Resistance (Unit 2)

Question 1

What is Potential Difference (PD)?

Answer 1

energy converted from electrical potential energy to some other form per coulomb of charge flowing from one point to the other.

Question 2

What is Potential Difference (PD) measured in?

Answer 2

Volts = JC-1

Question 3

In a Current (I) Voltage (V) Graph, when the temperature remains constant, what are the characteristics of a resistor?

Answer 3

-voltage and current are directly proportional
-resistance remains constant

Question 4

In a Current (I) Voltage (V) Graph, when the temperature remains constant, what are the characteristics of a FILAMENT LAMP?

Answer 4

-As Voltage increases current increases at a decreasing rate
-resistance increases

Question 5

What is Ohms Law?

Answer 5

-The current in a metal wire at constant temperature is proportional to the pd across it.
- V=IR

Question 6

Definition of Resistance?

Answer 6

• opposition to flow of charge,
• the voltage per unit current

Question 7

What is Resistance measured in?

Answer 7

Ohms = VA-1

Question 8

When do we use P= IV

Answer 8

any device, all forms of energy

Question 8

When do we use P= I^2 R = V^2/R

Answer 8

-when ALL electrical energy is transferred into heat energy
- only for pure resistor

Question 9

What creates electrical resistance?

Answer 9

collisions between free electrons and ions

Question 9

What is the relationship between resistance and temperature?

Answer 9

electrical resistance increases with temperature

Question 10

Unit for resistivity?

Answer 10

Ohm Metre

Question 11

What is the relationship between resistance and temperature in a metal?

Answer 11

-The resistance of a PURE metal increases linearly with increasing temperature

Question 12

How does when resistance increases, temperature also increases?

Answer 12

-At HIGHER temperatures the positive ions vibrate with greater amplitude
- the electron-ion collisions are more frequent, so the flow of electrons is slowed down (current is reduced) resistance increases

Question 13

What is Superconductivity?

Answer 13

a material when cooled down below its transition temperature has a resistance of zero

Question 14

What is transition temperature?

Answer 14

The temperature below which a material must be cooled in order
to have zero resistance and become superconducting

Question 15

Why are HIGH-TEMPERATURE SUPERCONDUCTORS?

Answer 15

-having transition temperature above the boiling point of nitrogen (–196 °C)
-we can use liquid nitrogen to turn materials into superconductors, 10 times cheaper than using liquid helium
-more PRACTICAL and COST EFFECTIVE

Question 16

Describe some uses of superconductivity.

Answer 16

• magnets to make vehicles float.
• MRI scanners use superconducting magnets to
  expose the human body to a strong magnetic field.
• Superconducting magnets are often used in particle
  accelerators to alter the paths of subatomic
  particles.

Question 17

What is EMF?

Answer 17

energy converted from some other form to electrical potential energy per coulomb of charge flowing through the source

Question 18

What is Potential Divider Equation?

Answer 18

V Out = R2 / R1+R2 x V In

Question 19

In a circuit what happens when a THERMISTOR is in low temperature?

Answer 19

-At low temperature
-High Voltage and Resistance at VARIABLE resistor
-Low Voltage AT FIXED RESISTOR

Question 20

In a circuit what happens when a THERMISTOR is in HIGH temperature?

Answer 20

-At high temperature
-High Voltage and Resistance at FIXED resistor
-Low Voltage AT VARIABLE RESISTOR

Question 21

What is Kirchoff’s first law?

Answer 21

-At a junction in a circuit, the sum of the currents entering the junction is equal to the sum of the currents leaving the junction

Question 22

What is Kirchoff’s Second Law?

Answer 22

In any closed loop in a circuit, the sum of the emf’s is equal to the sum of the pd’s around the loop

Question 23

How does LDR (Light Dependent Resistor) work?

Answer 23

• When light intensity increases, the resistance of LDR DECREASES

Question 24

What happens to LDR when dark?

Answer 24

High Resistance, High voltage
-Fixed Resistor has Low Voltage

Question 25

What happens to LDR when bright (day)?

Answer 25

-Low Resistance, Low Voltage
-Fixed Resistor has High Voltage

Question 26

Test to measure Ohm’s law? (how to draw circuit)

Answer 26

Circuit with variable resistor
-ammeter (connected in series)
-voltmeter connected to metal wire/ filament lamp (in parallel)

Question 27

Test to determine resistivity of a metal?

Answer 27

-circuit with ammeter, voltmeter, crocodile clips, nichrome wire, metre ruler vernier , vernier callipers
- record voltage and current at increasing 10 cm intervals using crocodile clip on wire
-Calculate the value of R for each length.
-Convert the values for length from cm to m.
-plot a graph of R (vertical axis) against L (horizontal axis).
-Calculate the cross-sectional area of the wire, A. using vernier callipers to record radius
-Calculate the gradient of your graph.
-Calculate resistivity, ρ, using this equation:
ρ=gradient×A

Question 28

Test to determine variation of resistance with temperature for a metal wire?

Answer 28

• water bath with heating element, ice , thermometer, stirrer, ohmmeter, copper coil
• put copper coil in boiling tube filled with oil and connect to ohmmeter
• increase water bath heat till reaches 100 degrees, stir continuously , record readings at 10 degree intervals and measure resistance, record readings again when cooling down to ensure accuracy and reliability
• use ice and water mix to reach 0 degrees
• plot resistance against temp, graph should be straight line (shows resistance increases LINEARLY with temperature)

Question 29

Test to determine speed of sound using stationary waves?

Answer 29

-1 litre measuring cylinder , glass, tube ,ruler ,set of tuning forks ,retort stand, boss and clamp large rubber bung
- strike tuning fork on rubber bung
- hold over tube
- move tube up and down until resonance position is obtained
- measure length of tube above water
- repeat with other tuning forks of known frequency
- measure each length of the tube above water until open end
- use Lander = 4L to work out wavelength and Wave speed = frequency x wavelength to work out speed of sound

Question 30

Conservation of charge?

Answer 30

-Electric charge cannot be created or destroyed, (though positive and negative charges can neutralise each other).Charge cannot pile up at a point in a circuit.