Question 8.3.2: Additional Answer Options

8.1

(Electrical) energy/work

Unit charge

Tip: the emf of the battery is the maximum energy/work provided by a battery per unit of charge passing through it.

2 marks

(2 x 1 mark)

8.2

How to approach this question: 

Option 1

• Calculate the total resistance of the top branch of the circuit.

• Next, calculate the total effective parallel resistance.

• While normally marks aren't awarded for simply showing the substitution of resistors in series, in this case, you should be awarded a mark for using 7Ω as the resistance for the top branch.
• It is always a good idea to show all your work, no matter how basic it may seem. This can help you avoid losing marks in cases where partial credit is awarded for the process.

4 marks

(4 x 1 mark)

8.3.1

How to approach this question: 

• You will need to do simultaneous equations to solve for emf and r since there are 2 unknown variables.
• It is possible to calculate the current using the external potential difference of 2,8V and the external resistance of 7Ω.
• Remember, when the switch is open, the current only flows through one branch of the parallel combination.

When the switch is open:

⁽¹⁾

⁽²⁾

• It is possible to use either formula below to calculate the first of the two equations needed to determine the internal resistance and emf.

⁽⁴⁾

⁽⁵⁾

Or

⁽⁴⁾

⁽⁵⁾

When the switch is closed:

• The total resistance of the parallel combination is 3,5 Ω as calculated in question 8.2 and the potential difference is 2,63V.

⁽¹⁾

⁽³⁾

⁽⁴⁾

⁽⁵⁾

Or

⁽⁴⁾

⁽⁶⁾

• Equating the two equations allows you to solve for the internal resistance.

⁽⁷⁾

⁽⁸⁾

Or

⁽⁷⁾

⁽⁸⁾

8 marks

(8 x 1 mark)

Marking criteria:

Calculation of current when the switch is open and when closed:

• ⁽¹⁾ Suitable formula for open or closed switches
• ⁽²⁾ Correct substitution when the switch is open
• ⁽³⁾ Correct substitution when the switch is close

Substitution into formula ε = I(R + r) or ε = V_ext + Ir:

• ⁽⁴⁾ Formula
• ⁽⁵⁾ Substitution in the formula for an open switch
• ⁽⁶⁾ Substitution in the formula for a closed switch

Calculating r:

• ⁽⁷⁾ Equating the equations
• ⁽⁸⁾ Final answer: 0,49 Ω

8.3.2

Option 1

Tip: the marks are awarded for the correct substitution of the value of the internal resistance into any formula used in question 8.3.1

There are more ways to answer this question. To view other options, click here.

2 marks

(2 x 1 mark)