1. Current and charge In this section you will learn
The 3 effects of a an electric current
Electric conductors and insulators
What carrys electric current.
Size of electric current, units and formula Q = I t
The charge ón the electron.
What conventional current is.
Distinguish between d.c. and a.c
How current is measured
Parallel and series circuits
Formula for current splitting up at a junction.

2. Current and charge Three effects of an electric Current
(1) Heating effect
Electricity is converted to heat. e.g kettle, immersion heater.
(2) Magnetic effect.
Electricity is converted to magnetism. e.g. electric motor, electric bell, loudspeaker.
(3) Chemical effect.
Electricity causing a chemical reaction. e.g. electrolysis, electroplating

3. Current and charge
Heating effect of a electric current

4. Current and charge
Magnetic effect of a electric current

5. Current and charge
Chemical effect of a electric current

6. Electric current
Electric current is due to the flow of charge.
In a solid conductor, the charge is carried by electrons.
In a solid conductor, an electric current is due to the flow of electrons.
In a fluid, it is due to the flow of ions.
© TPS 2010

7. All metals Electric current Conductors include: copper gold silver
lead
All metals
And water (not distilled) which is why you should not use mains appliances in the presence of water.
© TPS 2010

8. Glass, unless it is very hot, is one of the best insulators available.
Electric current
Insulators include:
Rubber
Plastic
Most solid non metals
Glass
Glass, unless it is very hot, is one of the best insulators available.
© TPS 2010

9. Current electricity is what happens when static electricity flows.
Electric current
Current electricity is what happens when static electricity flows.
In solids, this means when electrons flow and they will do this as they are repelled from a negative charge and move towards a positive charge. So
electron flow is from negative to positive.
BUT
Before Physicists really knew what was happening, they assumed that it flowed from positive to negative. So
conventional current is from positive to negative.
From now on, when we say current, we mean conventional flow.
© TPS 2010

10. 1 mA = 0.001A that is 1 / 1 000 th of an ampere
Electric current
Electric current (I) is measured in ampere (A) - I is the symbol used to indicate current.
The “amp” is a rather large unit for most electronic applications so we use the following sub-multiples:
1 mA = 0.001A that is 1 / th of an ampere
You already know that 1mm is 1/1 000th of a metre so there is nothing new here.
1 A = A that is 1 / th of an ampere
© TPS 2010

11. Current is the rate of flow of charge so:
Electric current
Current is the rate of flow of charge so:
current = charge / time i.e. I = Q / t so current is also measured in Cs-1
This is much easier to remember as
Q = It
Charge Q is measured in coulomb (C). It tells us how much attraction or repulsion there will be. The coulomb is a very large unit. The charge on an electron is 1.6 x 10-19C. That is C
Question:
How much charge flows through a 200mA lamp in 5 minutes?
© TPS 2010

12. How much charge flows through a 200mA bulb in 5 minutes?
Electric current
Example 1
How much charge flows through a 200mA bulb in 5 minutes?
Answer
We want to use Q = It
but we must have things in ampere and seconds first.
I = 200mA = 0.2A (just like 200mm = 0.2m)
t = 5 minutes = 5 x 60 = 300s
Q = It = 0.2 x 300 = 60 C
© TPS 2010

13. no. of electrons = charge = 1 = 6.25 x 1018 Charge on 1e- 1.6 x 10-19
Electric current
Example 2
The charge on the electron is 1.6 x How many electrons are needed to give a charge of 1 C? Find the no. of electrons passing a any point in a circuit, per second when 10 A flows.
Answer
no. of electrons = charge = = 6.25 x 1018
Charge on 1e x 10-19
10 A = 10 C /s = 10 (6.25 x 1018 ) = 6.25 x 1019 passing / second
© TPS 2010

14. DC stands for direct current.
Electric current
DC stands for direct current.
This is a current that flows in one direction all the time.
Current
Time
Steady dc
Current
Time
Varying dc
© TPS 2010

15. AC stands for alternating current.
Electric current
AC stands for alternating current.
This is a current that flows in both directions.
Current
Time
High frequency ac
Current
Time
Low frequency ac
© TPS 2010

16. Which side of the bulb should you put the ammeter in the circuit?
Electric current
Electric current is measured using an ammeter. Look at this circuit.
Positive
Negative I I
You should see a cell (not a battery) and a bulb (or lamp).
The current into the bulb is the same as the current out of the bulb. It has to be. It is a flow of electrons. Where else could they go? They don’t get used up!
Which side of the bulb should you put the ammeter in the circuit?
© TPS 2010

17. Direct current is measured using an ammeter. Look at this circuit.
Electric current
Direct current is measured using an ammeter. Look at this circuit.
Positive
Negative I I
It doesn’t matter, the current doesn’t split so it is the same all the way around the circuit.
© TPS 2010

18. Electric current
Electric current is measured using an ammeter. Look at this circuit.
Positive
Negative - + A I I
Notice that it does matter about the polarity of the meter. The positive side of the ammeter goes next to the positive side of the cell.
© TPS 2010

19. Electric current Positive Negative I I
Now we have two cells. The current will be greater and the bulb brighter. These two cells constitute a battery.
© TPS 2010

20. Current in a series circuit
The current is the same at every point in the wire in a series circuit. (components connected one after the other)
The charges or electrons do not get used up only their energy is used up.
(use virtual physics lab. to show this)
Regardless of where the ammeter is put in the circuit,
Between the bulbs
Between the battery and bulb
Between the battery and switch
The current is the same. (Why?)

21. Current at a Junction of Conductors
The sum of the currents = the sum of the currents
Entering a junction leaving the junction

22. Measuring Current with an Ammeter
The ammeter must always be connected in series. In a series circuit it does not matter where the ammeter is inserted since the current is the same everywhere.
Show series and parallel pre-made circuits. Use an ammeter to show the current is the same in series circuits.
Ammeters A1,A1, A1, and A4 all give the same reading for the current.

23. Measuring Current with an Ammeter
The bulbs are in parallel in the circuit below (side by side) In a parallel circuit it the reading on the ammeter depends on the path the ammeter is placed. I1 I1
At junction A the current splits
I = I I2
15.8 =
At junction B
I1 = I I4
6.75 = I A B I3 I2 I4