Electricity

Question 1

Define current

Answer 1

• The rate of flow of charged particles

Question 2

What is the equation for current?

Answer 2

• I = ΔQ / Δt

Question 3

What is the equation for Voltage?

Answer 3

• V = W / Q
• or
• V = E / Q

Question 4

Define Resistance

Answer 4

• R = V / I

Question 5

Define Voltage

Answer 5

• One volt is one Joule per coulomb
• It is what makes the current flow
• V=W/Q

Question 6

What is Ohms law?

Answer 6

• Special case where current is directly proportional to voltage for constant temperature

Question 7

What is the the ditribution of current a consequence of?

Answer 7

• Conservation of charge
• Total current into a junction = Total current out

Question 8

What is the distribution of potential difference a consequence of?

Answer 8

• Conservation of energy
• Total emf from cells = Total p.d in circuit

Question 9

Derive the equation for combining resistances in parallel

Answer 9

1. IT = I₁ + I₂ (conservation of charge)
2. V_T / R_T = V₁ / R₁ + V₂ / R₂
3. 1 / R_T = 1 / R₁ + 1 / R₂

Question 10

Derive the equation for combining resistances in series

Answer 10

1. V_T = V₁ + V₂ (Conservation of energy)
2. I_TR_T = I₁R₁ + I₂R₂ (I cancels as current is the same in series circuit)
3. R_T = R₁ + R₂

Question 11

What are the seven base units?

Answer 11

1. Second
2. Metre
3. Kilogram
4. Amp
5. Kelvin
6. Mole
7. Candela

Question 12

What does V and I do in series?

Answer 12

• V splits (Conservation of energy)
• I is same (Conservation of charge)

Question 13

What do V and I do in parallel

Answer 13

• V is same across each branch (conservation of energy)
• I splits between branches (conservation of charge)

Question 14

What are the equations for power

Answer 14

• P = IV
• P = I²R and P = V² / R

Question 15

What is the resistivity equation?

Answer 15

R = pl / A

Question 16

What is the equation which includes charge carrier density?

Answer 16

• I = nAve or I = nAvq

Question 17

What is the difference between potential difference and potential

Answer 17

• P.D is difference in potential between two points
• Potential is joules per coulomb at a point

Question 18

Define emf

Answer 18

• emf is the energy per coulomb converted into electrical energy by the cell

Question 19

Define internal resistance

Answer 19

• Resistance in battery itself which causes lost volts as heat

Question 20

Define terminal p.d

Answer 20

• Terminal p.d is the electrical energy converted per coulomb into other forms

Question 21

Define Resistivity

Answer 21

• A constant for a given material at a given temperature

Question 22

What does emf stand for?

Answer 22

• Electromotive force

Question 23

What are Thermistors and LDRs (in general)?

Answer 23

• Semiconducters
• They are resistors whose resistance is based on local conditions

Question 24

What is the rule for Thermistors for a negative thermal coefficient?

Answer 24

• As temperature increases, resistance decreases

Question 25

Explain why when temperature increases in Thermistors, the resistance decreases?

Answer 25

1. I = nAve 2. As temperature increases, drift velocity decreases as there are more collisions between electrons and lattice. Amplitude of oscillations increases. 3. But charge carrier density increases as more charge carriers are released hence why resistance decreases

Question 26

What is a use for Thermistors and LDRs?

Answer 26

* Sensors

Question 27

What happens if you increase the light intensity at LDR?

Answer 27

1. As more photons hit the LDR, more charge carriers are released 2. This means charge carrier density increases, current increases and resistance decreases

Question 28

What is the ideal voltmeter?

Answer 28

* Ideal voltmetere draws no current * Has ∞ resistance

Question 29

What is an ideal Ameter?

Answer 29

* Ideal Ameter has no voltage drop * Resistance of 0

Question 30

What is the real resistance of an Ameter?

Answer 30

* 0.5 Ohms

Question 31

What is real resistance of a voltmeter?

Answer 31

* 50 mega Ohms

Question 32

What happens to the potential as you travel along a wire with no components?

Answer 32

* It decreases linearly

Question 33

Draw a graph for distance along wire and potential difference?

Answer 33

Question 34

Learn this diagram on Potential Dividers!

Answer 34

Question 35

Equations for Energy?

Answer 35

* E = IVt or W = IVt * E = Pt

Question 36

Core practical 2: Determine electrical resistivity * Independant * Dependant * Equation * y = mx + c * Graph to plot

Answer 36

* Length * Resistance * R = pl / A * y = mx + c * y axis is R * x axis is l * m = resistivity/A

Question 37

Core practical 3: Determine emf and internal r of cell * Independant * Dependant * Equation * y = mx + c * Plot graph

Answer 37

* Current * Terminal p.d * E = V + Ir * y = mx + c * y axis = V * x axis = I * m = r * y intercept = emf