Current
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
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
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
Series Vs Parallel + _ Total I = I1 = I2 = I3 Total I = I1 + I2 +I3 I1
Effects of an Electric Current Heat Chemical Magnetic
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
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
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.
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
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
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
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
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
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
I V I V The VI Graphs! I V I V I V A B I V
RESISTANCE
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
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 .
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
Factors affecting Resistance of a conductor R = L = constant of proportionality called A resistivity Unit: ohm meter m = Rd 2 4L (For a wire with circular cross-sectional area)
Factors affecting Resistance of a conductor Resistivity of a substance is the resistance of 1m3 of the substance.
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)
Series Vs Parallel + _ Bulb + _
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 = 1 + 1 + 1 R R1 R2 R3 R1 R2 R3 http://lectureonline.cl.msu.edu/~mmp/kap20/RR506a.htm
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
Wheatstone Bridge Uses Temperature control Fail-Safe Device (switch circuit off) Measure an unknown resistance R1 = R3 (When it’s balanced) R2 R4 I r1 r2 r4 r3 A C B D
The Metre Bridge I r1 r2 a c b
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
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.
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
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
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.
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.
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.
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
Formulae Needed Electrical energy = resistance x current squared x time Power=voltage x current Power = energy / time