Flashcards in Physics Unit 5: Electricity Deck (15):

1

## Conductor

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conducts current

ex: metals

2

## Insulator

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cannot conduct current

-have high resistance

ex: nonmetals, substances with air pockets and holes

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## Resistance

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Electrical Resistance (R): a property of matter that describes how difficult it is for electric current to travel through a material.

It is the ratio of the potential difference across a load, relative to the current that flows through it

Units: Ohm

R = V/I

Rseries = R1 + R2 + R3 + . . . (three resistors can be reduced to a single resistor with a value equivalent to the sum of the three resistors)

1/Rparallel = 1/R1 + 1/R2 + 1/R3 + . . .

the more resistors you connect in parallel, the greater the current (total resistance decreases)

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## Resistivity

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quantifies how strongly a given material opposes the flow of electric current.

low resistivity: readily allows the flow of current

Resistance of a wire depends on four factors:

-length (L) in meters

-cross-sectional area (A) in m2 (pi*r^2)

-**resistivity (p) in ohm m

-temperature (resistance increases with temperature-->+ vibrate and prevent delocalized electrons from passing through)

Resistance = p*L/A

5

## Ohmic resistor

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The current through an ohmic resistor increases proportionally to the potential difference applied across the resistor (Constant resistance)

Current and potential difference graph

*non-ohmic resistors: resistance increases with temperature so the current does not increase proportiaonlly to potential difference

• Low resistor = high current

• High resistor = low current

*Ohmeters are connected in parallel and the circuit must be off

6

## Current (I)

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Electrical current (I) is a movement of electric charge that can occur in solids, liquids, and gases

-->conventional current: direction of position charge

(+ to -)

-->the rate at which charge flows

I = Q/changetime

• Current is required for an electrical device to operate

• Unit: Amperes (C/sec)

1 A = 1000 milliampere (mA)

• Current stays the same throughout a series circuit, but splits for parallel (and rejoins in the main circuit)

Iseries = I1 = I2 = I3 . . . (same throughout)

Iparallel = I1 + I2 + I3 + I4 . . .

• Measured using an ammeter → connected to the circuit in series (need electrons to flow through the ammeter to give an accurate reading)

7

## Potential Difference (V)

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Electric Potential Difference (V): the change in electric potential energy associated with charges at TWO DIFFERENT points in a circuit. The potential energy difference across its terminals for every Coulomb of Charge →voltage

• + voltage = voltage gain

• - voltage = voltage drop

V = ChangeinEe/Q

Voltmeter: electrical device that measures electric potential difference

• Only work if it is connected in parallel in the circuit

Vseries = V1 + V2 + V3 . .

:Vparallel = V1 = V2 = V3 . . .

*connecting wires and control devices (i.e. switch) are not supposed to affect the amount of electric potential (do not cause a voltage drop). However in real circuits, they do

8

## Electrical energy as it travels through the circuit...

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Electrical energy decreases as it travels through the circuit →gives electrical potential energy to loads such as light bulbs and resistors

• Loads cause a decrease in electric potential energy or a voltage drop

*When the charge reaches the negative terminal of the battery, it is at 0 volts and is ready to be re-energized

9

## Electrical load

### Electrical load is an electrical component of a circuit that consumes electric power (opposite of power source)

10

## Series vs. parallel circuits

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Series: voltages add up and currents remain constant

Parallel: voltages remain constant and currents add up

Series circuits: circuits that have one complete path

Parallel circuits: circuits that have more than one complete path

-when analyzing circuits separate series circuit from parallel circuit

11

## Kirchoff's Voltage

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Kirchhoff’s Voltage Law (KVL): In any complete path in an electric circuit, the total electric potential increase at sources is equal to the total electric potential decrease throughout the rest of the circuit

Ex: 6 V battery loses 6V throughout the circuit, but then gains 6 volts again back at the battery.

• The Sum of the voltage gains and drops in a complete path in a circuit is zero

12

## Kerchief's Current Law

### Kirchhoff’s current law (KCL): in a closed circuit, the amount of current entering a junction is equal to the amount of current exiting a junction

13

## Requirements for a circuit

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There must be a closed conducting path that extends from the positive terminal to the negative terminal.

-the light bulb must be included in the path in order to light up

There must also be an ENERGY supply capable doing work on CHARGE to move it from a low energy location to a high energy location and thus establish an electric potential difference across the two ends of the external circuit.

-->increases electric potential energy from low energy terminal to high energy terminal (in the battery)

By establishing this difference in electric potential, charge is able to flow downhill/naturally through the EXTERNAL circuit.

14

## Why is a voltmeter connected in...

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Parallel

Because voltmeters have high resistance and therefore does not take the current from the device being measured

It would obstruct the flow of current if connected in series

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