Electric Current A flow of electrons Chapter 19 (20 hons)

A charge in an electric field experiences a force. If the charge is in a conductor, it can move. A charge in an electric field experiences a force. If the charge is in a conductor, it can move. CURRENT: A flow of electrons. Symbol, I. Unit Ampere (A). CURRENT: A flow of electrons. Symbol, I. Unit Ampere (A). I = Δq / Δt I = Δq / Δt

Sources of voltage Batteries: Either wet (like car battery) or dry (like alkaline batteries) cells. Batteries: Either wet (like car battery) or dry (like alkaline batteries) cells. Generators: Like COMED – power to your house. Kinetic energy changes to voltage Generators: Like COMED – power to your house. Kinetic energy changes to voltage Photo-voltaic (Solar) cells: Sunlight is changed to voltage Photo-voltaic (Solar) cells: Sunlight is changed to voltage

Conventional current is defined in terms of movement of (+) charges even though they don’t move. So current goes from ____ to ____ Conventional current is defined in terms of movement of (+) charges even though they don’t move. So current goes from ____ to ____ Drift speed of electrons is actually very slow, however, current flows in a circuit almost instantly when a field (voltage) is applied. Drift speed of electrons is actually very slow, however, current flows in a circuit almost instantly when a field (voltage) is applied.

CIRCUIT: A closed path, around which a current can flow (see board) CIRCUIT: A closed path, around which a current can flow (see board) VOLTAGE : Electrical “pressure’ that causes current to flow. Symbol, V. Unit Volt, (v). Voltage is also known as potential difference. VOLTAGE : Electrical “pressure’ that causes current to flow. Symbol, V. Unit Volt, (v). Voltage is also known as potential difference.

Analogy of water and electricity WaterElectricity Pressure (p.s.i.) Potential diff. (V) Volume (L) Charge (Q) Flow Rate (G.P.M.) Current (A) Flow restriction Resistance (Ω)

Resistance RESISTANCE: Tendency of a circuit to oppose the current. Symbol R, Unit Ohm, Ω. Resistance is due to energy loss as the collisions between electrons and atoms are not totally elastic. RESISTANCE: Tendency of a circuit to oppose the current. Symbol R, Unit Ohm, Ω. Resistance is due to energy loss as the collisions between electrons and atoms are not totally elastic. Depends on Depends on Material Material Length Length Crossectional area Crossectional area Temperature (resistance proportional to it) Temperature (resistance proportional to it)

Insulators have _________ resistance. Insulators have _________ resistance. Conductors have _________ resistance. Conductors have _________ resistance. Superconductors have zero resistance. Superconductors have zero resistance. Ohm’s Law : The current in a circuit is proportional to the voltage and inversely proportional to the resistance Ohm’s Law : The current in a circuit is proportional to the voltage and inversely proportional to the resistance I= V R P.703 Q1-5 Hons: 3 probs. on resistivity sheet and P639 Q1 – Q8

Power and electric current As a current flows through a resistance, energy is converted from electrical potential energy to other forms (often thermal, but also kinetic, light etc.) As a current flows through a resistance, energy is converted from electrical potential energy to other forms (often thermal, but also kinetic, light etc.) Power = rate of energy conversion Power = rate of energy conversion P = I V = I 2 R = V 2 /R P = I V = I 2 R = V 2 /R where I is current through the resistance, V is the potential across the resistance.

Cost of Electric energy Energy = power X time Energy = power X time Electricity is charged (pun) by the kiloWattHour Electricity is charged (pun) by the kiloWattHour A kiloWattHour costs about $ ____ in Illinois. A kiloWattHour costs about $ ____ in Illinois. Do Page 710 Q1 & 2, P712 Q1 & 2. P713 Q1 to Q5 Do Page 710 Q1 & 2, P712 Q1 & 2. P713 Q1 to Q5 Hons: P Q21-26 and Q28,28. Hons: P Q21-26 and Q28,28.

What are electric circuits ? (CH20 reg) Circuits typically contain a voltage source, a wire conductor, and one or more devices which use the electrical energy. What is a series circuit? A series circuit is one which provides a single pathway for the current to flow. If the circuit breaks, all devices using the circuit will fail.

Resistance of a series circuit R Tot = R 1 + R 2 + …. R n The most important thing to remember about series connected resistances is that the CURRENT through each resistance is the same. Why ? Charge has only one path, it can’t just disappear. Homework: (Hons) P640 Q Reg. P739 Q1-6.

What is a parallel circuit? A parallel circuit has multiple pathways for the current to flow. If the circuit is broken the current may pass through other pathways and other devices will continue to work.

Resistance of a parallel circuit R tot = 1 / 1/ R 1 + 1/ R 2 + …. + 1/ R n The most important thing to remember about parallel connected resistances is that the VOLTAGE across each resistance is the same. Why ? Voltage at any point along a wire/conductor is considered the same.