Topic 2.5: Current and Circuits Questions

State what is meant by an electric current of 1 ampere.

An electric kettle is labelled 2300 W when connected to the 230 V mains supply.

Two resistors, 5.0 Ω and 15 Ω, are connected in series with a cell of negligible internal resistance.

The current in the circuit is 0.40 A.

Determine the emf of the cell.

State what is meant by the resistance of an electrical component.

A phone charger delivers a steady 5.0 W to a phone while charging it.

The phone takes 2.5 hours to charge fully.

Estimate the electrical energy delivered to the phone during one full charge, giving your answer in joules.

A length of constantan wire has a resistance of 6.0 Ω.

A second wire of the same material and the same cross-sectional area has three times the length of the first.

Deduce the resistance of the second wire.

Identify whether the current or the potential difference is the same for two resistors connected in parallel, and state which resistor — the larger or the smaller — carries the greater current.

State what is meant by the emf of a cell.

A cell is connected to a fixed external resistor.

Outline why the terminal potential difference of the cell is smaller than its emf when current flows.

A torch bulb carries a steady current of 0.25 A.

Determine the charge that flows through the bulb in 1.0 minute.

A potential difference of 12 V is applied across a fixed resistor and a current of 2.5 A flows through it.

Determine the resistance of the resistor.

Define potential difference between two points in a circuit.

A cell of emf 4.5 V and internal resistance 1.2 Ω is connected to a single resistor of resistance 7.8 Ω.

Calculate the current in the circuit.

A heating element of fixed resistance is run from a supply whose voltage can be adjusted.

The voltage across the element is reduced to half its original value.

State and explain what happens to the power dissipated by the element.

A power supply of emf 24 V has an internal resistance of 3.0 Ω.

It is connected to an external resistor R whose value can be changed.

Estimate the terminal p.d. of the supply when R is very much larger than the internal resistance (for example R = 297 Ω), and comment on how the terminal p.d. compares with the emf in this case.

A student connects a cell of emf 6.0 V to a 5.5 Ω resistor and measures a current of 1.0 A.

Determine the internal resistance of the cell, then state what current would flow if the internal resistance were instead negligible.

Two straight wires, X and Y, are made from the same metal and have the same uniform cross-sectional area.

When wire X is connected across a 12 V supply it dissipates 60 W.

When wire Y is connected across an 18 V supply it dissipates 90 W.

What is the length of wire Y in terms of the length L of wire X?

A 12 V battery is connected to a network of four identical resistors, each of resistance R. The network has two parallel branches between the same two points: one branch contains a single resistor R, and the other branch contains the remaining three resistors connected in series. The current in the single-resistor branch is I₁ and the current in the three-resistor branch is I₂.

What is the ratio I₂ : I₁?

A cell of emf ε and internal resistance r delivers a current I to an external resistor R.

Show that the power dissipated inside the cell is given by P = I²r, and calculate this internal power loss for ε = 9.0 V, r = 0.75 Ω and I = 1.6 A.

A 12 V car battery transfers 2.4 kJ of electrical energy to charge passing through a heating element.

Calculate the charge that flows through the element.