Physics Unit 5: Electricity Flashcards Preview

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Flashcards in Physics Unit 5: Electricity Deck (15):
1

Conductor

conducts current

ex: metals

2

Insulator

cannot conduct current
-have high resistance

ex: nonmetals, substances with air pockets and holes

3

Resistance

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)

4

Resistivity

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

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)

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)

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...

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

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

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

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...

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

15

Why is an ammeter connected in...

Series

Because ammeters have very low resistance and therefore allow current to pass through

If connected in parallel, the current would flow through he ammeter due to low resistance and cause it to become destroyed.