Topic 3 - Spec - Electric Circuits

Question 1

Define Current.

Answer 1

Current is Rate of Flow of Charge

Question 2

What is the Current Equation?

Answer 2

I = ΔQ / Δt

Question 3

What are the Components of this equation? V = W / Q

Answer 3

V - Voltage W - Work Done (I know it as E - Energy Transferred) Q - Charge

Question 4

What is the Definition for Resistance?

Answer 4

V / I

Question 5

What is Ohms Law?

Answer 5

The current through a metal wire is directly proportional to the P.D across it (Providing the TEMPERATURE is constant)

Question 6

What are materials that obey Ohms Law called?

Answer 6

Ohmic Conductors

Question 7

The Current in a […………..] circuit is the same at all points.

Answer 7

Series

Question 8

What is unusual about the Convectional Current flow and the Electron Flow?

Answer 8

Convectional Current flows from Positive to Negative Electron Flow flows in the opposite direction to Convectional Current from Negative to Positive

Question 9

What is Kirchhoffs First Law?

Answer 9

Current flowing into a junction equals the current flowing out of the junction.

Question 10

In a […………] circuit the P.D across each branch is the same.

Answer 10

Parallel

Question 11

Derive the equation for Resistance in Series.

Answer 11

1 - We know that for Resistors in series V (total) = V1 + V2 + V3 2 - we know that V = IR so replace V with IR IR (total) = IR1 + IR2 + IR3 3 - We know that for a Series circuit the current through each resistor is the same so we cancel out the I. 4 - This leaves us with R (total) = R1 + R2 + R3

Question 12

3 important facts you GUS should know about a Series Circuit.

Answer 12

1 - The current through each resistor s series is the same 2 - The total P.D across the resistors is the sum of the P.D’s across the separate resistors so : V (total) = V1 + V2 + V3 3 - The combined resistance in the circuit is the sum of the separate resistors R (total) = R1 + R2 + R3

Question 13

Derive the Equation for Resistors in Parallel.

Answer 13

1 - We know that the current is the same for Resistors in Parallel I (total) = I1 + I2 + I3 2 - We know that I = V/R so substitute this into the equation V/R (total) = V/R1 + V/R2 + V/R3 3 - Now divide each term by V This gives us 1/R (total) = 1/R1 + 1/R2 + 1/R3

Question 14

Define the Power of a Device.

Answer 14

Rate at which a device transfers energy

Question 15

What is the equation or Power which has Current in it?

Answer 15

P = IV

Question 16

How do we derive these equations? P = V²/R P = I²R

Answer 16

By using P = IV and combining Ohm’s Law V = IR

Question 17

What are the equations made from P = IV and V = IR

Answer 17

P = V²/R P = I²R

Question 18

What is the Total Electrical Energy Equation?

Answer 18

W = VIt

Question 19

What are the different variations of the Total Electrical Energy Equatoin, W = IVt ?

Answer 19

W = (V²/R) t W = I²Rt

Question 20

What is the IV graph for Ohmic Conductors?

Answer 20

Question 21

What is IV graph of a Filament Lamp?

Answer 21

Question 22

What is the IV graph for a Diode?

Answer 22

• The point at which the Diode allows current to pass is called the Threshold Voltage and it’s is about 0.6V in the Forward direction.
• Forward bias is the positive part of the IV graph - Reverse bias is the negative part of the IV Graph
• In the Reverse bias the resistance of the diode is very high and only allows a tiny amount of current to pass.

Question 23

What is the IV Graph for a Thermistor?

Answer 23

• This is a Negative Temperature Coefficient (NTC) resistor.
• Its shape is because as Voltage increase so does current, more current leads to an increase in temperature and therefore a decrease in resistance. So more current can flow because resistance decreases.

Question 24

What is the Resistivity Equation?

Answer 24

R = ρl/A

Rearranged to give

ρ = RA/l

ρ = Resistivity ( Ωm)

R = Resistance ( Ω)

A = Area (m*2)

l = Length (m)

Question 25

What is the Core Practical to find the Resistivity of a Wire?

Answer 25

1 - Find the cross sectional area of the wire, using a micrometer 2 - Clamp the wire to a ruler and connect the wire to the rest of the circuit 3 - Attach the flying lead to the test wire - Has a crocadile clip at the end so the length of wire in the circuit can be changed. 4 - Record the length of the test wire connected in the circuit, record the voltmeter and ammeter readings 5 - Use the readings to calculate the resistance in the wire R = V/I 6 - Repeat test in all sort of length ranges 7 - Plot a graph of Resistance against length and draw a line of best fit, the gradient is equal to R/I = ρ/A so get the resistivity times it by the Area. 8 - Remeber to keep the wire at a constant temperature. 8 -

Question 26

What is the Current equation with Drift Velocity in it?

Answer 26

I = nqvA I = Current (A) n = Number of Charge Carriers (m\*-3) q = Charge on each Carrier (C) v = Mean drift Velocity (ms-1) A = Cross-sectional Area (m\*2)

Question 27

What is the Charge on each Electron?

Answer 27

-1.6x10\*-19

Question 28

What are the Potential Divider Equations?

Answer 28

V (out) / V (in) = R1 / R1 + R2 or V (out) = V (in) x R1/R1 + R2

Question 29

Potential Divider Circuits which include Light Dependent Resistors (LDR) and Thermistors.

Answer 29

- With a Thermistor its resistance rises as it COOLS, this means V (out) increases - For an LDR as light intensity increases the resistance decreases.

Question 30

Define E.M.F.

Answer 30

Is the electrical energy supplied per coulomb of charge. ℰ = W/Q

Question 31

What is meant by Internal Resistance?

Answer 31

Is the energy that is wasted or 'lost' as the coulombs move against the resistance of the cell itself

Question 32

What is the Difference between E.M.F and Terminal P.D?

Answer 32

E.M.F measures how much charge is gained Terminal P.D is the voltage around the whole circuit taking into account Internal Resistance

Question 33

Determine the E.M.F and Internal Resistance of an electrical cell CORE PRACTICAL

Answer 33

- Create a circuit with a cell with internal Resistance, a variable resistor, a Voltmeter and Ammeter. - Vary the current in the circuit by changing the load resistance, measure the P.D - Plot the results on a V I graph - Use the equation Є = V + Ir and rearrange the equation to give V = -rI + Є - Since Є and r are constants we can use the straight line equation y = mx + c - So the intercept on the VERTICAL AXIS is Є - The GRADIENT is -r

Question 34

Explain the effect of temperature on resistance.

Answer 34

- Metals contain large numbers of free electrons and as these free electrons love through the lattice they collide with the vibrating metal ions. - This causes the metals temperature to increase, leading to the ions to vibrate faster with greater amplitude. - This increases the amount of collisions which oppose the flow of electrons and causes the metals resistance to increase.