# PHYSICS Electrical quantities (2) LEARNING OBJECTIVES Core State PHYSICS Electrical quantities (2) LEARNING OBJECTIVES Core State that the e.m.f. of an electrical source of energy is measured in volts State that the potential difference (p.d.) across a circuit component is measured in volts Use and describe the use of a voltmeter, both analogue and digital State that resistance = p.d. / current and understand qualitatively how changes in p.d. or resistance affect current Recall and use the equation R = V / I Describe an experiment to determine resistance using a voltmeter and an ammeter

Relate (without calculation) the resistance of a wire to its length and to its diameter Understand that electric circuits transfer energy from the battery or power source to the circuit components then into the surroundings Supplement Show understanding that e.m.f. is defined in terms of energy supplied by a source in driving charge round a complete circuit Recall that 1 V is equivalent to 1 J / C Sketch and explain the currentvoltage characteristic of an ohmic resistor and a filament lamp Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire Recall and use the equations P = IV and E = IVt

Recap emf Current is the rate of flow of electrons around a circuit. The higher the current, the faster the electrons are travelling. The unit of current is the amp, and in a circuit an ammeter is used to measure current. Recap emf VOLTAGE is the amount of energy given to electrons as they travel around the circuit. Current is the rate of flow of electrons around a circuit. The higher the current, the faster the electrons are

travelling. The unit of current is the amp, and in a circuit an ammeter is used to measure current. Recap emf VOLTAGE is the amount of energy given to electrons as they travel around the circuit. Voltage is also known as POTENTIAL DIFERENCE (PD) Current is the rate of flow of electrons around a circuit. The higher the current, the faster the electrons are travelling. The unit of current is the amp, and in a circuit an ammeter is used to measure current.

Recap emf VOLTAGE is the amount of energy given to electrons as they travel around the circuit. Voltage is also known as POTENTIAL DIFERENCE (PD) Current is the rate of flow of electrons around a circuit. The higher the current, the faster the electrons are travelling. The unit of current is the amp, and in a circuit an ammeter is used to measure current. Unit of voltage or PD is the volt. Supplement 1 volt = 1 joule of

potential energy is given to each coulomb of charge (1J = 1 J/C) emf VOLTAGE is the amount of energy given to electrons as they travel around the circuit. Voltage is also known as POTENTIAL DIFERENCE (PD) The cell produces its highest potential difference when not connected in a circuit. This maximum PD is known as the electromotive force (EMF) of the cell. The battery cell gives electrons potential energy. This energy is then passed on to the components in the cell

emf VOLTAGE is the amount of energy given to electrons as they travel around the circuit. Voltage is also known as POTENTIAL DIFERENCE (PD) The cell produces its highest potential difference when not connected in a circuit. This maximum PD is known as the electromotive force (EMF) of the cell. The battery cell gives electrons potential energy. This energy is then passed on to the components in the cell As soon as the cell is connected in a circuit the potential difference drops because of energy

wastage inside the cell. Just a reminder A single cell A battery, made up of several cells. A battery is a series of joined cells, although it is commonly used for a single cell as well. VOLTAGE is the amount of energy given to electrons as they travel around the circuit. Voltage is also known as POTENTIAL DIFERENCE (PD) The cell produces its highest potential difference when not connected in a circuit. This maximum PD is known as the electromotive force (EMF) of the cell. The battery cell gives electrons

potential energy. This energy is then passed on to the components in the cell As soon as the cell is connected in a circuit the potential difference drops because of energy wastage inside the cell. Measuring voltage (PD) in a circuit. Measuring voltage (PD) in a circuit. Voltage is measured using a VOLTMETER Measuring voltage (PD) in a circuit. To measure the voltage across a component in a circuit the voltmeter must be placed in parallel with it.

