Electricity Current

• defined as the rate of flow of electric charge
• There is electric current only when there are moving electric charges
• I = Q / t
• where Q is the amount of charge flows (in C), and t is the time taken (in s)
• SI unit is ampere, A
• The ammeter, milliammeter, and microammeter are current-measuring instruments and must be connected in series in the circuit

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Potential difference V and electromotive force, emf

• The potential difference or voltage, V between 2 points is defined as the work done, W in taking 1 C of positive charge from the lower potential to the point of higher potential.
• In a circuit, it is the energy sources that supply energy, not the electric charges.
• The energy that drives the free electrons around the circuit is known as emf
• More than 1 cell can be connected in a circuit (series/parallel)
  

Cells in series

• the combined emf used to drive the electric charges is the sum of all the individual cell's emf
• with more cells, the circuit will have more power to drive the electric charges
  

Cells in parallel

• the combined emf used to drive the electric charges is the emf of one individual cell (each cell contributes an equal amount of emf)
• with more cells, the circuit will have longer time to drive the electric charges
  

Potential difference = work done / charge transferred

• V = E / Q
• where V is potential difference, E is energy, Q is charges flow 
• SI unit is volt
  

Example
To transfer 2 C of charge from points X to Y in an electrical circuit 50 J of energy is needed. What is the potential difference between X and Y?

Solution
Potential difference between X and Y = 50 / 2 = 25 V

• 1 volt is defined as the potential difference between two points such that one joule of work is done in transferring 1 C of charge from one point to the other.
• The voltmeter and millivoltmeter are voltage-measuring instruments and must be connected in parallel to the component across which the potential difference is being measured.
• The emf of an electrical source like a battery is equal to the electrical energy provided by the source for every coulomb of charge which flows round the circuit.
• The emf of an electrical source can also be defined as the potential difference across the terminals of the source in an open circuit

Ohm's Law and Resistance

• Ohm's Law states that the current I, passing through a conductor is directly proportional to the potential difference, V between its ends provided that the physical conditions and temperature of the conductor remain constant.
• a resistor is a conductor with known value of resistance. It can be used to control (reduce) the size of current flowing in a circuit.
• Resistance, is therefore a measure of how difficult it is for the current to pass through the circuit.
• V / I = constant

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• Conductors or resistors which obey Ohm's Law are called ohmic.
• Eg. pure metal, copper sulphate solution with copper electrodes, metal alloy
• Those which do not obey Ohm's Law are called non-ohmic.
• The resistance, R of an electrical component is defined as the ratio of the potential difference, V across the component to the current, I flowing through it.

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Rheostat

• a variable resistor used to vary the control of electric current

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• The voltmeter is connected in parallel
• Use the rheostat to adjust the size of the current to a convenient value. Hence, record the readings shown on the ammeter and voltmeter
• adjust the rheostat to take 5 sets of readings of I and V
• Calculate the resistance from the equation R = V / I

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Factors affecting resistance of a wire

1. Length

• for a wire of uniform cross-sectional area, the resistance is directly proportional to the length of the wire
• hence, the longer the wire, the higher the resistance
  

2. Cross-sectional area

• for a wire of fixed length, its resistance is inversely proportional to the cross-sectional area
• so, the thinner the wire, the higher the resistance
  

3. Material 

• resistance depends on the kind of substance
• copper is a good conductor and is used for connected wires
• nichrome has more resistance and is used in the heating elements of electric heater
  

4. Temperature

• for metallic wires, as temperature increases, the resistance increases
• but for some materials like silicon and germanium as temperature increases, the resistance decreases

Electric Circuits

An electric circuit is a complete or closed path through which electric charges flow from one terminal of an electrical source to the other, passing through one or more circuit components.

Series circuit

• It has only one path for the current to flow.
• the sum of voltages across individual components in the circuit is equal to the voltage across the terminals of the electrical source or the whole circuit.
• Application: voltage divider
  

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Parallel circuit

• It has more than 1 path for the current to flow
• The sum of the currents flowing in the separate branches of a parallel circuit is equal to the current from the source.
• Application: electrical household connections

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Short circuit

• A short circuit occurs when a large current flows due to the very little or negligible resistance of the circuit
• A short circuit leads to
• overheating of wires which may cause electric fires
• damage of the electrical source (eg battry) and other circuit components
• To prevent short circuits, use fuse
  

        - fuses break the circuit if the current flowing through them exceed their respective ratings.

Diode

- A diode allows the electric current to flow in only ONE direction

• The follow arrow on the diode symbol shows that it is forward biased - the current flows easily
• The reverse arrow shows that the diode is reverse biased - the current is nearly zero
  

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Rectifier

• in a direct current or d.c. circuit, the current only flows in one direction, ie from positive to negative
• in a alternating current or a.c. circuit, the power supply can be controlled in such a way that the current alternates between forward and reverse directions, ie from positive to negative for a short period, then from negative to positive for another short period
• since a diode only lets current flow in the forward direction and stops all the reverse current, an a.c. can be changed into a d.c. by using a diode
• the conversion of an a.c. into a d.c. is called rectification
• the diode used to achieve rectification is called rectifier
• in half-wave rectification, the diode conducts in the forward half cycle of the a.c. (forward biased) and cuts off the reverse half cycle of the a.c. (reverse biased)
  

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