Model Answers Test 2

Question 1

Define potential difference

Answer 1

Potential difference is the energy transferred (or work done) between two points in a circuit per unit charge (V = W/Q).

Question 2

Define emf

Answer 2

Emf is the work done per unit charge by the power supply or cell, converting energy into electrical potential energy of the charges.

Question 3

Define current

Answer 3

Current is the rate of flow of charge (I = AQ/At).

Question 4

Derive the equation linking current with the number of electrons N flowing past a point in a time At

Answer 4

The charge flow AQ = Ne where e is the magnitude of the charge on each electron, e=1.6x10-19 C.

= Ne / At

Question 5

Explain how to calculate the number of electrons in a certain amount of charge

Answer 5

Number of electrons = total charge / charge on one electron.

N = Q/ 1.6 x10-19

Question 6

Define resistance

Answer 6

Resistance is the ratio of potential difference to current (R=V/I).

Question 7

Derive the formula for the ratio of currents down parallel branches of a circuit

Answer 7

The pd across each branch is the same V1 = V2.

11R1 = 12R2; So 11/12 = R2/R1.

Question 8

Derive the formula for resistors in series

Answer 8

Vtotal = V1 + V2 + V3 (due to energy conservation).

Rtotal = R1 + R2 + R3.

Question 9

Derive the formula for resistors in parallel

Answer 9

Itotal = I1 + I2 + I3 (due to charge conservation).

1/Rtotal = 1/R1 + 1/R2 + 1/R3.