Voltage is measured using a VOLTMETER Measuring voltage (PD) in a circuit. To measure the voltage across a component in a circuit the voltmeter must be placed in parallel with it. Voltage is measured using a VOLTMETER Measuring voltage (PD) in a circuit. Series and parallel circuits In a series circuit the total voltage (PD) of the supply is shared between the various components, so the voltages around a series circuit always

add up to equal the source voltage. Voltage is measured using a VOLTMETER Measuring voltage (PD) in a circuit. Series and parallel circuits In a series circuit the total voltage (PD) of the supply is shared between the various components, so the voltages around a series circuit always add up to equal the source voltage. Voltage is measured using a VOLTMETER In a parallel

circuit all components get the full source voltage, so the voltage is the same across all components Whenever a current flows around an electrical circuit there is resistance to the electrons. Whenever a current flows around an electrical circuit there is resistance to the electrons. Copper connecting wire is a good conductor, it offers little

resistance to the electrons, and a current passes through it easily. Nichrome is not such a good conductor, it has a bigger resistance to the electrons, and less current will flow. Whenever a current flows around an electrical circuit there is resistance to the electrons. Resistance is calculated using this equation: Copper connecting wire

is a good conductor, it offers little resistance to the electrons, and a current passes through it easily. Nichrome is not such a good conductor, it has a bigger resistance to the electrons, and less current will flow. resistance = voltage R = V current

I The unit of resistance is the ohm (Greek letter omega) Whenever a current flows around an electrical circuit there is resistance to the electrons. Resistance is calculated using this equation: Copper connecting wire is a good conductor, it offers little resistance to the electrons, and a current passes through it easily. Nichrome is

not such a good conductor, it has a bigger resistance to the electrons, and less current will flow. resistance = voltage R = V current I The unit of resistance is the ohm (Greek letter omega) eg. If a PD of 8V is needed to make a current of 4A flow through a wire. Resistance = 8 / 4 = 2 Remember, remember .. The

equation linking V, I and R V = I x R V I I = V / R R R = V / I Factors affecting resistance. Factors affecting resistance. Temperatur e Lengt

h of wire Factors affecting resistanc e Materia l Cross sectiona l area Lengt h of wire Factors affecting resistance. Temperature

Factors affecting resistanc e Materia l For metal conductors, resistance increases with temperature. For semiconductors, it decreases with temperature. Cross sectional area Lengt h of wire Factors affecting resistance.

Temperature Factors affecting resistanc e Materia l For metal conductors, resistance increases with temperature. For semiconductors, it decreases with temperature. When a current flows through a wire, resistance causes a heating effect. This principle is used in heating elements and in Cross sectional area Lengt h of

wire Factors affecting resistance. Temperature Factors affecting resistanc e Cross sectional area Materia l For metal conductors, resistance increases with temperature. For semiconductors, it decreases with temperature. When a current flows through a

wire, resistance causes a heating effect. This principle is used in heating elements and in Electrons collide with atoms as they pass through conductors, losing energy. The atoms vibrate more, causing a heating effect Temperature Factors affecting resistance. Lengt h of wire Factors affecting resistanc

e Cross sectional area Materia l A B Wires A and B have the same cross-sectional area and are at the same temperature. Wire B is twice as long as wire A, and has twice the resistance. Temperature Lengt h of wire Factors

affecting resistance. Factors affecting resistanc e Cross sectional area Materia l Wires A and B have the same cross-sectional area and are at the same temperature. Wire B is twice as long as wire A, and has twice the resistance. A B Resistance

length Resistance is directly proportional to Temperature Factors affecting resistance. Cross sectional area Factors affecting resistanc e Lengt h of wire Materia l

A B Wires A and B have the same length and are at the same temperature. Wire B is twice the cross-sectional area of A, and has half the resistance. Temperature Factors affecting resistance. Cross sectional area Factors affecting resistanc e

Lengt h of wire Materia l Wires A and B have the same length and are at the same temperature. Wire B is twice the cross-sectional area of A, and has half the resistance. A B Resistance 1 area (area = cross-sectional area)

