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Explainer Video

Tutor: Lex

Summary

Alternating current and voltage

In a nutshell

An alternating current is a type of current which periodically reverses direction. The root mean square values are a type of average used for sinusoidal waveforms.

Equations

description	equation
R.m.s. voltage	$V_{rms}=\dfrac{V_0}{\sqrt{2}}$
R.m.s. current	$I_{rms}=\dfrac{I_0}{\sqrt{2}}$

Variable definitions

quantity name	symbol	derived units	si base units
$voltage$	$V$	$V$	$kg\ m^2\ s^{-3}\ A^{-1}$
$current$	$I$	$A$	$A$

Alternating current

An alternating current (AC) is a type of electric current which periodically reverses direction. It is used in generators, motors and power distributors due to being more efficient than direct current (DC), which is current flowing in only one direction.

An alternating current follows a waveform such as the one shown below:

Physics; Electromagnetism; KS5 Year 12; Alternating current and voltage

1.	Current
2.	Time

The frequency of the current is the number of times the generators spin per second. The UK mains electricity has a frequency of $50\ Hz$ .

Root mean square voltage and current

Since alternating current follows a waveform it has peak values. These are the maximum values, regardless of sign, of sinusoidal waves.

The peak voltage $V_0$ is the maximum voltage value, regardless of sign, of an alternating current.
The peak current $I_0$ is the maximum value, regardless of sign, of an alternating current.

To determine the average voltage or current of an alternating current, a simple mean would not work as it will be $0$ due to it being a wave. Scientists therefore use the root mean square (r.m.s.) values which are a different type of averaging technique.

The r.m.s. voltage $V_{rms}$ can be calculated with the formula:

$V_{rms}=\dfrac{V_0}{\sqrt{2}} \\$

The r.m.s. current $I_{rms}$ can be calculated with the formula:

$I_{rms}=\dfrac{I_0}{\sqrt{2}}$

Example

The UK mains electricity has a peak voltage of $\pm325\ V$ , calculate its r.m.s. value.

First, write down what you know:

$V_0=325\ V$ 

Next, write down the formula for the r.m.s. voltage:

$V_{rms}=\dfrac{V_0}{\sqrt{2}}$ 

Substitute the values and calculate the r.m.s. voltage:

$V_{rms}=\dfrac{325}{\sqrt{2}}=230\ V$ 

The r.m.s. voltage of the UK mains electricity is $\underline{230\ V}$ .

Oscilloscopes

An oscilloscope is a type of voltmeter where you can manipulate how readings are displayed. You can do this by changing the time-base ( $x-axis$ ) and the amplitude ( $y-axis$ ). An oscilloscope can be used to obtain information about an alternating current, such as peak values for current and voltage.

1.	Display
2.	Time-base dial
3.	Amplitude dial

Learn with Basics

Length:

Unit 1

Electrical circuits

Unit 2

Household electricity

Jump Ahead

Optional

Unit 3

Alternating current and voltage

Final Test

Create an account to complete the exercises

FAQs - Frequently Asked Questions

What is an alternating current?

An alternating current is a type of current which periodically reverses direction.

What is a root mean square value?

A root mean square value is a type of average used for sinusoidal waveforms.

What is the peak value of a sinusoidal waveform?

The peak values of a sinusoidal waveform are the maximum values, regardless of sign, of the wave.

