1. Current

2. Electric Current (I) This is the flow of electric charge
In a metal conductor it is the flow of electrons
Size of current in a conductor is the amount of charge passing any point of that conductor per second
Q = It
Unit Amp A

3. Electric Current + _ Electrons flow from – to +
Conventional current flows from + to – i.e. flow of positive charge + _ e-
d.c. direct current flows in one direction in a closed circuit, it is caused by a power supply
a.c. alternating current is when the current constantly reverses direction e.g. mains current alternates 100 times per second

4. Measuring Electric Current
An Ammeter is used to measure current and is always connected in series in the circuit
A Galvanometer is a sensitive ammeter/microammeter
A Multimeter can also be set to measure current

5. Series Vs Parallel + _ Total I = I1 = I2 = I3 Total I = I1 + I2 +I3 I1

6. Effects of an Electric Current
Heat
Chemical
Magnetic

7. The Effects of an Electric Current
Heating
Connect the circuit as in the diagram. Note the temperature on the thermometer. Switch on the power supply. Note the current. After 5 minutes note the temperature. The current causes the temperature to rise. A

8. The Effects of an Electric Current
Magnetism
Connect the circuit as shown. Arrange the wire so that it is parallel to the compass. Switch on the power. The compass turns perpendicular to the wire demonstrating the magnetic effect of an electric current. A A

9. The Effects of an Electric Current
Chemical effect
Set up the circuit as shown. Switch on the power supply and note the current. Copper can be seen to deposit on the cathode demonstrating the chemical effect of an electric current.

10. Effects of an Electric Current
Electrolysis is the chemical effect of an electric current
Voltameter consists of electrodes, an electrolyte and a container
Inactive electrodes are electrodes that don’t take part in the chemical reaction e.g. platinum in H2SO4
Active electrodes are electrodes that take part in the chemical reaction e.g. copper in CuSO4

11. Relationship between V and I for conductors
Metallic conductor
Electrons are the charge carriers. Current is proportional to voltage.
Ohmic
Filament bulb
Electrons are the charge carriers.
As the filament gets hot its resistance increases and this restricts the current. I V
Non Ohmic

12. Relationship between V and I for conductors
Semiconductor
Electrons and positive holes are the charge carriers.
The voltage has to reach a certain value for the diode to switch on when in forward bias. After this point the current increases. In reverse bias the voltage does not affect the drift current. I V

13. Relationship between V and I for conductors
Active electrodes
eg copper electrodes in copper sulphate solution.
Positive and negative ions are the charge carriers
Ohmic
Inactive (Inert) electrodes
eg carbon electrodes in hydrochloric acid
Positive and negative ions are the charge
carriers. A voltage is produced between the plates which must be overcome before current will flow. I V
Non Ohmic

14. Relationship between V and I for conductors
Vacuum If the cathode gets hot electrons can be released by thermionic emission and they will move under the influence of the potential difference. Electrons are the charge carriers. I V

15. Relationship between V and I for conductors
Gas
Positive and negative ions and electrons are the charge carriers.
Initially increasing the voltage increases the current. At A all available charge carriers are moving. At B further charges have been created by collisions and the current again increases. I V A B

16. I V I V
The VI Graphs! I V I V I V A B I V

17. RESISTANCE

18. Resistance (R)
This is the ratio of the p.d. across a conductor to the current flowing through it.
V α I
V = RI
Where R is a constant called resistance
Unit: Ohm 

19. Factors affecting Resistance of a conductor
Resistance depends on
Temperature
The resistance of a metallic conductor increases as the temperature increases e.g. copper
The resistance of a semiconductor/insulator decreases as the temperature increases e.g. thermistor .

20. Factors affecting Resistance of a conductor
2. Length
Resistance of a uniform conductor is directly proportional to its length.
i.e. R  L
3. Cross-sectional area
Resistance of a uniform conductor is inversely proportional to its cross-sectional area.
i.e. R  1 A

21. Factors affecting Resistance of a conductor
R = L  = constant of proportionality called
A resistivity
Unit: ohm meter  m
 = Rd 2 4L
(For a wire with circular cross-sectional area)

22. Factors affecting Resistance of a conductor
Resistivity of a substance is the resistance of 1m3 of the substance.

23. Measuring Resistance Ohmmeter (portable but inaccurate)
Measure current through and voltage across the resistor and R=V/I (easy to use but inaccurate)
Wheatstone bridge (portable but expensive)
Metre bridge (Accurate but cumbersome)

24. Series Vs Parallel + _
Bulb + _

25. Resistors in Series and Parallel
In Series the total resistance is
R = R1 + R2 + R3 R1 R2 R3
In Parallel the total resistance is
1 =
R R1 R2 R3 R1 R2 R3

26. Ohm’s Law
This states that for certain conductors (mainly metals) the current flowing through them is directly proportional to the p.d. across them at a constant temperature.
V = IR

27. Wheatstone Bridge Uses Temperature control
Fail-Safe Device (switch circuit off)
Measure an unknown resistance
R1 = R3 (When it’s balanced)
R R4 I r1 r2 r4 r3 A C B D

28. The Metre Bridge I r1 r2 a c b

29. Joules Law
Joule’s Law states that the rate at which heat produced in a conductor is directly proportional to the square of the current provided its resistance is constant
i.e. P = I 2R

30. Transmission of Electricity
From Joule’s law the larger the current the more heat produced hence a transformer can be used to increase voltage and lower current
i.e. P = V I
In order to prevent power lines from energy loss due to heating, electricity is transmitted at a very high voltages and low currents.

31. Domestic Electric Circuits
Electricity entering the home is supplied at 230V a.c.
2 wires enter the house from the mains: Live + neutral and pass through the meter box
These 2 wires pass into a distribution box with fuses

32. Domestic Electric Circuits
Radial circuit are used for appliances that take a large current. e.g. cooker, electric shower
Ring circuit are used for connections to sockets.
Lights are connected in parallel and a
number of them are connected to the
same fuse

33. Safety in house circuits
Switch: should always be connected in the live wire
Fuse: piece of wire that will melt when a current of a certain size passes though it. Connected to the live wire.

34. Safety in house circuits
MCBs: miniature circuit breakers are found in the distribution box. They are bimetallic strips(for small currents) and electromagnets (for large currents). They can be reset when the switch trips and are faster than fuses.
RCDs: residual current devices protect sockets and people against electrocution by detecting a difference between current in live and neutral wire.

35. Safety in house circuits
Bonding: All metal taps, pipes, water tanks etc are connected to the earth
Earthing: Earth wire prevents electrocution from touching metal parts of appliances by providing a path of least resistance when faults occur.

36. E.S.B
Kilowatt-hour kWh
This is the amount of energy used by a 1000 W appliance in one hour
The ESB charge bills based on the no. of units, kWh, used in the home

37. Formulae Needed
Electrical energy = resistance x current squared x time
Power=voltage x current
Power = energy / time