Electricity 2

Question 1

Explain why adding an extra resistor in parallel decreases the total circuit resistance

Answer 1

you are adding an extra path for current

Question 2

Explain why adding an extra resistor in series increases the total circuit resistance?

Answer 2

current must now pass through this new resistor as well

Question 3

Prove the resistance rule for series

Answer 3

• Vt = V1 + V2 + …
• V = IR
• IRt = IR1 + IR2 + …
• Rt = R1 + R2 + …

Question 4

Prove the resistance rule for parallel

Answer 4

• It = I1 + I2 + …
• I = V/R
• V/Rt = V1/R1 + V2/R2 + …
• V = V1 = V2
• 1/Rt = 1/R1 + 1/R2 + …

Question 5

What is a superconductor?

Answer 5

a material that has zero resistance/resistivity below a very low, critical teperature

Question 6

Give two things superconductors can be used to make?

Answer 6

• high power electromagnets (MRI)

- power cables (cables for high current)

Question 7

Derive the equation: Ɛ = I(R + r)

Answer 7

• Ɛ = terminal pd + lost volts
• Ɛ = IR + Ir
• Ɛ = I(R + r)
• R = external resistance, r = internal resistance

Question 8

Define EMF

Answer 8

the total energy supplied by the cell per coulomb of charge

Question 9

What is the rule for total internal resistance for n number of cells in series?

Answer 9

r(total) = nr

Question 10

What is the rule for total internal resistance for n number of cells in parallel?

Answer 10

r(total) = r/n

Question 11

What are potential dividers?

Answer 11

a combination of resistors in series connected across a voltage source, to produce a required p.d

Question 12

What is the relationship between resistance and temperature for a thermistor?

Answer 12

resistance is inversely proportional to temperature

Question 13

Give three uses of a thermistor

Answer 13

monitor temperature in:

• laptops
• car engines
• heat alarms
• kettles

Question 14

What is the relationship between resistance and light intensity for and LDR?

Answer 14

inversely proportional

Question 15

If there are 2 resistors in series, what does the ratio of V1:Vsupplied equal?

Answer 15

R1:Rtotal

Question 16

What is the relationship between ∑R1 and Rtotal in a series circuit?

Answer 16

directly proportional

Question 17

What is the relationship between ∑R1 and Rtotal in a parallel circuit?

Answer 17

inversly proportional

Question 18

Define internal resistance

Answer 18

the resistance created in a power source when electrons collide with atoms inside the power source and lose energy

Question 19

Define lost volts

Answer 19

the energy wasted per coulomb overcoming the internal resistance of a power source

Question 20

What is the relationship between the resistance of a resistor and the potential difference across a resistor, in a series circuit?

Answer 20

the fraction of the supply p.d. a resistor takes is equal to its fraction of the total resistance