Electricity

Question 1

Equation for current

Answer 1

I = Q/t

Question 2

Definition of current

Answer 2

Charge passing a point per unit time

Question 3

Definition of Potential Difference

Answer 3

Joules per columb - energy per unit charge

Question 4

Definition of Resistance

Answer 4

R = V/I

Question 5

Ohm’s Law

Answer 5

V = IR

Question 6

Kirchoff’s First Law

Answer 6

The total current entering a junction is equal to the total current out of a junction (conservation of charge)

Question 7

Kirchoffs Second Law

Answer 7

The total pd around a closed loop is equal to the pd of the supply

Question 8

General equation for resistors in series

Answer 8

Rt = R1 + R2 …..

Question 9

General equation for resistors in parallel

Answer 9

1/Rt = 1/R1 + 1/R2 ….

Question 10

Derive the formula for resistors in series

Answer 10

V = IR Vt = V1 + V2 + V3 … IRt = IR1 + IR2 + IR3 since in series current is constant Rt = R1 + R2 + R3

Question 11

Derive the formula for resistors in parallel

Answer 11

I = V/R It = I1 + I2 ….. V/Rt = V/R1 + V/R2 …. since voltage is constant is parallel 1/Rt = 1/R1 + 1/R2

Question 12

Describe the IV graphs for an ohmic resistor, a filament lamp and a diode

Answer 12

Question 13

Give the equation linking resistivity and resistance

Answer 13

R= ρl/A

Question 14

Give the units for resistivity

Answer 14

Ohm Meters

Question 15

State the equation linking current and drift velocity

Answer 15

I = nAvq

I = current (A)

n = charge density (m^-3) - number of charge carriers per m^3

q = charge on each charge carrier (C)

v = drift velocity (ms^-1)

A = cross sectional area (m^2)

Question 16

Derive the question for drift velocity

Answer 16

Number of charge carriers per m^3 is An

Total charge = qAn

Total charge per second = vqAn = I

Question 17

Describe a potentiometer

Answer 17

A potentiometer is a device that produces a p.d. ranging from 0V to the voltage of the supply

Question 18

Give the equation associated with potentiometers

Answer 18

(voltage required)/(voltage of supply) x length of potentiometer = length required

Question 19

Derive the equation for potential dividers

Answer 19

In series I is constant

V = IR
Vsupply = I(R1 + R2)

Vout = IR2

Vsupply/(R1 + R2) = Vout /R2

R2/(R1+R2) = Vout/Vsupply

Question 20

What happens in an LDR when the intensity of the light increases?

Answer 20

Resistance decreases - negative coefficient

Question 21

Define Internal Resistance

Answer 21

Internal resistance is the resistance of the cell

Question 22

Define lost volts

Answer 22

The volts are volts used to move the electrons through the cell

Question 23

Give the equation for emf

Answer 23

E = IR + Ir

Question 24

Explain what happens to resistance when temperature increases

Answer 24

Increasing) current leads to temperature increase / leads to thermistor ‘heating up’ More conduction electrons / more electrons released / more free electrons / more charge carriers / charge carrier density increased / n increases

Question 25

Explain what happens to the resistance of an LDR when intensity is increased

Answer 25

LDR being a semi conductor
The semi-conductor in LDRs will reduce resistance due to the energy gained from light (Heat if thermistors)
The energy lets some electrons move around freely (same as the free electrons in metals which allow them to conduct).

so MORE LIGHT ENERGY=MORE FREED ELECTRONS=BETTER CONDUCTOR=DECREASED RESISTANCE

Question 26

Give equations for Power with regard to electricity

Answer 26

P = VI

P = V^2/R

P = RI^2