What two conditions must be met before charge will flow round a circuit?

There must be a source of potential difference, and the circuit must be complete with no gaps.

A cell is joined to a lamp but one wire has come away from the lamp. Why is there no current?

The circuit is not closed, so there is no complete path for charge to flow round.

A closed loop of copper wire on its own carries no current. Which condition is missing?

There is no source of potential difference to push the charge round.

What is the unit of current, and which instrument measures it?

The ampere, A, measured with an ammeter.

OCR GCSE Combined Science Physics J250

Electricity

Topic overview Charge, and producing static electricity Transfer of electrons in charging and discharging Current as a rate of flow of charge Charge, current and time Series and parallel circuits, and circuit symbols Current, potential difference and resistance Resistors whose resistance changes, and investigating them Linear and non-linear circuit elements from graphs Net resistance in series and parallel Circuits for measurement, and electrical power

Energy

Topic overview Conservation of energy, and energy stores Energy changes on heating, and calculating them Energy of a moving body, a spring and a raised object Dissipation, and energy transfer in domestic devices Power ratings, efficiency, and increasing it Reducing unwanted energy transfer, and the rate of cooling

Forces

Global Challenges

Topic overview Typical speeds and everyday accelerations Reaction time, stopping distance and the dangers of large decelerations Stopping distance, acceleration and collision force calculations Energy resources, and trends in their use The national grid, and step-up and step-down transformers Why the national grid is efficient, and power in a transformer Mains electricity: a.c., d.c. and the domestic supply Live, neutral and earth wires, and electrical danger

Magnetism and Magnetic Fields

Topic overview Permanent and induced magnets, and magnetic poles Magnetic fields, and the dipping compass The magnetic effect of a current, and solenoids The motor effect and Fleming's left-hand rule The force on a conductor, and how a motor turns

Matter

Topic overview How the atomic model changed, and the structure of the atom The size of atoms, and defining density Density of the states of matter, and density calculations Conservation of mass in changes of state Heating a system, and specific heat capacity Calculations with specific heat capacity and latent heat Gas molecules, temperature and pressure

Radioactivity

Topic overview Protons, neutrons and isotopes Unstable nuclei and the emissions they give out Balancing nuclear equations Electron energy levels, and radiation from atoms Half-life and the randomness of decay Penetration of alpha, beta and gamma Contamination and irradiation, and reducing the hazard

Waves in Matter

Topic overview Amplitude, wavelength, frequency and period The wave equation Transverse and longitudinal waves Modelling waves with ripples and sound Electromagnetic waves as transverse, and energy transfer Frequency and wavelength across the spectrum What our eyes detect, and light as an electromagnetic wave Uses of electromagnetic waves Hazards of ultraviolet, X-rays and gamma rays Producing and receiving radio waves Absorption, transmission, refraction and reflection Investigating reflection and refraction of light

Electricity

Current as a rate of flow of charge

3 things to know
Paper 11 (Physics)

Overview

Current as the rate of flow of charge, the electrons that carry it in a metal, the two conditions for charge to flow, the ampere and the ammeter, conventional current, and why the current is the same at every point in a single loop and is not used up in a lamp.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why is the current the same all round a single loop, and what happens to it when a bigger resistor is fitted?

Key term

What is electric current?

Key term

Which particles flow to make a current in a metal wire?

Questions from this subtopic

Each one with the model answer underneath.

1
What is electric current?
Model answer
The rate of flow of electric charge.
2
Which particles flow to make a current in a metal wire?
Model answer
Electrons, which are free to move through the metal.
3
What two conditions must be met before charge will flow round a circuit?
Model answer
There must be a source of potential difference, and the circuit must be complete with no gaps.
4
A cell is joined to a lamp but one wire has come away from the lamp. Why is there no current?
Model answer
The circuit is not closed, so there is no complete path for charge to flow round.
5
A closed loop of copper wire on its own carries no current. Which condition is missing?
Model answer
There is no source of potential difference to push the charge round.
6
What is the unit of current, and which instrument measures it?
Model answer
The ampere, A, measured with an ammeter.

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Electricity

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Current means the rate of flow of electric charge.

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