Topic 2.5: Current and Circuits Questions

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.

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.

A torch bulb carries a steady current of 0.25 A.

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

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.

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

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.

Define potential difference between two points in a circuit.

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.

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

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.

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.

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 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.

A torch cell of emf 1.5 V has an internal resistance of 0.30 Ω.

It is connected to a small bulb and a current of 0.40 A flows.

Calculate the terminal p.d. across the cell, and state how much energy per coulomb is lost inside the cell.

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?

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?

In a Van de Graaff generator, a rubber belt carries a uniform surface charge density of 2.0 × 10⁻⁵ C m⁻² on one face.

The belt is 0.15 m wide and moves at a constant speed of 8.0 m s⁻¹, carrying its charge up to the dome where the charge is removed into a wire.

What is the current in the wire?

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.