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#### Current Electricity and Electrical Quantities

##### Current Electricity
• Electric current is the flow of charged particles
• Charge at a point is the product of the current at that point and the time for which the current flows
• Q = It
• Coulomb is the charge flowing per second past a point at which the current is one ampere
• Charge is Quantized: values of charge are not continuous, they are discrete
• All charges are multiples of 1e: 1.6 × 10-19 C
• Potential Difference (p.d.): Two points are a potential difference of 1 V if the work required to move 1 C of charge between them is 1 Joule
• Volt: is joule per colulomb
• P = VI
• W = VQ
• P = I²R
• P = /R
###### Current-Carrying Conductors • Electrons move in a certain direction when p.d. is applied across a conductor causing current
• Deriving a formula for current:
• I = Q/t
• t = L/v
• Volume of container = LA
• Number of free electrons = nLA
• Total charge = Q = nLAq
• ∴ I = nLAq/L/v
• ∴ I = Anvq
• where:
• L = length of conductor
• A = cross-sectional area of conductor
• n = number of free electrons per unit volume
• q = charge on 1 electron
• v = average electron drift velocity
###### Current P.D. Relationships • Ohm’s Law states that the current in a component is proportional to the potential difference across it, provided all physical conditions (e.g. temperature) stay constant
###### Resistance
• Resistance is the ratio of potential difference to the current
• Ohm is volt per ampere
• V = IR
• Resistivity is the resistance of a material of unit cross-sectional area and unit length
• R = ρL/A
##### Electrical Quantities
###### Electric Charge
• You can detect an electrostatic charge using a leaf electroscope
• If a charged object is placed near the cap, charges are induced
• The metal cap gets one type of charge (positive or negative) and the metal stem and gold leaf get the other type of charge so they repel each other • There are 2 types of charges: positive and negative
• Unlike charges attract and like charges repel
• Electric Field is the region in which an electric charge experiences a force
• Conductors are materials that let electrons pass through them
• Metals are the best electrical conductors as they have free electrons
• Insulators are materials that hardly conduct at all
• Their electrons are tightly held to their atoms and hardly move, bu they can be transferred by rubbing
• The SI unit of charge is the Coulomb (C)
###### Electric Field Lines ###### Induced Charge
• An induced charge is a charge that ‘appears’ on an uncharged object because of a charged object nearby
• For example, if a positively charged rod is brought near a small piece of aluminum foil, electrons in the foil are pulled towards the rod, which leaves the bottom of the foil with a net positive charge
• The attraction is stronger than repulsion because the attracting charges are closer than the repelling ones
###### Current
• Current is the flow of charge, its SI unit is the Ampere (A)
• An ammeter measures the current in a circuit and is connected in series
• Current is a rate of flow of charge
• Charge (C) = Current (A) × Time (s)
• Q = It
• Current follows the path of least resistance
• The conventional current is the opposite of what actually happens: • red = conventional current
• green = actual current
• 1 ē = 1.6 × 10-19
• 1 C = 6.25 × 1018
###### Electromotive Force (EMF)
• The maximum voltage a cell can produce is called the electromotive force (EMF), measured in volts
• When a current is being supplied, the voltage is lower because of the energy wastage inside the cell
• A cell produces its maximum p.d. when not in a circuit and not supplying current
###### Potential Difference (P.D.)
• Potential difference is also known as voltage
• Voltage is the amount of energy the cell gives the electrons it pushes out.
• Voltage is measured in volts (V) and is measured by a voltmeter (connected in parallel). If a cell has 1 Volt, it delivers 1 Joule of energy to each coulomb of charge (J/C)
• Voltage = Energy/Charge
• V = E/C
###### Resistance
• Resistance (Ω) = Voltage/Current = V/I
• Factors affecting resistance:
• Length; R ∝ L
• Cross-sectional area; R ∝ 1/A
• Material: If the material is a better conductor, there would be less resistance
• Temperature:
• For metal conductors, the higher the temperature, the more the resistance
• For semi-metal conductors, the higher the temperature, the less the resistance
###### V-I Characteristics of a Resistor • Ohm’s Law states that the voltage across a resistor is directly proportional to the current through it
• This is only true if the temperature of the resistor remains constant
###### Electrical Energy
• 1 Watt is 1 J/s
• Electrical power = Voltage (V) × Current (A)
• P = VI
• Electrical Energy = Voltage × Current × Time
• E = VIt
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