Electrical Current Electrical current (measured in amps) is the rate of flow of charge. Electrical current is a flow of electric charge through a medium. This charge is typically carried by moving electrons in a conductor such as wire, or… It can also be carried by ions in an electrolyte, or by both ions and electrons in plasma.

What is 1 ampere? (a.k.a. 1 amp?) It is 1 coulomb of charge passing through a wire cross-section over 1 second. How many electrons? It is about 6.2x10 18 electrons passing a cross-section during one second Current I=Q/t 1 A = 1 C/s The unit is the ampere (A). Current I=Q/t 1 A = 1 C/s The unit is the ampere (A).

Electrical Current Electrical current in amps is the rate of flow of charge. Conventional d.c. current flows out of the positive terminal of a battery, around a circuit, and into the negative terminal. In metals, the charge carriers are electrons, which move in the opposite direction.

Modelling current (using people!) This shows: Purpose of current is to move energy from a supply to a user Current keeps going. Current is NOT used up in components.

Electrical Current Current is measured using an ammeter - a device used in series that does not impede current flow but which has a deflection proportional to current. Current I=Q/t The unit is the ampere (A). 1 A = 1 C/s Current I=Q/t The unit is the ampere (A). 1 A = 1 C/s

Quick experiment Prove to yourself that current in a loop circuit is the same everywhere.

Potential Difference Potential difference (p.d.) in volts is the “driving force” that makes current flow around a circuit. Quite similar to a gravitational potential difference of a waterfall Potential difference is V. The units is the volt (V). 1 V=1 J/C Potential difference is V. The units is the volt (V). 1 V=1 J/C

Potential Difference It is the energy delivered to the components of the circuit per unit charge.

Potential Difference It is the energy delivered to the circuit per unit charge. Often called “voltage”. The voltage generated by the chemical reactions in a battery is called the electromotive force (e.m.f.).

Potential Difference Potential difference is measured using a voltmeter, which goes in parallel with the circuit. V Potential difference has symbol V. The units are volts (V). 1 V=1 J/C V = W/Q Potential difference has symbol V. The units are volts (V). 1 V=1 J/C V = W/Q

Visual models of pd and emf: (1) A simple circuit

Visual models of pd and emf: (2) A series circuit

Visual models of pd and emf: (3) A parallel circuit

Visual models of pd and emf: (4) A more complex circuit

Sketch these! Then try Q1

Electrical Resistance All conductors offer resistance to the flow of electrical current. Symbol - In electric circuits resistors is usually shown as:

Electrical Resistance An electrical component specifically made to limit current is called a resistor. Resistance is the pd required for unit current to flow. If V is proportional to I, R is constant: Ohms law.

Electrical Power How to estimate how much electrical power was used or delivered? We know the voltage supplied and the current used. What about power?

Electrical Power Electrical power is the energy delivered per unit time in a circuit. The power is the voltage times the current. The unit is the watt, W. 1 W = 1 J/s. Power P=VI Example: The iron uses 8 A. What is its power? Since the standard voltage supply is 240V, then the iron’s power is 8A x 240V = 1920W (Almost 2kW).

Electrical Power Using equation P=VI and Ohm’s law I=V/R lets express power in several different forms. P=IV=(V/R)V=V 2 /R Lets rearrange Ohm’s law as V=IR P=IV=I(IR)=I 2 R Power, P=VI =I 2 R=V 2 /R The unit is the watt (W). Power, P=VI =I 2 R=V 2 /R The unit is the watt (W). I=V/R V=IR

Electrical Power In a resistor, the power delivered manifests itself as heating caused by electrons colliding with atoms and giving them energy. Power, P=VI =I 2 R=V 2 /R The unit is the watt (W). Power, P=VI =I 2 R=V 2 /R The unit is the watt (W). Why do we have huge energy release in short circuit? If voltage is fixed then the power is inversely proportional to the resistance: P=V 2 /R.

Electrical Power Electrical power is the energy delivered per unit time in a circuit. The power is the voltage times the current. The unit is the watt, W. 1 W = 1 J/s. Power P=VI Since V=IR then the power is P=I 2 R=V 2 /R. In a resistor, the power delivered manifests itself as heating caused by electrons colliding with atoms and giving them energy. Power, P=VI =I 2 R=V 2 /R The unit is the watt (W). Power, P=VI =I 2 R=V 2 /R The unit is the watt (W).

Try Qs 2, 3 and 4

Resistance and resistivity Resistance R is a property of a particular resistor. Resistivity  is a property of material of which the resistor is made. We can write: R=  L/A  is the resistivity in  m A cross-sectional area, L length

Resistance and Resistivity The unit of resistance is the ohm (  ). What is the unit of resistivity? Let’s rearrange R=  L/A as   =R A/L.  A is the cross sectional area, thus, it is measured in m 2. L is the resistor length, thus, it is measured in m.  Thus, the unit A/L is m, while the resistance unit is ohm.  Thus, the resistivity units are ohm m. (Ωm)

Resistance and Resistivity All conductors offer resistance to the flow of electrical current. R=  L/A  is the resistivity in  m A cross-sectional area, L length The unit of resistance is the ohm (  ). An electrical component specifically made to limit current is called a resistor. The current that a potential difference drives through a resistor is given by I=V/R –ohms law.