ELECTRICITY

Question 1

What is current?

Answer 1

The rate of flow of charged particles (electrons)

Question 2

What is the equation for current in terms of charge?

Answer 2

I = Q/t

Question 3

How do you define voltage/potential difference and give the equation in terms of the definition

Answer 3

The work done per unit of charge to move a charge between two points in an electrical field

V = W/Q

Question 4

What is Ohm’s law?

Answer 4

I is proportional to V when temp is constant (V = IR)

Question 5

What is Kirchhoff’s First Law?

Answer 5

The total current entering a junction is equal to the total current leaving a junction

(Same at all point in series, splits in parallel)

Question 6

What is Kirchhoff’s Second Law?

Answer 6

The sum of EMFs in a closed loop is equal to the total number of potential drops in that loop

(The total voltage going into a loop = the total voltage coming out of the loop

Question 7

Explain why a current-potential difference graph for an ohmic conductor is like that

Answer 7

Current increases in proportion to voltage

Question 8

Explain why the current-potential difference graph of a filament bulb is like that

Answer 8

As voltage increases, the bulb gets hotter and resistance increases (KE up, particles vibrate faster)

Question 9

Explain why the current-potential difference graph for a diode looks like that

Answer 9

Current only travels in one direction, very high resistance in other direction

Question 10

Explain why the current-potential difference graph for a thermistor looks like that

Answer 10

Increase in PD = increase in current = increase in temp
When temp goes up, resistance goes down, so more current can flow

Question 11

EMF VS Terminal PD

So explain the equation E = I(R-r)

Answer 11

EMF = the max potential difference a cell can provide when there is no current flowing (open circuit)
Terminal pd = the total voltage across terminals of the circuit when there is a current flowing

E = I(R-r) = E = IR + Ir
= EMF = terminal pd + voltage lost in battery

Question 12

How to find EMF and internal resistance from a graph from CPAC3?

Answer 12

EMF = y intercept
-r = gradient

Question 13

How do you define power, give equation and derive it (as well as P = I^2R)

Answer 13

Rate at which energy is transferred (P = E/t)
P = IV, P = (Q/t) * (E/Q), P = E/t
P = IV, V = IR, P = I^2R

Question 14

Why are ammeters connected in series and voltmeters in parallel?

Answer 14

Ammeters = minimal resistance, max current can flow through
Voltmeters = max resistance, no current can flow

Question 15

Why are ammeters connected in series and voltmeters in parallel?

Answer 15

Ammeters = minimal resistance, max current can flow through
Voltmeters = max resistance, no current can flow

Question 16

Derive Rt = R1 + R2 + R3 for a series circuit

Answer 16

In series:
Vt = V1 + V2 + V3
ItRt = I1R1 + I2R2 + I3R3
Bc series, current is the same all throughout
It = I1 + I2 + I3
IRt = IR1 + IR2 + IR3
IRt = I(R1 + R2 + R3)
Rt = R1 + R2 + R3

Question 17

Derive 1/Rt = 1/R1 +1/R2 + 1/R3 in parallel

Answer 17

In junction, current going in = current going out
It = I1 + I2 + I3
Vt/Rt = V1/R1 + V2/R2 + V3/R3
V going into loop = V coming out
So Vt = V1 + V2 + V3
Vt/Rt = Vt/R1 + Vt/R2 + Vt/R3
1/Rt = 1/R1 + 1/R2 + 1/R3

Question 18

Define r and EMF and give the equation

Answer 18

R = internal resistance. Resistance inside power supply
EMF = voltage supplied to circuit when current = 0
V (across terminals) = EMF - Ir

Question 19

Derive EMF = I(R+r)

Answer 19

Vt = VR
IR = EMF - Ir
IR + Ir = EMF
I(R+r) = EMF