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# 6 d electronics 291110 2

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Introduction to electricity - Higher physics
E.M.F., internal resistance, open / closed / short circuit, Wheatstone bridge

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### 6 d electronics 291110 2

1. 1. Electricity 6D: Monday 29th November
2. 2. Electromotive Force (e.m.f.) • The energy given to a coulomb of charge by a source of electrical energy is called the e.m.f. of the source • This is measured in J C-1 or volts
3. 3. Internal resistance, r • When a power supply is part of a closed circuit, it must itself be a conductor. All conductors have some resistance. A power supply has internal resistance, r, measured in Ohms • Energy will be wasted in getting the charges through the supply (heat from supply is noticeable) and so energy at the output (the terminal potential difference) will fall r E
4. 4. • When supply of e.m.f., E, and internal resistance, r, is connected to a circuit of resistance, R, it causes a current, I, to flow through circuit: rE R I I Applying Ohm’s law to this circuit gives: Voltage = current x resistance e.m.f. = I (R + r) E = IR + Ir Or E = V + Ir Ir = lost volts: the voltage dropped across the internal resistance, r V = terminal voltage: the voltage across the terminals of the power supply
5. 5. Open circuit • When no current is taken from the power supply, no energy is wasted • The terminal potential difference is therefore the maximum available and equals the e.m.f.
6. 6. General circuit • With switch open, voltmeter gives the e.m.f. (an open circuit) • With switch closed, the voltmeter reading will fall (lost volts) – Voltmeter now giving the output voltage, the terminal potential difference, t.p.d. rE R V
7. 7. Experiment • Alter variable resistor and take readings of terminal voltage, V, and current, I • Plot V versus I rE V A V I the e.m.f. the gradient = - internal resistance
8. 8. Short circuit current • The greater the current, the more energy will be dissipated in the power supply until eventually all the available energy (e.m.f.) is wasted and none is available outside the power supply. This maximum current is the short circuit current – the current which would flow if the terminals were connected with a short piece of thick wire (zero resistance: R=0). • I shortcircuit= e.m.f / internal resistance = E/r
9. 9. Wheatstone bridge circuit • Wheatstone bridge consists of 4 resistors: V R3 R4 R1 R2 A B If VOA = VOB there is no p.d. between A and B so no current flows. The potentials at A and B depend on the ratio of the resistors that make up each of the two voltage dividers. The voltmeter forms a ‘bridge’ between the two voltage dividers to make up a bridge circuit. The bridge is said to be balanced when the resistors in the two potential dividers are in the same ratio as each other and voltmeter reads zero: O R1 R2 = R3 R4
10. 10. Finding an unknown resistance • Circuit is set up with three resistors of known values – one needs to be a variable resistor • The unknown resistor is connected into circuit as fourth resistor • Variable resistor is adjusted until the voltmeter reads zero, i.e. the bridge is balanced • Substitute known values into equation to calculate unknown resistance • Note: Balance condition does not depend on supply voltage • The meter must be sensitive to detect zero precisely • Circuit is sometimes drawn in a diamond shape or rotated through 90 degrees R1 R2 = R3 R4
11. 11. Wheatstone bridge – slightly out of balance • When one of the resistors is varied slightly above and below its balance value, the voltmeter shows a reading which is directly proportional to the change in resistance • This direct proportionality (or linear) relationship allows Wheatstone bridge to be used as as type of measuring device when operated near to its balance point Out of balance voltage Out of balance resistance + + - - 0