Electric current, potential difference and resistance AICE Chapter 10 Electric current, potential difference and resistance

Electric Current Electricity – we do not speak on his name. Not a proper term, its used for electrical energy or electrical power or electric current Conventional current – the scientific convention that the direction of the current goes from positive to negative

Electric current The current occurs in a wire because the negative charged electrons are free to move about. These are called conduction or free electrons The atoms are bound together very closely and one electron breaks free from each atom to become a conduction electron A plain wire not connected has an equal number of free electrons and ions it has a neutral charge

Electric current When the cell is connected to the wire, electrical force on the conduction electrons that makes them travel along the length of the wire. Electrons flow toward the positive terminal, opposite of the conventional current direction. Blame Ben Franklin for guessing wrong on the positive and negative titles.

Charge carriers Sometimes a current is a flow of positive charges Beam of protons in a particle accelerator Sometimes a current is due to both positive and negative charges When charged particles flow through a solution A solution that conducts energy is called an electrolyte and it contains both positive and negative ions. Any charged particles which contributes to an electric current are known as charge carriers.

Current and Charge When charged particles flow past a point in a circuit, we say that there is a current in the circuit. Electrical current is measured in amperes (A) Charge is measured in coulombs (C) Current = charge/time The Electric current is the rate of flow of electric charge past a point

Charge continued Manipulated the equation can be read as Charge = current x time One coulomb is the charge which flows past a point in a circuit in a time of 1 s when the current is 1 A ΔQ= IΔt Q is charge (coulomb), I is the current (ampere), t is time (seconds)

Charged particles Electrons have a tiny charge Approx. -1.6 x 10-19C This charge is represented with –e This is called the elementary charge Protons have a positive charge, +e This is equal and opposite to that of the electron. Ions carry charges that are multiples of +e and -e

Voltage The total voltage across the power supply is equal to the sum of the voltages across resistors Electrical energy is transferred to the charge by the power supply Charge flows around the circuit transferring some energy in the first, then the second resistor.

Voltage A voltmeter indicates the energy transferred to the component by each unit of charge. Placed across the power supply measure the electromotive force or e.m.f (only remember e.m.f!) long term not important The total work done when unit charge goes around a complete circuit Place across the resistors measure the potential difference The energy given up by unit charge as it moves from point A to point B

Electrical resistance The potential difference of voltage V – the greater the potential difference, the greater the current for a given lamp The greater the resistance, the smaller the current for a given potential difference Electrical resistance is the ratio of the potential difference to the current

Resistance R = V/I Resistance (ohms, Ω) = Voltage (volt V) / current (ampere A) Copy table 10.2 One ohm is equal to 1 volt per ampere 1 Ω = VA-1

Electrical Power Power is the rate at which energy is transferred (chapter 5) P = ΔW/Δt Power is measured in watts (don’t confuse with W, that energy, joules) Rate at which energy is transferred in an electrical component is related to two components. The current I in the component The potential difference V across the component

Lets Derive! ΔW=VΔQ P = ΔW/t = (VΔQ)/t = V(ΔQ/t) (ΔQ/t) is the current I, therefore: P=VI Power = potential difference x current Watts = volts x amperes

Fuses A fuse is a device that is fitted in an electrical circuit; it is usually there to protect wiring from excessive currents High currents cause wires to get hot, which can lead to damaged wires and fires Fuses are intentionally designed with a thin wire that melts and breaks if the current exceeds this value. This is different from a circuit breaker, because a circuit breaker can be reset and reused, a fuse cannot.

Power and resistance Current I in a resistor of resistance R transfers energy to it. The resistor dissipates heat. The p.d. V across the resistor is given by V = IR Combine this equation with the equation for power, P = VI We get two new equations from this P = I2R P = V2/R

Calculating Energy Last combination P= current x voltage Energy = power x time Energy transferred = current x voltage x time ΔW = IVΔt Energy is always measured in joules.