2.3 Direct Current Circuits

Question 1

Kirchhoff’s first law

Answer 1

The sum of currents coming into a point in a circuit equals the sum of currents going out from it

Question 2

Current in each branch must add to equal output

Answer 2

in parallel

Question 3

Potential difference is the same across branches

Answer 3

in parrallel

Question 4

Before a meter is connected

Answer 4

• a voltmeter’s resistance is so high that the current through it is negligible
• each ammeter’s resistance is s low that inserting it in series doesn’t lower the current

Question 5

In series PDs add up, and the current is the same so

Answer 5

IR = IR1 + IR2…

Question 6

Resistances add up in

Answer 6

series

Question 7

In parallel the currents through each branch add to give the total so

Answer 7

V/R = V/R1 + V/R2…

Question 8

So in parallel total resistance is

Answer 8

1/R = 1/R1 + 1/R2…

Question 9

Potential divider

Answer 9

V1/V total = R1/ R total : the ratio of the PDs is equal to the ratio of the resistances

Question 10

The EMF of a cell or battery is

Answer 10

the energy that changes category from chemical to electrical potential per unit charge passing through the cell

Question 11

Unit of EMF

Answer 11

JC^-1 = V

Question 12

The condition for no further transfer of charge is

Answer 12

that the PD across the cell terminals is equal to E- so on open circuit V = E

Question 13

For internal resistance, V=

Answer 13

E-Ir

Question 14

EMF of battery=

Answer 14

sum of emfs of cells in series

Question 15

Internal resistance of battery=

Answer 15

sum of internal resistance of cells

Question 16

Using V=E-Ir gives:

Answer 16

1. I = E/ (R+r)
2. V = E-(V/R)r
3. P = (E^2R)/(R+r)^2

Question 17

PRACTICAL: Determination of the internal resistance of a power supply

Answer 17

1. By measuring V and I (the first readings should be with the switch open so that I=0)
2. By measuring I and the external resistance
3. By measuring V and the external resistance