Temperature Factors affecting resistance. Materia l Factors affecting resistanc e Cross sectional area Some wires have much more resistance for a given length. For example a 10cm length of nichrome has a much higher resistance than copper of the same length and cross-sectional area. Nichrome is said to have a

higher resistivity. Lengt h of wire Temperature Factors affecting resistance. Materia l Factors affecting resistanc e Lengt h of wire Cross

sectional area Some wires have much more resistance for a given length. For example a 10cm length of nichrome has a much higher resistance than copper of the same length and cross-sectional area. Nichrome is said to have a higher resistivity. Typical resistivity (/m)m) Constantan 49 x 10-8 Manganin 44 x 10-8 Nichrome 100 x 10-8

Tungsten 55 x 10-8 The Greek letter rho () is the resistivity constant for any given material) Lengt h of wire Factors affecting resistance. Temperature Factors affecting resistanc e Materia l

Combining the resistance equations Cross sectional area Lengt h of wire Factors affecting resistance. Temperature Factors affecting resistanc e Materia l

Combining the resistance equations Resistance length area Cross sectional area Lengt h of wire Factors affecting resistance. Temperature Factors affecting resistanc e

Cross sectional area Materia l Combining the resistance equations Resistance length area R = x l A Lengt h of wire Factors affecting resistance.

Temperature Factors affecting resistanc e Cross sectional area Materia l Combining the resistance equations Resistance length area R = x l A

= R x A l Lengt h of wire Factors affecting resistance. Temperature Factors affecting resistanc e Cross sectional area Materia l

Combining the resistance equations Comparing different wires, A and B, made from the same material (so is the same for each wire) at the same temperature. R = x l A = R x A l Lengt h of wire Factors affecting resistance. Temperature Factors affecting resistanc e

Cross sectional area Materia l Combining the resistance equations Comparing different wires, A and B, made from the same material (so is the same for each wire) at the same temperature. ResistanceA x AreaA = ResistanceB x AreaB LengthA LengthB R = x l A = R x A l More about

resistors Resistor 1 kilohm (k) = 1000 1 megohm (M) = 1 000 000 More about resistors Resistor 1 kilohm (k) = 1000 1 megohm (M) = 1 000 000 Variable resistor Used for varying current, for example in light dimmer switches More about resistors Resistor

1 kilohm (k) = 1000 1 megohm (M) = 1 000 000 Variable resistor Used for varying current, for example in light dimmer switches Thermistor High resistance when cold, but much lower resistance when hot. Eg. Digital thermometer More about resistors Resistor 1 kilohm (k) = 1000 1 megohm (M) = 1 000

000 Variable resistor Used for varying current, for example in light dimmer switches Thermistor High resistance when cold, but much lower resistance when hot. Eg. Digital thermometer Light dependent resistor (LDR) High resistance in the dark but a low resistance in the light. Eg. Controlling light switches

More about resistors Resistor 1 kilohm (k) = 1000 1 megohm (M) = 1 000 000 Variable resistor Used for varying current, for example in light dimmer switches Thermistor High resistance when cold, but much lower resistance when hot. Eg. Digital thermometer Light dependent resistor (LDR)

High resistance in the dark but a low resistance in the light. Eg. Controlling light switches Diode Extremely high resistance in one direction, but low in the other. Controls flow of current Ohms Law A 19 Century th scientist who first investigated the electrical properties of wires, and the relationship between V, I and R I (the symbol for current) = intensite du courant

Ohms Law How current varies with voltage (PD) for a metal conductor. Circuit diagram: battery Variable resistor Ammeter Voltmeter V Nichrome wire A

Water bath to keep nichrome at constant temperature Ohms Law How current varies with voltage (PD) for a metal conductor. Circuit diagram: battery Variable resistor Ammeter Voltmeter

V Nichrome wire A Water bath to keep nichrome at constant temperature V I R = V/I 2.0V 0.4A 5.0

4.0 0.8 5.0 6.0 1.2 5.0 8.0 1.6 5.0 10.0 2.0 5.0 Ohms

Law How current varies with voltage (PD) for a metal conductor. Circuit diagram: battery Variable resistor Ammeter Voltmeter V A V I