Home

Physics

Electromagnetism

Alternating current and voltage

Videos

Summary

Exercises

Select Lesson

Exam Board

Select an option

Oscillations

Simple harmonic motion

Displacement, velocity and acceleration for simple harmonic motion

Energy within simple harmonic motion

Damping and resonance

Simple harmonic oscillator and pendulum

Gravitational Fields

Gravitational fields

The law of gravitation

Kepler's laws

Gravitational potential and gravitational potential energy

Nuclear physics

Einstein's mass-energy equation

Nuclear fission and nuclear reactors

Nuclear fusion

Radioactivity

Radioactivity: alpha, beta and gamma radiation

Half-life and radioactive decay

Cosmology

Units for astronomical distances

Hubble's law

The evolution of the Universe

Thermal physics

Measuring temperature and temperature scales

Solids, liquids and gases

Internal energy

Specific latent heat and specific heat capacity

Ideal gases

The ideal gas law

Kinetic theory of gases: Root mean square speed

Black-body radiaton and stellar luminosity

Particle physics

The nuclear model of the atom

Magnitudes of the atom

Fundamental particles

Quarks and anti-quarks

Particle accelerators and detectors

Electromagnetism

Magnetic fields and electromagnetism

Magnetic flux density

Charged particles in a magnetic field

Faraday's and Lenz's Laws

Uses of electromagnetic induction

Alternating current and voltage

Capacitance

Capacitors and capacitance

Energy stored in a capacitor

Charging and discharging capacitors

Electric fields

Coulomb's law

Electric field between two parallel plates

Motion of charged particles in an electric field

Electric potential and potential energy

Circular Motion

Angular velocity and the radian

Centripetal acceleration

Centripetal force

Quantum physics

The photon model

The photoelectric effect

The photoelectric effect equation

Wave-particle duality

Energy levels and spectra

Lenses

Power of a lens

Ray diagrams, the thin lens equation and magnification

Waves

Progressive waves

Waves: displacement-time and distance-time graphs

Refraction and Snell's law

Reflection and the critical angle

Intensity of a wave

Diffraction and polarisation

The principle of superposition

Young's double-slit experiment

Stationary waves

Harmonics

Materials

Hooke's law

Elastic and plastic deformation

Stress, strain and Young's modulus

Stress-strain graphs

Fluids

Density and pressure

Fluid flow and viscosity

Terminal velocity

Electrical circuits

Circuits: diagrams and components

Potential difference and electromotive force

Resistance and Ohm's Law

Resistivity

I-V characteristics

Conductors and semiconductors

Combining resistors and Kirchhoff's Laws

Internal resistance

Potential dividers

Current and charge

Current and charges

Conventional current and electron flow

Mean drift velocity

Newton's laws of motion and momentum

Momentum: definition, conservation and calculations

Collisions in two dimensions

Newton's laws of motion

Energy

Forms of energy, conservation and work done

Kinetic energy and gravitational potential energy

Power and efficiency

Motion

Scalar and vector quantities

Resolving vectors

Displacement and velocity

Acceleration and velocity-time graphs

Equations of motion

Acceleration and free fall

Projectile motion

Mass, weight and centre of mass

Resolving forces

Triangle of forces

The principle of moments

Working as a physicist

Physical quantities and units

How to plan an experiment

Hazards, risks and results

Analysing data

Evaluating data

Explainer Video

Tutor: Lex

Summary

Alternating current and voltage

In a nutshell

An alternating current is a type of current which periodically reverses direction. The root mean square values are a type of average used for sinusoidal waveforms.

Equations

description	equation
R.m.s. voltage	$V_{rms}=\dfrac{V_0}{\sqrt{2}}$
R.m.s. current	$I_{rms}=\dfrac{I_0}{\sqrt{2}}$

Variable definitions

quantity name	symbol	derived units	si base units
$voltage$	$V$	$V$	$kg\ m^2\ s^{-3}\ A^{-1}$
$current$	$I$	$A$	$A$

Alternating current

An alternating current follows a waveform such as the one shown below:

1.	Current
2.	Time

The frequency of the current is the number of times the generators spin per second. The UK mains electricity has a frequency of $50\ Hz$ .

Root mean square voltage and current

Since alternating current follows a waveform it has peak values. These are the maximum values, regardless of sign, of sinusoidal waves.

The peak voltage $V_0$ is the maximum voltage value, regardless of sign, of an alternating current.
The peak current $I_0$ is the maximum value, regardless of sign, of an alternating current.

The r.m.s. voltage $V_{rms}$ can be calculated with the formula:

$V_{rms}=\dfrac{V_0}{\sqrt{2}} \\$

The r.m.s. current $I_{rms}$ can be calculated with the formula:

$I_{rms}=\dfrac{I_0}{\sqrt{2}}$

Example

The UK mains electricity has a peak voltage of $\pm325\ V$ , calculate its r.m.s. value.

First, write down what you know:

$V_0=325\ V$ 

Next, write down the formula for the r.m.s. voltage:

$V_{rms}=\dfrac{V_0}{\sqrt{2}}$ 

Substitute the values and calculate the r.m.s. voltage:

$V_{rms}=\dfrac{325}{\sqrt{2}}=230\ V$ 

The r.m.s. voltage of the UK mains electricity is $\underline{230\ V}$ .

Oscilloscopes

1.	Display
2.	Time-base dial
3.	Amplitude dial

Learn with Basics

Length:

Unit 1

Electrical circuits

Unit 2

Household electricity

Jump Ahead

Optional

Unit 3

Alternating current and voltage

Final Test

Create an account to complete the exercises

FAQs - Frequently Asked Questions

What is an alternating current?

An alternating current is a type of current which periodically reverses direction.

What is a root mean square value?

A root mean square value is a type of average used for sinusoidal waveforms.

What is the peak value of a sinusoidal waveform?

The peak values of a sinusoidal waveform are the maximum values, regardless of sign, of the wave.

Alternating current and voltage

Videos

Summary

Exercises

Select Lesson

Exam Board

Oscillations

Gravitational Fields

Nuclear physics

Radioactivity

Cosmology

Thermal physics

Particle physics

Electromagnetism

Capacitance

Electric fields

Circular Motion

Quantum physics

Lenses

Waves

Materials

Fluids

Electrical circuits

Current and charge

Newton's laws of motion and momentum

Energy

Motion

Working as a physicist

Explainer Video

Summary

Alternating current and voltage

​​In a nutshell

description

equation

​​quantity name

symbol

derived units

si base units

Alternating current

Root mean square voltage and current

Example

Oscilloscopes

Learn with Basics

Unit 1

Electrical circuits

Unit 2

Household electricity

Jump Ahead

Unit 3

Alternating current and voltage

Final Test

FAQs - Frequently Asked Questions

What is an alternating current?

What is a root mean square value?

What is the peak value of a sinusoidal waveform?

Alternating current and voltage

Videos

Summary

Exercises

Select Lesson

Exam Board

Oscillations

Gravitational Fields

Nuclear physics

Radioactivity

Cosmology

Thermal physics

Particle physics

Electromagnetism

Capacitance

Electric fields

Circular Motion

Quantum physics

Lenses

Waves

Materials

Fluids

Electrical circuits

Current and charge

Newton's laws of motion and momentum

In a nutshell

quantity name

In a nutshell

quantity name