Unit 1 Electricity Electric Fields and Potential Difference CfE Higher Physics Unit 1 Electricity Electric Fields and Potential Difference

Learning Intentions State that voltage is defined as the energy transformed per unit of charge. State the relationship V = Ew/Q. Carry out calculations involving the relationship between energy, voltage and charge. State that the energy transformed from an external source to the circuit is known as the electromotive force (e.m.f.).

Electric Fields An electric field is a region of space in which charged particles experience a force. When an electric field is applied to a conductor the free electric charges are caused to move. Electric fields can be represented by field lines. These lines show the direction in which a positive charge experiences the force. The closer together the lines the stronger the force.

Field Patterns Positive point charge Negative point charge + - Positive point charge Negative point charge + - + - Positive and negative point charges Parallel charged plates

Potential Difference When a charged particle is moved in an electric field work is done. The potential difference (p.d.) between two points is a measure of the work done in moving one coulomb of charge between the two points. The SI unit of potential difference is the volt, V. V = Q Ew

Unit 1 Electricity Current, Charge, Voltage, Resistance and Power CfE Higher Physics Unit 1 Electricity Current, Charge, Voltage, Resistance and Power

Charge Charge is a property of a particle. Charge can either be positive (+ve) or negative (-ve). Electrons have negative charge, protons have positive charge and neutrons have no charge. The SI unit of charge is the coulomb, C. For example the charge on an electron is –1.6 x 10-19 C. The symbol used for charge is Q.

Current An electric current is a flow of charge. In a metal it is electrons (negative charges) that are free to flow. The SI unit of current is the ampere, A. A current of one ampere is when one coulomb of charge passes a point in the circuit each second. The symbol used for current is I. current = charge time I = t Q

Voltage Voltage is the energy used up or given to each unit of charge as it passes through an electrical component. The SI unit of voltage is the volt, V. A voltage of one volt is when one joule of energy is given to or used by each coulomb of charge as it passes through a component. The symbol used for voltage is V. voltage = energy charge V = Q E

Resistance Resistance is the opposition of an electrical component to charge passing through it. The SI unit of resistance is the ohm, Ω. A resistance of one ohm is when one ampere of current passes through a component when one volt is applied across it. The symbol used for resistance is R. resistance = voltage current R = I V

Series Circuits When components are placed one after the other, in a line, they are said to be in series. The current in a series circuit is the same at all points (conservation of charge) The voltages across components in series add up to the supply voltage (conservation of energy).

Series Circuits (continued) Resistance in a series circuit: Since Vs = V1 + V2 + … and V=IR so IsRT = I1R1 + I2R2 + … but Is = I1 = I2 = … so RT = R1 + R2 + …

Parallel Circuits When components are placed in separate branches they are said to be in parallel. The currents through components in parallel add up to the supply current (conservation of charge). The voltages across components in parallel is the same (conservation of energy).

Parallel Circuits (continued) Resistance in a parallel circuit: Since Is = I1 + I2 + … and V1 R1 Vs RT V2 R2 = + + … V R I = so but Vs = V1 = V2 = … so 1 R1 RT R2 = + + …

Power Electrical power can be calculated from current and voltage: power = current x voltage P = IV Proof: Also: P = IV P = I (IR) power = current x voltage. P = I2R charge time x energy charge power = And: P = IV P = (V/R)V power = energy time P = V2/R

Applications of Potential Dividers CfE Higher Physics Applications of Potential Dividers

Voltage Divider A voltage divider circuit consists of two resistors placed in series: R2 R1 V1 V2 0 V Vs In a voltage divider circuit the ratio of the voltages is equal to the ratio of the resistances. V1 = R2 V2 R1

Voltage Divider (continued) The voltage across the resistors in a voltage divider circuit can be calculated using the formulae: V1 = R1 R1+R2 Vs V2 = R2 R1+R2 Vs and (Note also: V1 + V2 = Vs)

Wheatstone Bridge A Wheatstone bridge circuit consists of two voltage dividers placed in parallel with a voltmeter forming a bridge between them. R2 R1 0 V Vs R4 R3 V R2 R1 0 V Vs R4 R3 V or

Balanced Wheatstone Bridge A Wheatstone bridge circuit is said to be balanced when there is no potential difference across the bridge (i.e. when the voltmeter reads zero). In this situation the ratio of the voltages in the two voltage dividers must be the same. R1 = R4 R2 R3

Worked Example Calculate the value of resistor X that enables the following Wheatstone bridge circuit to be balanced: 1.2 kΩ 2 kΩ 0 V 6 V 820 Ω Ω V R1 = R4 R2 R3 2,000 = 820 1,200 X X = 2,000 x 820 1,200 X = 1,370 Ω

Unbalanced Wheatstone Bridge In an initially balanced Wheatstone bridge circuit as the value of one resistor is changed by a small amount the out of balance p.d. across the bridge is proportional to the change in resistance. ∆R p.d.

Unit 1 Electricity E.m.f and Internal Resistance CfE Higher Physics Unit 1 Electricity E.m.f and Internal Resistance

Lesson Starter

Solution to 1       Answer: B

  Solution to 2   Answer: B

Electromotive Force (e.m.f.) The electromotive force (e.m.f.) of a source is the electrical potential energy supplied per unit charge which passes through the source. Electromotive force can be generated by chemical cells, solar cells, thermocouples and dynamos. The SI unit of electromotive force is the volt. The symbol used for e.m.f. is E.

Internal Resistance When current is drawn from an electrical source some energy is wasted inside the source due to it’s internal resistance. A real electrical source can be considered as a source with an e.m.f. in series with a small resistance, r. E r

Lost Volts The energy per unit charge that is wasted inside the electrical source is called the “lost volts”, Vlost. The greater the current the more energy that is wasted. Vlost = Ir

Terminal Potential Difference The energy per unit charge available at the terminals of the electrical source is called the terminal potential difference, Vtpd. Vtpd = E - Vlost When a load resistance, R, is placed across the terminals of the source: E r I R Vtpd = IR

Equations E = Vtpd + Vlost Vtpd = IR Vlost = Ir E = IR + Ir E = I(R+r) and Vlost = Ir E = IR + Ir E = I(R+r)

Worked Example When a current of 1.2 A is drawn from a cell of e.m.f. 1.6 V the voltage measured at the terminals of the cell drops to 1.45 V. What is the internal resistance of the cell? E = Vtpd + Vlost Vlost = Ir 1.6 = 1.45 + Vlost 0.15 = 1.2 r r = 0.125 Ω Vlost = 0.15 V

Measuring e.m.f. and Internal Resistance The following circuit allows the e.m.f. and internal resistance of an electrical source to be measured: E r R A V A series of readings of current and terminal potential difference are made while varying the load resistance, R.

Measuring e.m.f. and Internal Resistance (continued) A graph of terminal potential difference against current is then drawn: Vtpd I This gives a straight line graph of the form: y = mx + c or Vtpd = mI + c.

Measuring e.m.f. and Internal Resistance (continued) Comparing the equation of this line: Vtpd = mI +c to the equation for internal resistance: Vtpd = E - Ir Gives us E = c and r = -m e.m.f. = y-axis intercept (Vtpd when current is zero). Internal resistance = the negative of the gradient.

Short Circuit Current The short circuit current is the current that passes when there is no load resistance (R = 0 Ω). E = Ir +IR E = Ishortr + 0 Ishort = E r

Common Explain questions