Home / iGCSE Physics (0625) 4.2.3 Electromotive force and potential difference Paper 3 -Exam Style Questions- New Syllabus

iGCSE Physics (0625) 4.2.3 Electromotive force and potential difference Paper 3 -Exam Style Questions- New Syllabus

Question

Two identical resistors, ( R_1 ) and ( R_2 ), are connected to a 24 V battery, as shown in Fig. 7.1.
The value of each resistor is 50 (\Omega).
(a) Calculate the combined resistance of ( R_1 ) and ( R_2 ) when they are connected as shown in Fig. 7.1.
(b) Show that the current in the circuit is approximately 0.25 A.
(c) Determine the potential difference (p.d.) across ( R_1 ).
(d) Calculate the power transferred in ( R_1 ).
(e) A student connects ( R_1 ), ( R_2 ) and the battery to make a different circuit. The resistors ( R_1 ) and ( R_2 ) are connected so their combined resistance is as small as possible. Draw a circuit diagram to show how ( R_1 ) and ( R_2 ) are connected to the battery.

Most-appropriate topic codes (Cambridge IGCSE Physics 0625):

• Topic $4.3.2$ — Series and parallel circuits (Parts $\mathrm{(a)}$, $\mathrm{(e)}$)

• Topic $4.2.2$ — Electric current (Part $\mathrm{(b)}$)

• Topic $4.2.3$ — Electromotive force and potential difference (Part $\mathrm{(c)}$)

• Topic $4.2.5$ — Electrical energy and electrical power (Part $\mathrm{(d)}$)

▶️ Answer/Explanation

(a)
For the correct answer: $100\ \Omega$
In a series circuit, the total resistance is the sum of individual resistances. Since the resistors are connected end-to-end in a single loop, the combined resistance is calculated as $R_{total} = R_1 + R_2 = 50\ \Omega + 50\ \Omega = 100\ \Omega$.

(b)
For the correct answer: $0.24\ \text{A}$ (which is approx. $0.25\ \text{A}$)
Using Ohm’s Law, the current $I$ is equal to the voltage $V$ divided by the total resistance $R$. With a battery voltage of $24\ \text{V}$ and a total resistance of $100\ \Omega$, the calculation is $I = 24\ \text{V} / 100\ \Omega = 0.24\ \text{A}$, which rounds to approximately $0.25\ \text{A}$.

(c)
For the correct answer: $12\ \text{V}$
The potential difference across a single resistor in series is found using $V = I \times R$. Substituting the circuit current of $0.24\ \text{A}$ and the resistance of $50\ \Omega$ gives $V_1 = 0.24\ \text{A} \times 50\ \Omega = 12\ \text{V}$, representing half the total supply voltage.

(d)
For the correct answer: $2.88\ \text{W}$
Power is the rate of energy transfer, calculated using the formula $P = I^2R$ or $P = V \times I$. Using the values for $R_1$, the power is $P = (0.24\ \text{A})^2 \times 50\ \Omega = 0.0576 \times 50 = 2.88\ \text{W}$, indicating the energy dissipated as heat per second.

(e)
For the correct answer: Resistors connected in parallel.
To minimize combined resistance, the components must be connected in parallel so the current has multiple paths. The circuit diagram should show $R_1$ and $R_2$ placed in separate branches, both connected directly across the terminals of the $24\ \text{V}$ battery.

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