombs per second)

Define R

the ratio of potential difference applied across a material to the current going through the material

what is the unit for emf

epsilon (E)

what is IR

Vterm, the external voltage drop outside the cell

what is Ir

the internal voltage drop across the cell

what is E\=I(R+r) saying?

total energy supplied around a circuit by the cell is equal to the internal voltage drop across the cell plus the external voltage drop outside the cell

When does emf \= Vterm?

When no current is flowing- infinite resistance, an ideal cell, or an open circuit

When does emf \= IR + Ir?

When external resistance R is much greater than internal resistance r

When does emf \= Ir

during a short circuit, when I \= Imax

what is a short circuit

When maximum current flows through the circuit

What is Ohm's law?

when a conductor is at a constant temperature the current flowing through it is proportional to the potential difference across it (V is proportional to I)

What is an ohmic device? give an example

a device that obeys ohm's law- has a constant resistance. EX: a resistor

What is the slope on a graph at any given point with I as the y value and V as the x value?

1/R

R= V/I at any given point, regardless of ohmic or non-ohmic characteristics

What is a non-ohmic device? Give an example

A device that doesn't obey ohm's law- resistance isn't constant. EX: a filament lamp

Describe the slope of a filament bulb with I as the y-value and V as the x-value

starts at a constant rate, then decreases into a plateau after a certain point (when the lightbulb filament heats up)

Which variable increases faster on an I-V graph for a non-ohmic device?

V increases faster than I

What is the relationship between slope and resistance for an I-V graph of a non-ohmic device?

no relationship (except at start when slope is linear)

What is a diode?

a semiconductor device that only lets current flow in one direction

What is a light-dependent resistor (aka light sensor)?

a photo-conductive cell. When light strikes it, charge carriers are released- the more light that strikes it, the more carriers are released, and the more resistance decreases.

Relationship: as light intensity increases, effective resistance decreases

A circuit is set up so a regular resistor is above a light-dependent resistor. A voltmeter is connected in parallel with the regular resistor.

As light intensity increases, what happens to the reading on the voltmeter and the total current across the circuit?

The voltmeter's reading would increase. The voltage across the light-dependent resistor would decreases because the voltage drop across a resistor is proportional to its resistance. Because the total voltage must match the voltage supplied by the cell, the voltage drop across the regular resistor must increase.

Current would decrease as Rtotal decreases

What are some practical applications for LDRs

night lights, security lights

What is a thermistor/temperature sensor

a sensor that works based on temperature. As the thermistor gets hotter, more charge carriers are released and its resistance goes down

A circuit is set up so a regular resistor is below a thermistor. A voltmeter is connected in parallel with the regular resistor.

As temperature increases, what happens to the reading on the voltmeter and the total current across the circuit?

The reading on the voltmeter increases. The voltage across the thermistor would decrease as its resistance decreases, and so the voltage drop across the regular resistor would have to increase so the total voltage drop across the two is still equal to Vterm.

The total current through the circuit would decrease as the total resistance decreases.

What is a practical application for a thermistor

stovetop heat lights (lights that turn on when a stove is hot)

What is the symbol for a light-dependent resistor

What is the symbol for a thermistor

Picture a circuit with a lamp in series with a variable resistor (0-10 ohms), and a battery with an emf of 12 volts. Describe the current and voltage drop across the lamp when R \= 0

Lamp gets maximum current and potential difference

Realistically, it still doesn't get 12 volts because of the internal resistance of the battery and the variable resistor

Picture a circuit with a lamp in series with a variable resistor (0-10 ohms), and a battery with an emf of 12 volts. Describe the current and voltage drop across the lamp when R \= max

The potential difference is divided between the lamp and the variable resistor. The lamp gets the minimum amount of current and potential difference.

This minimum amount can never be zero though.

What is a potentiometer?

a type of variable resistor with 3 contact points

Define variable resistor and identify its symbol

a resistor that allows you to change how much resistance it supplies

What are potentiometers commonly used for?

a potential divider to measure the I-V characteristics (the relationship between current and voltage) of a device

Draw a diagram of a dimmer switch with a potentiometer, labeling contact points

check the diagram on page 19.

Should have a circle with a knob indicating the dimmer switch, 3 circuit lines representing a resistive strip connecting the circle to contact 3 points of a rectangle which symbolizes the potentiometer

Picture a potentiometer connected to a dimmer switch. Think of the 3 points of contact as A, B, and C. As the slider is rotated towards point C, what happens to the resistance between points AB, BC, and AC?

AB: resistance increases

BC: resistance decreases

AC: stays the same (its the total resistance)

What is the symbol for a potentiometer

When you turn the knob of a dimmer switch, you're changing where the arrow is.

Total resistance stays the same

Resistance at each point changes as arrow moves

A lamp is connected in parallel with variable resistor, so that the connecting arrow is above the resistor entirely. Describe the total resistance and the voltage, current, and brightness of the lamp

Total resistance is at its lowest (because they're completely in parallel)

The lamp has its maximum voltage drop. Vl = Vtotal (except for internal resistance of the batteries/wires)

The lamp has its maximum current and brightness

A lamp is connected in parallel with a variable resistor so that the connecting arrow is halfway down the variable resistor. Describe the voltage, current, and brightness of the lamp

The voltage of the lamp is less than the total voltage of the circuit

The lamp is dim

The current across the lamp is less than the total current in the circuit

A lamp is connected in series with a variable resistor so that the connecting arrow is beneath the variable resistor. Describe the voltage, current, and brightness of the lamp

There is no voltage drop across the lamp (assuming lamp has 0 internal resistance)

There is no current across the lamp

Lamp goes out

What happens to bulb brightness as voltage drop across it increases?

it gets brighter

What happens to bulb brightness as current across it increases?

it gets brighter

What happens to bulb brightness as total resistance increases?

it gets dimmer

Solve through the final hard quizlet problem to make sure you really understand solving circuits with matricies

whew done

What equation would you use to determine how much energy is in a battery given its mAhr and v?

E = qV


1. find q- we know that A = j/c and 3600s in 1 h
2. solve