Alternating Current

Question 1

what is the general setup of an ac generator

Answer 1

• a coil of wire is placed between a north and south pole
• the coil of wire is connected to a circuit with other components like a bulb
• carbon brushes and slip rings are what make that connection

Question 2

the north pole is on the left and the south pole is on the right. the current in the coil of wire flows from the left side of the coil, into the field, and back to the right side. what direction is the coil of wire spinning in (clckwise or anti-clockwise)

Answer 2

clockwise

Question 3

using flemings left hand rule, what is the direction of the force that the left side of the coil would be experiencing

Answer 3

• the first finger field is pointing from the left to the right
• on the left side of the coil, the current is flowing away from us
• so the second finger conventional current would be pointing towards us
• making the thumb motion point upwards

Question 4

using flemings left hand rule, what is the direction of the force that the right side of the coil would be experiencing

Answer 4

• with the right side, the current is flowing towards us
• so the second finger would be pointing away from us
• meaning the thumb motion would be pointing downwards

Question 5

what is the result of these two subsequent forces

Answer 5

• a turning force is created

- which would rotate the coil of wire clockwise

Question 6

if a coil of wire is rotating in a magnetic field, how would the induced emf change

Answer 6

• an emf would be induced constantly

- buts its direction will keep alternating

Question 7

why would the emf change like this

Answer 7

because the direction of the coils position relative to the field keeps alternating

Question 8

what does the alternating polarity of the induced emf mean for the current flowing through the wire

Answer 8

• the current will also be alternating

- producing an alternating current

Question 9

what would a graph of current on the y axis over time on the x axis look like

Answer 9

• a waveform / sin graph

- alternating from the +ve end to the -ve end

Question 10

what is the time period of a wave

Answer 10

the time it takes for one complete oscillation (one wavelength long) to occur

Question 11

how would you measure the time period of an ac current on its graph

Answer 11

• look for a clear start and end point for one oscillation of the wave
• measure the time between the points using the time axis

Question 12

what would you then do to work out the frequency of the ac current

Answer 12

• frequency = 1 / time period

- so you would divide 1 by the time period you measured

Question 13

what should you be wary of when reading the time period off of the time axis

Answer 13

• the units the time is in
• it would be seconds or milliseconds or anything else
• so you would need to convert into seconds

Question 14

what is the common frequency of uk main electricity

Answer 14

50Hz

Question 15

what does this frequency of 50Hz tell you about the generators in the power stations, considering the definition of frequency

Answer 15

• frequency is the number of (complete) oscillations per second
• implying the generators are rotating 50 times a second

Question 16

what is the peak value on the graphs of ac current / voltage over time (as they both follow a sinusoidal waveform)

Answer 16

• the maximum value the line reaches

- regardless of sign / direction

Question 17

what is an oscilliscope

Answer 17

• a voltmeter

- that gives off a visual / graphical reading of pd over time

Question 18

what kind of graph would show up on an oscilloscope it it was connected to an alternating pd

Answer 18

a sinusoidal graph

Question 19

considering the setup of an oscilloscope, why does a sinusoidal graph appear

Answer 19

• the pd reading is shown by a small dot on the screen
• the vertical position of the dot shows the pd value
• the horizontal position is simply time
• as time goes by, the dot will travel up and down as the pd alternates
• while it gets moved forwards as time passes
• the combination of these creates the sinusoidal graph

Question 20

what variables on the x and y axis on an oscilloscope do you need to calibrate

Answer 20

• the x axis time-base (dot speed control)

- the y axis volts-per-division (pd sensitivity control)

Question 21

what does volts-per-division mean

Answer 21

• the amount of pd one square indicates

- not the squares area, but its length

Question 22

what is the root-mean-square value

Answer 22

the calculated average of alternating supplies

Question 23

why is it important to know the root-mean-square value of an alternating supply

Answer 23

• so you can compare it to dc supplies consistently

- and for safety reasons

Question 24

what is the equation for the root-mean-square value of voltage and current

Answer 24

• I(rms) = I(0) / root 2

- V(rms) = V(0) / root 2

Question 25

what does the 0 in I(0) and V(0) mean

Answer 25

• their peak values

- probably read from their graphs

Question 26

what is the main feature of a power over time graph with alternating current or pd you need to remember

Answer 26

• the line never drops below 0 (into the -ve end)
• as P = I^2 * R or V^2 / R, P can never be -ve
• as a - ve pd or current would just be squared