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.

Electrical CurrentElectrical 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.

Electrical Current

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 6.2x1018 electrons passing a cross-section during one second

Current I=Q/t1 A = 1 C/sThe unit is the ampere (A).

Current I=Q/t1 A = 1 C/sThe unit is the ampere (A).

Electrical CurrentElectrical 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 CurrentCurrent 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/tThe unit is the ampere (A).1 A = 1 C/s

Current I=Q/tThe unit is the ampere (A).1 A = 1 C/s

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

Potential DifferencePotential 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 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 has symbol V.The units are volts (V).1 V=1 J/CV = W/Q

Potential difference has symbol V.The units are volts (V).1 V=1 J/CV = W/Q

Potential DifferencePotential 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.

Resistance symbol R.The units are ohms (W).V=IR

Resistance symbol R.The units are ohms (W).V=IR

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 PowerElectrical 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 PowerUsing equation P=VI and Ohm’s law I=V/R lets express power in several different forms.

P=IV=(V/R)V=V2/R

Lets rearrange Ohm’s law as V=IR

P=IV=I(IR)=I2R

Power, P=VI =I2R =V2/RThe unit is the watt (W).

Power, P=VI =I2R =V2/RThe unit is the watt (W).

I=V/R

V=IR

Electrical PowerIn a resistor, the power delivered manifests itself as heating caused by electrons colliding with atoms and giving them energy.

Power, P=VI =I2R=V2/RThe unit is the watt (W).

Power, P=VI =I2R=V2/RThe 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=V2

/R.

Electrical PowerElectrical 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=VISince V=IR then the power is P =I2R =V2/R.In a resistor, the power delivered manifests itself as heating caused by electrons colliding with atoms and giving them energy.

Power, P=VI =I2R =V2/RThe unit is the watt (W).

Power, P=VI =I2R =V2/RThe unit is the watt (W).

Try Qs 2, 3 and 4

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

Resistance: symbol R.R= rL/A

The units for resistance are ohms (W).V=IR

Resistance: symbol R.R= rL/A

The units for resistance are ohms (W).V=IR

Resistance and ResistivityThe unit of resistance is the ohm (W).What is the unit of resistivity?Let’s rearrange R=rL/A as

r=R A/L. A is the cross sectional

area, thus, it is measured in m2. 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 is R.R= rL/A

The units for resistance are ohms (W).The units for resistivity are ohm mOhm’s law: V=IR

Resistance is R.R= rL/A

The units for resistance are ohms (W).The units for resistivity are ohm mOhm’s law: V=IR

Resistance and ResistivityAll conductors offer resistance to the flow of electrical current. R=rL/A is the resistivity in mA cross-sectional area, L lengthThe unit of resistance is the ohm (W).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.

Resistance is R.R= rL/A

The units are ohms (W).V=IR

Resistance is R.R= rL/A

The units are ohms (W).V=IR