5: Electricity

Question 1

What is terminal p.d?

Answer 1

The voltage directly across a power supply (with internal resistance)

Question 2

What are the equations for emf?

Answer 2

emf=current multiplied by the total resistance
(ε=I(R+r))
emf=energy supplied/charge

Where:
ε=emf
I=current
R=circuit resistance
r=internal resistance

Question 3

What are two applications of superconductors?

Answer 3

-Used in power cables to reduce energy loss via heating to zero
-Used in strong magnets

Question 4

What is the emf?

Answer 4

-The electrical energy transferred by a power supply per unit charge
-The work done required to push a charge around a circuit

Question 5

What is Kirchhoff’s first law?

Answer 5

-The current into a junction must equal the current leaving that junction

Question 6

What is Kirchhoff’s second law?

Answer 6

-The sum of all voltages in a loop equals zero

Question 7

What is a superconductor?

Answer 7

-A material which has zero resistance at or below a critical temperature

Question 8

How does resistance change for a Light Dependent Resistor?

Answer 8

-As light intensity increase the resistance decreases

Question 9

What is a potential divider?

Answer 9

-A circuit that turns a changing resistance into a change potential difference

Question 10

How is charge conserved in a circuit?

Answer 10

-The current is equal at each point in a series circuit

Question 11

How is energy conserved in a circuit?

Answer 11

-Total voltage input equals the sum of voltages across components

Question 12

How do voltages sum when cells are connected in series?

Answer 12

-The voltages add together

Question 13

How do voltages sum when cells are connected in parallel?

Answer 13

-The overall voltage remains the same

Question 14

How do currents sum when cells are connected in series?

Answer 14

-There is no change in overall current

Question 15

How do currents sum when cells are connected in parallel?

Answer 15

-The current will increase as the total resistance decreases

Question 16

How do resistances sum when cells are connected in series?

Answer 16

-The resistances add together

Question 17

How do resistances sum when cells are connected in parallel?

Answer 17

-Combining cells in parallel will decrease the overall resistance

Question 18

Why does increasing current increase resistance for a filament lamp?

Answer 18

-As current flows electrical energy is converted to thermal energy
-This causes the metal ions to vibrate more
-Which in turn further impedes the movement of electrons thus making it harder of current to flow

Question 19

How would you determine the resistivity of a metal (experiment)?

Answer 19

-Measure the diameter with a micrometer in several places and calculate a mean
-Calculate the cross-sectional area by π*(d^2)/4
-Vary the wire length and record voltage with a voltmeter and current with an ammeter
-Repeat this for multiple lengths of wire and calculate the resistance for each length
-Repeat the experiment twice more to calculate mean resistances
-Plot a graph of resistance against length
-The resistivity is the gradient multiplied by the area

Question 20

What are the power equations?

Answer 20

P=V.I
P=(V^2)/R
P=(I^2)*R

W=P.T

Where:
W=work done
P=power
V=voltage
I=current
R=resistance

Question 21

How do resistors sum in a series cirucit?

Answer 21

-RT=R1+R2…

Question 22

How do resistors sum in a parallel circuit?

Answer 22

-1/RT=1/R1+1/R2…

Question 23

How does resistance change for a thermistor?

Answer 23

-As temperature increase the resistance decreases

Question 24

How are thermistors used in temperature sensors?

Answer 24

-As temperature of thermistor increases the resistance will decrease
-This will decrease the total resistance and increase the current
-This will increase the voltage across the fixed resistor and decrease the voltage across the thermistor
-This allows you to measure a voltage for the thermistor and when the voltage is too low you will know the temperature is too high

Question 25

How do you calculate resistivity?

Answer 25

-Resistivity equals (Resistance*Area)/Length

Question 26

How much resistance does an ideal voltmeter have?

Answer 26

-Infinite resistance

Question 27

How much resistance does an ideal ammeter have?

Answer 27

-Zero resistance

Question 28

What is ohm’s law

Answer 28

-For a constant temperature the current is directly proportional to the potential difference

Question 29

What is resistance?

Answer 29

-A measure of how difficult it is for current to flow through a component
-R=V/I
Where:
V=voltage
I=current
R=resistance
-Measure in ohms

Question 30

What is potential difference?

Answer 30

-The work done per unit charge
-V=W/Q
Where:
V=voltage
W=work done
Q=charge
-Measure in volts

Question 31

What is current?

Answer 31

-The rate of flow of charge
-I=ΔQ/Δt
Where:
I=current
Q=charge
t=time
-Measured in amps

Question 32

Information that can’t be put on flashcards

Answer 32

-Graph for filament lamp
-Graph for semiconductor diode
-Graph for Ohmic conductor
-Circuits