R = V/I 2.0V 0.4A 5.0 4.0 0.8 5.0 6.0 1.2 5.0 8.0 1.6 5.0

10.0 2.0 5.0 2.0 Nichrome wire Water bath to keep nichrome at constant temperature Current (A) 0 Voltage (V) 10.0

Ohms Law 1. A graph of current against voltage is a straight line through the origin. 2. If the voltage doubles then the current doubles, etc 3. In this experiment, V/I always has the same value. Ohms Law Current is proportional to the voltage. Current Voltage 1. A graph of current against voltage is a straight line through the origin. 2. If the voltage doubles then

the current doubles, etc 3. In this experiment, V/I always has the same value. Provided temperature is constant Ohms Law Current is proportional to the voltage. Current Voltage 1. A graph of current against voltage is a straight line through the origin. 2. If the voltage doubles then the current doubles, etc 3. In this experiment, V/I always has the same value. So what happens if

temperature changes? For a tungsten filament lamp, as the current increases, the temperature rises and the resistance increases. Current is not directly proportional to So what happens if temperature changes? And for the diode . For a tungsten filament lamp, as the current

increases, the temperature rises and the resistance increases. Current is not directly proportional to Current is not proportional to the voltage. If the voltage is reversed, the resistance increases greatly, so effectively making sure that current only flows in one direction in the And finally Understand that electric circuits transfer energy

from the battery or power source to the circuit components then into the surroundings And finally Chemical energy is transformed into potential energy in the electrons, and in the bulb this is changed into thermal (heat) energy. Understand that electric circuits transfer energy from the battery or power source to the circuit components then into the surroundings And finally

Chemical energy is transformed into potential energy in the electrons, and in the bulb this is changed into thermal (heat) energy. The rate at which energy is transformed is known as POWER. The unit of power is the watt (W). Understand that electric circuits transfer energy from the battery or power source to the circuit components then into the surroundings And finally Chemical energy is transformed into potential energy in the electrons, and in the bulb

this is changed into thermal (heat) energy. The rate at which energy is transformed is known as POWER. The unit of power is the watt (W). Understand that electric circuits transfer energy from the battery or power source to the circuit components then into the surroundings P = I x V P I V V = P/I I = P/V 1 kilowatt (kW) = 1000 watts

Understand that electric circuits transfer energy from the battery or power source to the circuit components then into the surroundings And finally 2200W (2.2kW) 450W 11W 80W And finally Recall and use the equations P = IV and E = IVt Supplemen t

And finally Recall and use the equations P = IV and E = IVt Supplemen t Power = energy transformed time taken And finally Recall and use the equations P = IV and E = IVt Supplemen t Power = energy transformed

time taken P = E t And finally Recall and use the equations P = IV and E = IVt Supplemen t Power = energy transformed time taken P = E t E =P x t And finally

Recall and use the equations P = IV and E = IVt E =IxV x t Supplemen t Power = energy transformed time taken P = E t E =P x t And finally Recall and use the equations P = IV and E = IVt

E =IxV x t Joules per second Supplemen t Power = energy transformed time taken P = E t E =P x t LEARNING OBJECTIVES Core State that the e.m.f. of an electrical source of energy is measured in volts State that the potential difference (p.d.) across a circuit component is measured in volts Use and describe the use of a voltmeter, both analogue and digital

State that resistance = p.d. / current and understand qualitatively how changes in p.d. or resistance affect current Recall and use the equation R = V / I Describe an experiment to determine resistance using a voltmeter and an ammeter Relate (without calculation) the resistance of a wire to its length and to its diameter Understand that electric circuits transfer energy from the battery or power source to the circuit components then into the surroundings Supplement Show understanding that e.m.f. is defined in terms of energy supplied by a source in driving charge round a

complete circuit Recall that 1 V is equivalent to 1 J / C Sketch and explain the currentvoltage characteristic of an ohmic resistor and a filament lamp Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire Recall and use the equations P = IV and E = IVt PHYSICS Electrical quantities (2)