1. Exam 1 Results Sores are curved: Score based out of 98 total points
AVG
75.8
Median
77.6
StdDev
14.8
High Score 99
Will reweight final grades as follows:
Lowest exam grade: 5%
Other 2 exams: 11% each

2. Ammeters and Voltmeters
An ammeter is a device that measures the current (amps) anywhere in a circuit
To use it, you must route the current through it
A perfect ammeter should have zero resistance
A voltmeter is a device that measures the potential difference (volts) between any two points in a circuit
To use it, you can simply connect to any two points
A perfect voltmeter has infinite resistance A V

3. Charging and Discharging Capacitors
The combination RC =  is called the time constant
It’s the characteristic time it takes to discharge
We can work out the current from
In this circuit, the capacitor is initially uncharged, but at t = 0 the switch is closed. What happens? Q C R + – E I

4. *Actually, this is alternating current, later chapter
Household Wiring
All household appliances consume electrical power
Think of them as resistors with fixed resistance R
Devices are designed to operate at 120 V*
Often, they give the wattage at this voltage
Can easily get the effective resistance from
To make sure power is given to each device, they are all placed in parallel
*Actually, this is alternating current, later chapter
Fuse box A + –
Inside House
If you put too many things on at once, a lot of current is drawn
The wires, which have some resistance, will start to get hot
To avoid setting the house on fire, add a fuse (or a circuit breaker)

5. Why three wires?
If a device is functioning properly, you need only two wires
“Live” and “Neutral” wires
Toaster
If the live wire accidentally touches the casing, the person can be electrocuted
The wrong solution – connect the neutral to the casing
Now imagine the neutral wire breaks
The person again can be electrocuted
The right solution: Add a third “ground” wire connected directly to ground
Normally no current will flow in this wire
If the hot wire touches the casing, it will trigger the fuse/circuit breaker and protect the person

6. Magnetism Magnetic Fields Magnetic Forces
Certain objects and circuits produce magnetic fields
Magnetic fields, like electric fields, are vector fields
They have a magnitude and a direction
Denoted by B, or B(r)
They have no effect on charges at rest
They produce a force on moving charges given by
Perpendicular to magnetic field
Perpendicular to velocity
Magnetic field strengths are measured in units called a tesla, abbreviated T
A tesla is a large amount of magnetic field

7. The Right-Hand Rule
To figure out the direction of magnetic force, use the following steps:
Point your fingers straight out in direction of first vector v
Twist your hand so when you curl your fingers, they point in the direction of B
Your thumb now points in the direction of v B
If q is negative, change the sign
v  B
Vectors in the plane are easy to draw
Vectors perpendicular to the plane are hard
Coming out of the plane
Going into the plane B v

8. Magnetic fields do no work on pure charges
Work and Magnetic Fields
How much work is being done on a point charge moving in a magnetic field?
Work = force  distance
Divide the distance into little tiny steps, divide by time
Magnetic fields do no work on pure charges
But recall F  v
 = 90 and cos = 0 F B q v

9. Magnetic fields do no work on pure charges
Work and Magnetic Fields
How much work is being done on a point charge moving in a magnetic field?
Work = force  distance
Divide the distance into little tiny steps, divide by time
Magnetic fields do no work on pure charges
But recall F  v
 = 90 and cos = 0 F B q v

10. This version works even when you take relativity into account
Cyclotron Motion
Consider a particle of mass m and charge q moving in a uniform magnetic field of strength B B v
Motion is uniform circular motion
Centripetal force formula: v F F F q F
This version works even when you take relativity into account v v
Let’s find how long it takes to go around: