HIGHER PHYSICS ELECTRICITY

D.C.

Current flows through the terminals in only one direction

A.C.

Current repeatedly changes direction and instantaneous value with time

Example of a D.C. supply

Battery

Example of an A.C. supply

UK mains

D.C. supply on an oscilloscope

* It appears as a straight line on an oscilloscope
* This shows the output voltage is constant

A.C. supply on an oscilloscope

* It appears as a wave on an oscilloscope
* This shows the output voltage varies with time

peak voltage

In an A.C. supply the max voltage is called the peak voltage

r.m.s value

The r.m.s value of an alternating voltage/current is equal to the value of the direct voltage/current which gives the same power output

What is the UK mains rms?

230V

What is the frequency of UK mains?

50Hz

Finding peak voltage on an oscilloscope

* Count the number of boxes from the centre of the wave to the top
* Multiply this by the y-gain setting to get peak voltage

Finding frequency on oscilloscope

* Count the number of boxes for one complete wave (i.e. one wavelength)
* Multiply by time base setting to get the period of the wave, T
* Use equation f = 1/T

Electric current

The electric charge transferred per unit time

Potential difference

A measure of the energy given to each coulomb of charge that passes through the power supply

Resistance

A measure of the opposition to the flow of charge in an electrical current

Current and voltage in a series circuit

Is = I1 = I2 = I3

Vs = V1 + V2 + V3

Current and voltage in a parallel circuit

Is = I1 + I2 + I3

Vs = V1 = V2 = V3

Electrical power

The electrical energy transformed per second

EMF

The energy given to a coulomb of charge by a voltage supply

Internal resistance

A power supply has a small resistance itself. It is a conductor and all conductors have some resistance

Lost volts

Small loss in voltage due to heat wasted when current flows through the power supply

Terminal potential difference (tpd)

The potential difference measured across the terminals of the power supply and therefore also the maximum voltage available for the external circuit

Ideal supply

A supply with zero internal resistance, provides a constant emf

Short circuit

A circuit with an external resistance of zero

Open circuit

A circuit with a break in it so no current flows