Alternating Current 1

8/2/2019 Alternating Current 1

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Alternating Current

Electricity Lesson 11



Learning Objectives

• To know what is meant by alternating current.

•

To know how to calculate the rms value of analternating current.

•To know how to calculate the power suppliedby an alternating current.



Recap

• What is the difference between an alternating

current and a direct current?

•

phet simulation (AC connected to 1 ohmresistor).



Definition

• From the specification book:-

•

An alternating current (ac) is a current thatrepeatedly reverses its direction.



Questions

• What is frequency? What is the unit of

frequency?

• What is the frequency of mains electricity?

•

Calculate the period of mains electricity.



Answers

• Frequency is the number of cycles per second.

•

The unit of frequency is the hertz (Hz)

• Mains electricity is an alternating current with

a frequency of 50 Hz too fast to noticeflickering.

frequency

1period s

f

T 020.0

Hz50

11



Peak Value

• The peak value of an alternating current (or pd) is

the maximum current (or pd) which is the same in

either direction.

• One complete alternation is called a cycle (NOTwavelength).

• The graph is called a sinusoidal waveform or a sine

wave.



Peak Current

• This depends on the peak pd and the

components in circuit.

• If the peak pd was 260 V and the resistance of

the components in the circuit is 56 Ω, what

would the peak current be?



Peak Current

• This depends on the peak pd and the

components in circuit.

• If the peak pd was 260 V and the resistance of

the components in the circuit is 56 Ω, what

would the peak current be?

A64.4 55

V260

R

VI



Why do we use ac?



Important Physics Ideas

• Conservation of energy (power in=power out).

•

Faraday's law of induction (changing magneticfield induces a electric current).

•

Ampere’s law (electric current passing througha wire results in magnetic field around it).



Why do we use ac?

• Power dissipated in the wires =I2/R.

• So we want to transmit electricity using low I.

•

Transformers can be used to change the pd of ac.• Power going into transformer = Power going out of

transformer (Pi = Po) (IiVi=IoVo).

•

An electric current only gets induced in a wire whenthere is a changing magnetic field.

• There is only a changing magnetic field if there is achanging electric field i.e. alternating pd.



Observing AC

• An oscilloscope can be used to display the

waveform of the alternating pd from a signal

generator. Demo

• What effect does increasing the output pd

have?

• What effect does increasing the frequency

have?



These features are shown on the

graph:



Heating Effect of AC

• Imagine a heater connected to an ac supply.

• It would repeatedly heat up then cool down.

• The power, P, supplied to a heater of resistance R:-

• Recall that direction of the current doesn’t matter sothe power supplied varies with the square of thecurrent.

RIP

2



Heating Effect of AC

• When I is at its maximum, I0, max. power is supplied.

• When I is zero, zero power is supplied.

• The mean power supplied:-

• This is from the symmetrical shape of the power

curve.

RIP2

021

mean



Root Mean Square Value

• The root mean square value of an alternating current

is the value of direct current that would give the same

heating effect as the alternating current in the same

resistor.

RII2

0212

rms R2

021

2

rms II

2

II 0rms

2

VV 0rms

R2rmsIP



Question 1

• What is the rms voltage of an ac mains with a

peak voltage of 325 V?

• What would be the power supplied to a 70

ohm resistor?



Answer

• Vrms = 325 / √2 = 230 V

•

R2

rmsIP

R

VI

rmsrms



Root Mean Square Value

• The values of voltage and current are constantly

changing in AC, unlike in DC in which they are

steady. We can measure AC voltages in two ways:-

• Measure the peak to peak voltage, easily done on acathode ray oscilloscope (CRO).

• Measure the root mean square (rms) value, or the

effective value.

• We use the rms value because its use allows us to do

electrical calculations as if they were direct

currents.



Summary

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Alternating Current 1