#### 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 =
^{V²}/_{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

- I =

**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}

- R =

**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 × 10
^{18}ē

**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